Hybrids of Corylus avellana, C. americana and C. cornuta are a potential crop for the Upper Midwest. Current nitrogen (N) recommendations for hazelnut production are based on research from Oregon and may not be applicable to these hybrids in the Upper Midwest due to differing soils, climate, genetics, and growing systems. In 2003 we established N-rate trials in three new plantings and in four three- to six-year old plantings. In the new plantings, a strong negative linear effect of N rate on transplant survival was observed. In the second year we added additional trials on same-aged plants that had not previously been fertilized and found no effect on survival. In the established plantings, results showed that N responses of hybrid hazelnuts fit patterns for other woody crops: there were no N-responses on soils with high organic matter or on soils with suspected P or K deficiencies. Where N-responses were observed, they suggested that the N requirements of hybrid hazelnuts in the Upper Midwest are very low relative to those of European hazelnuts in the Pacific Northwest. Leaf analyses suggested that about 1.9% N should be considered the threshold between deficiency and sufficiency and 2.2% N should be regarded as adequate.
In another experiment, we hypothesized that N application when the bushes were most fully leafed out would result in highest N uptake efficiency. We used 15N-labeled ammonium nitrate to measure NUE from soil applications in mid-April, late April, late May, early August, and mid-September. Nitrogen applied in either mid- or late April never comprised more than 5% of the total N in shoots or leaves, suggesting that N used for early leaf expansion came primarily from stored reserves. Applications made after April demonstrated that N was quickly translocated to rapidly growing plant parts: N applied in May went to leaves, N applied in August went to developing nuts, N applied in September went to catkins, and N applied applied in August and September appeared in new shoots the following April at higher levels than it did above ground the previous October, showing that N applied late in the season may be stored below ground over the winter. Nitrogen use efficiency was highest for August and September applications at one site and August and mid-April applications at the other, implying that summer is generally the best time to apply N for most efficient uptake. This may be because cool season weeds took up much of the N applied in the spring. However, overall NUE was low, only 5% for August applications, suggesting a need for development of other methods of improving NUE.
Hybrid hazelnuts are a potential new crop for the Upper Midwest of the United States. These are hybrids between Corylus avellana L., the common European hazelnut, which is the basis for commercial production worldwide, and two species of native American hazelnuts, Corylus americana Walter, the common American hazelnut, and Corylus cornuta Marsh, the beaked hazelnut. The two American species may confer to the hybrids resistance to Eastern Filbert Blight (EFB), a disease that threatens to decimate the hazelnut industry in the Pacific Northwest, as well as cold hardiness and tolerance to the extreme weather conditions of the Upper Midwest (Rutter and Shepard, 2002).
Alternatives to row crops are needed that combine economic potential with ecological sustainability. The benefits of woody perennials are well appreciated in tropical agroecosystems; Thevathasan and Gordon (2004) have found similar benefits in Ontario Canada, in the Northern temperate region, as well. They found that hybrid poplars intercropped with row crops increased soil organic carbon, improving N cycling efficiency and thus reducing N leaching and nitrous oxide emissions, compared with annual monocrops. Carbon sequestration was four times higher per year than for annual crops. Earthworm populations and diversity of beneficial insects and birds were increased. In addition, energy costs for management of woody perennials are lower, primarily because annual tillage is not needed and fertilizer requirements are lower. It is likely these benefits may be realized with hybrid hazelnuts, whether they are grown as field scale cash crops, small scale multi-purpose plantings in field and homestead windbreaks, living snow fences, or as riparian buffers (Josiah, 2001).
Hazelnuts also have economic potential. Currently only 4% of the world crop of hazelnuts is produced in the United States (O’Conner, 2006), and only 20% of the hazelnuts consumed in the United States are produced in this country. So there is a large un-met market demand, which is likely to grow as new hazelnut products are developed (Rutter and Shepard, 2002). Recent developments in our understanding of human health may also help grow demand: the oil in hazelnuts, as in all nuts, is high in healthful monounsaturated fatty acids, which help reduce the risk for heart disease (Richardson, 1997; Willet and Stampfer, 2003). Hazelnut oil also has characteristics desirable for industrial uses, including biodiesel (Xu et al., 2007), which may also increase demand. With 2008 prices up to $2.16/lb wholesale for unshelled nuts, and $11.95/lb retail for shelled nuts, there is good economic potential — if production costs can be kept down.
Thus a viable hazelnut industry in the Upper Midwest would provide an alternative energy and food crop to help farmers diversify economically while enhancing ecological sustainability. However, the ecological benefits of growing hazelnuts could be undermined if inappropriate N fertilization practices are used. Anecdotal observations by growers suggest that hybrid hazelnuts are heavy N feeders, but this has not been substantiated. The costs of overapplication of N are high: too much fertilizer N at the wrong time may stress or kill young seedlings, and reduce yield and nut quality and excess N may become a pollutant and is economically wasteful (Sanchez et al., 1995). According to Weinbaum et al. (1992), orchard crops have among the lowest N uptake efficiency (NUE) of any agricultural crops. The objective of our research program is to improve the viability of hazelnuts as an alternative crop for the Upper Midwest region by developing N fertilization recommendations that balance crop requirements with environmental goals. To that end we conducted two sets of studies, first some N rate studies, and second some N timing studies using 15N tracers.
In Minnesota, the traditionally recommended time to apply N to woody crops is spring (Rosen and Eliason, 2005). But this is problematic because it is a busy time of year for many growers and because the soil frequently is too wet for tractor traffic. Moreover, applying N when soils are cold and roots are inactive may contribute to low NUE (Dong et al., 2001).
Concerns that N applications in later summer and early fall (Aug. to Sept.) may stimulate late season shoot growth and delay stem hardening, leading to winter damage, discourage many growers from applying N from early August to mid-September, even though Pellett and Carter (1981) showed that this occurs only if plant N concentrations are very high, as indicated by leaf N above optimal concentrations. Waiting to fertilize until after leaf senescence, when the possibility of growth stimulation has passed, may not give the plants adequate time for N absorption before cold and wet soil conditions become conducive to N leaching or runoff (Kowalenko, 1996). Although some researchers have found that N uptake can occur in the dormant period (Grasmanis and Nicholas, 1971), Aguirre et al. (2001) found that N applied to apples (Malus domestica) after leaf senescence is not utilized very efficiently.
The most efficient time for N uptake by woody crops appears to be when plants are fully leafed out and actively growing. Titus and Kang (1982) reported that apple trees take up N continuously throughout the growing season, with a peak in summer. In peaches (Prunus persica), Munoz et al. (1993) found very little N uptake during dormancy through bud break, maximum uptake during rapid shoot growth and fruit expansion, and reduced uptake after August, when translocation to storage was high. Weinbaum et al. (1978) showed a clear correlation between NUE of prune trees (Prunus domestica) and the presence of leaves, partly because N uptake from the soil and N assimilation in the plant require photosynthetic energy from leaves, and partly because leaves generate a transpirational pull for nutrients. In Oregon, Olsen (2001) found that European hazelnuts take up soil-applied N most efficiently during active spring growth, which is from May to June in that environment.
Our objective was to determine the effect of N application time on fertilizer NUE in hybrid hazelnuts in Minnesota, using 15N-enriched fertilizer applied on five dates: mid-April, late April, late May, early August and mid-September. We hypothesized that NUE would be low early and late in the season and highest in the middle.
Our long-term goals are to improve the economic viability of small farms by giving farmers profitable alternatives to row crops, and to improve the environmental sustainability of food production by developing perennial cropping systems. The intermediate objective of this project was thus to develop nitrogen (N) recommendations for hybrid hazelnuts, for both plant establishment and nut production, that are specific to the growing conditions of the Upper Midwest and to the hazelnut cropping system proposed here in order to enhance the viability of hybrid hazelnuts as an alternative cash crop for farmers of the Upper Midwest. We also hoped to demonstrate the potential for using hazelnuts to take up excess N in environmentally sensitive areas, such as riparian buffers.
The immediate objectives of this project were: 1) to evaluate N responses to variable rates of N applications to hybrid hazelnuts in the field;
2) to evaluate leaf and soil analysis as diagnostic tools upon which to make recommendations; and
3) to determine the best timing of N applications for optimal N use efficiency. Although research has been conducted on the nutrition of European hazelnuts elsewhere, there are several compelling reasons to expect that this research may not be applicable to the hybrid hazelnuts developed for the Upper Midwest because of differences in soil, climate and genotype.
NEW PLANTINGS FERTILIZED THE YEAR OF PLANTING
SITES: We conducted N rate trials from 2003 through 2005 in three new hybrid hazelnut plantings in Minnesota. All three sites were located at University of Minnesota Experiment Stations on three different soil types: a loam (Chanhassen), a loamy sand (Becker), and a silt loam with disturbed horizons (Rosemount). These sites had relatively low soil organic matter, and were amended with P and K as needed before planting, based on rates recommended for woody crops in Minnesota (Rosen and Eliason, 2005).
Planting material consisted of five genetic lines of half-sib seedling hybrid hazelnut bushes (Badgersett Research Corporation in Canton, Minn.). They had been grown in 13 cm tall plug containers and were transplanted to the field at two to three months of age.
TREATMENTS AND PLOT LAY-OUT
Treatments were six N rates: 0, 0.1, 0.2, 0.4, 0.8, and 1.2 oz/plant. They were replicated five times at each site, in a randomized complete block design. That is, each of five blocks contained six plots, one for each treatment. Plots consisted of six plants in a row, one of each of four half-sib lines of seedlings, plus an additional plant on each end as a buffer, each of which received the same N treatment. Plants were spaced 5 ft apart within rows and 12.5 ft between rows. The buffers were planted in mid-May 2003, and the others were planted in late June, all by hand.
Plants were watered immediately after planting, and as needed for the first two years. We seeded perennial ryegrass (Lolium perenne L.) between rows around the same time we planted the hazelnuts; it was kept mowed. We maintained a 2 ft diameter weed-free circle by hoeing shallowly around each plant.
NEW PLANTINGS WITH DELAYED INITIATION OF FERTILIZATION
In 2004 and 2005 we added two new small N rate trials at Becker, using plants that had been transplanted one year previously, to evaluate the effects of delaying N applications for a year after planting. In the first trial we used N rates of 0, 0.1, 0.2, or 0.4 oz/plant, and in the second we used N rates of 0, 0.2, 0.4, 0.8, and 1.2 oz/plant. Plot management, and N application methods were the same as for the trials started in 2003.
YOUNG ESTABLISHED PLANTINGS
We also conducted N rate trials from 2003 through 2005 in four established seed-propagated hybrid hazelnut plantings in Minnesota. All genetic material came from Badgersett Research Corporation in Canton, MN. Two plantings were on-farm, planted in 1997 and 2000, and two were on University of Minnesota experiment station land, planted in 2000 (Table 2). For the latter, our experimental design reflected previous experiments.
SITES AND MAINTENANCE
Chippewa County: Hazelnuts were planted on this sugar beet/corn farm in 2000, with a spacing of 5 by 12.5 ft. It was maintained weed-free both in-row and between row with tillage and hoeing.
STAPLES (CENTRAL LAKES AG CENTER)
Hazelnuts were planted at this sandy site in 2000 with a spacing of 5 by 12.5 ft. It was mulched within rows with woodchips and mowed between rows. This site received overhead irrigation at the same rate and frequency as the adjacent corn field.
ROSEMOUNT: The hazelnuts at Rosemount, where the soil tested low in K, had been planted in 2000 for an experiment to determine the effect of K fertilization on hazelnut transplant growth and survival, with 0, 100, and 200 lbs/acre K applied at planting. Bushes were spaced 5 by 5 ft. Although no K-rate effects were observed when our experiment started in 2003, we set out our treatments in rows perpendicular to the K rates and genetic line, and included K rate as a covariate in our final statistical analysis. Weeds had been controlled with herbicide through spring 2002, but not again until 2003 when we began hoeing beneath hazelnut plants and mowing between them.
FILLMORE COUNTY: Hazelnuts were planted on CRP land on this homestead in 1997. Bushes were spaced 6 by 12.5 ft. No in-row weed control had been used in recent years; between-row weed control consisted of mowing.
PLOT LAYOUT: Plots consisted of three plants in a row, which were all fertilized at the same rate; data were collected only on the middle plant in a plot. Treatments were replicated from three to eleven times, depending on how many plants were available at a site, using a randomized complete block design. At all sites we continued management practices that had been established by previous managers.
Treatments were the same six N rates as for the new plantings: 0, 0.1, 0.2, 0.4, 0.8, and 1.2 oz/plant, with additional rates of 0.05 or 1.6 oz/plant if enough plants were available. Because plant spacing varied between sites, these rates vary when expressed as lbs/acre, but the intermediate rate of 0.4 oz/plant was approximately 40 lbs/acre. At Chippewa, based on lack of response in the first years, we increased our rates in 2005 by shifting all plots, except for the control plots, to the next higher rate, up to a top rate of 1.6 oz/plant. An application error at Rosemount in 2005 resulted in the conversion of the 0.05 oz/plant rate to 0.1 oz/plant and the 0.1 oz/plant rate to 1.6 oz/plant.
In both new and established plantings, N was applied as ammonium nitrate (NH4NO3) once a year from 2003 through 2005, with the same N rate repeated on each plot all three years. The new plantings were first fertilized in mid-July 2003, about two weeks after planting, but in subsequent years they were fertilized in spring at early leaf expansion. The established plantings were also fertilized at early leaf expansion in all three years. In the new plantings we surface-applied fertilizer in a circle about 6 ins from the main stem of each plant, whereas in the established plantings it was scattered beneath the drip line of each plant. At all sites it was incorporated with a hoe if possible. At Staples it was incorporated into the coarse woodchip mulch, whereas at Fillmore it was not possible to incorporate it due to dense perennial weed cover.
Before the start of the experiments we collected one soil sample from every block for determination of pH, organic matter, extractable P and exchangeable K at a commercial lab. Every year about a month after fertilization, we also collected soil samples from each plot at all sites to a depth of 12 ins to determine soil nitrate and ammonium.
We collected leaf samples in late July/early August every year, except that we did not collect any from the new plantings in 2003 because these plants were still too small. We collected the third fully expanded leaf from the apex of stems in full sun, at least twenty leaves per sample, consolidated them by plot, and analyzed them for N concentration.
In the established plantings, we measured bush height and width in early spring 2003 before bud-break to establish a baseline bush size. Likewise, we measured baseline plant height before the first N application in the two trials at Becker in which N fertilization was delayed for a year after planting. Subsequently, we measured bush height and width every fall after leaf drop through 2005 at all sites, and again in 2006 at Becker and Chanhassen. Height was measured from the soil surface to the highest live bud. Width was the average of two perpendicular measurements taken at the top of the plants, from which we calculated bush volume as a cylinder (V = πr2∙h).
In Aug. 2005 we excavated all the surviving plants three blocks planted for destructive sampling at Becker. We used a jet of water to wash the soil from the roots of each plant in situ, gently working the roots free from depths down to 2 ft, and lifted them out intact. We dried the plants, divided them into above- and below-ground portions, weighed them, and calculated root:shoot ratios.
Bushes began bearing nuts in 2003 at Fillmore, 2004 at Chippewa and Staples, and 2005 in the established bushes at Rosemount. In 2004, 2005, and 2006 we hand-harvested nuts and, after husking them, determined in-shell yield on a mass per bush basis. We shelled 20-nut subsamples to determine the proportion of nut mass comprised by kernel (“kernel percent”) and average mass of individual kernels (“kernel size”). We estimated kernel yield as the product of in-shell yield and percent kernel. The new plantings at Chanhassen began bearing nuts in 2007; there we merely recorded number of nut clusters produced on each bush, if any.
NITROGEN TIMING TRIALS
Experimental Sites and Experimental Design
In 2005 we conducted 15N tracer studies at two sites in Southeast Minnesota, near Amherst and Fillmore. Both sites were on slightly acid silt loam soils with moderately low organic matter. Bushes at both sites were seed-propagated. One-half of the bushes at Amherst were 13 years old and one-half were nine years old in 2005; all bushes at Fillmore were eight years old. Bushes at both sites were planted in hedge rows with no weed control other than mowed alleyways. Consequently, competition from weeds, mostly cool season grasses, was high. The experimental design was a randomized complete block, with four blocks arranged by landscape position, and by age and genetic line of bushes. Each block contained five treatment bushes and one control bush, which we selected for evenness of size. At least one untreated bush was present between each experimental bush. We measured initial height and width of each bush.
EXPLANATION OF HOW 15N TRACERS WORK
In nature, N atoms usually have 14 protons, giving them an atomic mass of 14. However, 0.366% of N atoms carry an extra proton, giving them an atomic mass of 15. Thus, the atomic mass of N atoms on average is 14.366. It is possible, in the lab, to increase the percentage of heavy N atoms to create 15N enriched N compounds. If these are used in fertilizer, it is possible to trace their movement through the soil and into plants by then measuring the average atomic mass of the N contained within those plants. It is thus possible to distinguish between the N that came from the fertilizer and the N that came from other sources.
APPLICATION OF TRACER SOLUTION
Nitrogen application dates were Apr. 11, Apr. 26, May 26, Aug. 1, and Sept. 10, 2005. At each application date, we applied 0.2 oz of 15N labeled ammonium nitrate in half a gallon of water, to one bush in each block. This supplied half the N of the rate, 0.4 oz/plant, that we found to be best for this age of hazelnuts in our N rate study. We poured the solution in a ring around each plant about 8 inches from the base and washed it into the soil with an additional 2 gallons of water.
Each time tracer was applied, except for the first time, we also sampled leaf and bark material from all bushes to which tracer had previously been applied, as well as from the control bushes, which had received no N. At Fillmore we also sampled immature nuts in early August and mature nuts in September. Commercial harvest of nuts prevented collection of nut samples at Amherst. In October we sampled new catkins at both sites. We dried and ground all samples, then had them analyzed with a mass spectrometer.
On April 24, 2006, the year after N application, we coppiced all bushes at Amherst at ground level to determine above-ground biomass at budbreak, after first measuring height and width. We chipped the entire bushes with a garden shredder, then dried and weighed them to determine above-ground biomass. Subsamples were processed as described above for analysis with a mass spectormeter. On May 31 we harvested stump sprouts that were emerging where we had coppiced bushes and analyzed them as described above. We did not coppice the bushes at Fillmore that spring; instead we sampled newly emerging shoots as we had done the previous year. Then, on Nov. 22, 2006, one year after the final tracer application, we harvested whole bushes for above-ground biomass and 15N content, after first measuring their height and width, as we had done at Amherst that spring. At that time we also collected nuts and autumn leaves.
We calculated the proportion of the total N in a plant part (leaf, or bark, or whole plant) that came from the 15N that had been applied. Nitrogen uptake efficiency (NUE) was calculated as the proportion of the 15N that had been applied that was taken up by the plants. We calculated bush growth as the difference between bush volume before and after the experiment, assuming a cylindrical shape where volume = height∙π(width/2)2. We also analyzed growth by regression on above-ground dry mass, with initial bush volume as a predictor.
As expected, soil inorganic N, both as nitrate and as ammonium, was highly correlated with N application rates on all sampling dates and at all sites, although the slopes differed among sites and sampling dates. In the established plantings, soil inorganic N in control plots was roughly correlated with soil organic matter. Based on total inorganic N measured in control plots in summer 2005, we estimate that N available to the hazelnuts in the top 12 ins of soil, without fertilization, was about 143, 48, 43, and 12 lbs/acre at Chippewa, Rosemount, Fillmore, and Staples, respectively.
In the plantings in which N had been applied two weeks after transplanting, survival declined steeply with increasing N rates at all three sites (p 0.91). The symptoms that preceded seedling death, marginal leaf necrosis followed by leaf death and abscission, were observed within two weeks of N application at Becker, and by fall at the other two sites. At all sites, N rates of 0.4 oz/plant or higher significantly reduced survival, but even the lowest N rate, 0.1 oz/plant, significantly reduced survival at two of the sites (p < 0.05). In contrast, in the plantings in which N applications were postponed until a year after transplanting, there was no mortality attributable to N fertilizer.
This mortality could have been caused by high soil ammonium concentration, by salinity, or by the soil acidifying effect of ammonium nitrate. We are reasonably confident that soil acidification was not an important cause because the pH in the control plots at Rosemount, where survival was 88%, was 5.1, which was lower than the 5.3 pH in the high N rate plots at Becker, where survival was only 9%. Moreover, Adiloglu and Adiloglu (2005) reported that hazelnuts in Turkey are grown on soils with pH as low as 4.3.
The first symptoms observed, marginal chlorosis and necrosis, could have been due to either salt damage or ammonia toxicity. The few plants that survived the high N rates into the second year had small, dark green leaves that were misshapen, cupped, or wrinkled, which are symptoms of ammonium toxicity (Pilon, 2006). Nitrogen concentration in these leaves in 2004 was 2.5% or higher, which is considered excessive (Olsen, 2001). However, that the mortality was first observed at Becker, where soluble salts in samples collected two weeks after N application were in the saline range, supports salinity as a cause.
Three reasons for the high mortality may be that fertilizer was concentrated near the plants instead of broadcast, that it was applied only two weeks after transplanting, and that a highly soluble form of fertilizer, ammonium nitrate, was used. The buffer seedlings, that were transplanted a month earlier than the others, died at a similar rate as later-planted seedlings, so waiting one and a half months before fertilizing is not long enough for young seedlings. Although fertilizing one year old transplants did not reduce survival, there was no benefit to it, so our findings support Olsen’s (2001) recommendation to avoid N fertilization in the first year.
The highest N rates significantly inhibited above-ground growth of the surviving seedlings, as measured by height, at Rosemount and Becker. At Rosemount the only N rate that did not significantly reduce height relative to the controls was the lowest one, 0.1 oz/plant. However, positive responses to intermediate N rates began to appear at Becker at the end of the third year, when plant height was significantly greater in the plots that received 0.4 oz/plant than in the control plots (p < 0.05), whereas height declined with higher rates. The growth response to applied N was flat at Chanhassen, where plants grew taller than at the other sites, regardless of treatment. Above-ground plant height was not closely related to soil inorganic N at any site.
Whole plants excavated at Becker after three growing seasons showed the same growth patterns as were found for the shoots: total dry weight increased for the 0.1 oz N/plant rate, but declined for higher N rates. High N inhibited shoot growth and root growth equally; thus N fertilization had no effect on root:shoot ratios. Roots comprised an average of 67% of total woody biomass, regardless of N rate. Our data do not support concerns about N fertilization stimulating shoot growth at the expense of root growth, leading to plants less able to withstand stresses such as drought and herbivory, as commonly reported for other woody species. These concerns may still be valid in situations conducive to strong positive shoot growth responses, which did not occur in our experiment.
NEW PLANTINGS WITH DELAYED INITIATION OF FERTILIZATION
At the end of two growing seasons there were no significant N rate responses in the first trial of N first applied one growing season after transplanting. In the second trial bush height increased with N applications up to 0.4 oz/plant, but decreased with higher N rates. For plants fertilized for the first time in the second season after transplanting, we conclude that N is still not beneficial, at least initially.
CONCLUSIONS ABOUT GROWTH OF NEW PLANTINGS
To summarize, there was a positive N response at only one site out of three, the site with sandy soil, the best rates were only 0.1 to 0.4 oz/plant/yr, and the responses only became apparent after three years. In conclusion, it appears that the N requirements of hybrid hazelnuts are very low for their first three years, even on low organic matter soils, and that it is best to apply either no N or very low N in the first two years after transplanting. Our results support the recommendations given for many other woody crops to incrementally increase N application rates each year according to size or age of plant (Sanchez et al., 1995). Application at rates in excess of demand merely reduces N uptake efficiency (Weinbaum et al., 1992).
YOUNG ESTABLISHED PLANTINGS
The most significant predictor of growth at all sites was initial bush volume: the larger a bush was at the beginning of the experiment, the more it grew. There were dramatic differences in growth rates and patterns of response between sites.
CHIPPEWA: Hazelnut bushes grew larger, maintained leaves that were larger and darker green, and bore nuts more abundantly at Chippewa than at other sites. However, there were no growth responses to applied N at Chippewa in any year. This lack of response to N was not surprising, given the high organic matter of the soil combined with high mineralization rates due to repetitive tillage. We estimated that 143 lbs/acre of inorganic N was available from the soil in the top 12 ins in 2005. Assuming that root spread was equal to canopy spread, the average bush at Chippewa would have had access to 1.3 oz of inorganic N in the top 12 ins of soil, which is much more than 0.25 oz, the total amount of N we estimated to be contained in that size of bush. In actuality, hybrid hazelnut root spread is much greater than canopy spread, and rooting depth is much greater than 12 ins, so the bushes at Chippewa had access to much more than 1.3 oz of N, and thus were not likely to deplete the available N supply soon. However, demand for N and other nutrients is likely to increase as yields increase and nutrients are exported with the harvest (Weinbaum and Van Kessel, 1998; Youssefi et al. 2000).
At the other three sites, N was limiting to growth, as expected due to lower organic matter soils, although this limitation did not become apparent until the second or third year after N rate trials began.
STAPLES: As expected, the strongest N response was at Staples, due to its sandy, low organic matter soil. A positive linear growth response became apparent at Staples at the end of the second year (p = 0.0063) and continued into the third (p = 0.0007), when the bushes fertilized with the highest N rate continued to be significantly larger than the controls (p = 0.0341). Again assuming root spread equal to canopy spread and a rooting depth of 12 ins, we estimated that only 12 lbs/acre or 0.03 oz/plant would have been available from the soil at Staples without fertilization, which is substantially less than 0.25 oz, the amount of N we would normally expect to be contained in that age of bush. This explains why there was a response to N at Staples: lack of N was limiting growth. Unfortunately, the lack of a plateau response at Staples prevented us from basing N rate recommendations on results there.
ROSEMOUNT: Although we observed no responses to the K fertilization that preceded our research during the first two years of our research, in the third year it became apparent that K was limiting the response to N fertilization. Where no K had been applied at planting there was no response to applied N, whereas where the high rate of K had been applied there was a positive linear growth response to applied N (p = 0.0013). Our hypothesis that K was limiting is supported by an average leaf K of only 0.3 %, which is considered severely deficient by Oregon standards for hazelnuts (Olsen, 2001). Where K was not limiting, in the K-fertilized plots, N was limiting, as expected because the calculated amount of N available without fertilization was 48 lbs/acre, or 0.1 oz/plant, is less than the 0.25 oz N expected to be contained in that age of plant.
In K-fertilized plots, the treatment with the largest bushes at the end of the experiment was the one in which the N rate had been increased from 0.1 to 1.6 oz/plant in the third year (p = 0.0010). By contrast, the bushes in the second-best treatment, those that had received 0.8 oz/plant all three years, had only grown half as much as those in the 0.1/1.6 oz treatment, even though over the three years a total of 33% more N was applied in the 0.8 oz treatment than in the 0.1/1.6 oz treatment. This suggests that the bushes in the 0.8 oz treatment were not able to take up or utilize all the fertilizer supplied to them from this relatively high rate in the first two years, possibly because their root systems were still too small to intercept it: N uptake ability is strongly affected by root volume (Ran et al., 1994). These results further support recommendations commonly made for woody plants, to increase N rates with increasing size and age of plant (Sanchez et al., 1995). This is why recommendations for European hazelnuts in Oregon are based on age of plant (Olsen, 2001).
FILLMORE: The bushes at Fillmore grew more slowly than at other sites. Although the planting was three years older than the others, and started out the largest, at the end of the three years the bushes at Fillmore were comparable in size to Staples and Rosemount, and were smaller than those at Chippewa. The leaves also were notably smaller and paler, and senesced earlier in the fall.
This slow growth could have been due to the fact that the soil was low in both P and K. Leaf K averaged within the severely deficient range at 0.38%, as defined for Oregon hazelnuts (Olsen, 2001), and leaf P was 0.12%, which is barely above the deficiency threshold of 0.10%. Unfortunately, soil K did not vary across the planting, so we could not evaluate its effect on growth, but soil P did vary by block. In the third year a strong interaction between soil P and applied N became apparent (p = 0.0008): there was a negative N rate growth response in the four blocks with low soil P (p = 0.0353), whereas in the four blocks with moderate soil P there was a plateau response to applied N (p = 0.0497). With adequate levels of P, all N rates of 0.4 oz N/plant or more produced bushes that were significantly larger than the controls at the end of the third year (p < 0.0459), whereas there were no significant differences between the 0.4 oz/plant N rate and higher rates.
That P may have been limiting crop response to N at Fillmore would have been surprising because a P response in nut crops is not common (Proebsting and Serr, 1954) and because the levels of soil P that appeared to be limiting, 15 – 18 ppm Bray-P, are considered only moderately low for other woody crops (Rosen and Eliason, 2005). However, plants frequently respond to low soil P by increasing root growth relative to shoot growth, which enables them to extract scarce P from a larger soil volume (Ericsson, 1995). The plants in the lower P soils at Fillmore may have been allocating resources below ground. Thus, an above-ground N response may be manifested later, after they have established large root systems to compensate for low P.
Yet another factor limiting growth at Fillmore could have been competition from weeds growing close to the bushes. We observed rings of darker green weeds around bushes fertilized at the higher rates, and tissue analysis of weeds in 2004 confirmed that they were utilizing fertilizer N: weed N concentration and thus N content both increased linearly with applied N (p = 0.0016 and p < 0.0001, respectively). However, much of the N taken up by weeds the first year should have been released as they died and decomposed in subsequent years. Sicher et al. (1995) affirmed that deep-rooted weeds may sequester and release N in a similar fashion as cover crops, thus actually enhancing the long term efficiency of N uptake by releasing it slowly, in greater synchrony with the ability of the crop to take it up. As the hazelnuts grow larger and shade out the weeds they should be better able to take advantage of N thus released. However, this should not be taken as justification to ignore weed control. Many authors (Merwin and Ray, 1997; Hangs et al., 2003) have found that weeds close to woody crops delay growth, not so much because of competition for N, but because of competition for moisture (Davis et al., 1999). Fertilization may intensify this competition for moisture by stimulating weed growth (Campbell et al., 1994). However, de Montard et al. (1999) found that competition for moisture was alleviated in later years as tree roots colonized soil horizons deeper than the competition.
We did not observe the high N response we expected in the low organic matter soils at Fillmore, where we estimated that only 43 lbs/acre, or 0.1 oz/plant was available without fertilization in the top 12 ins of soil under the plant canopy. This is less than half of the 0.25 oz N content that we measured in the above-ground biomass of other bushes of the same age at Fillmore in our tracer experiment (see below). This demonstrates the ability of woody plants such as hybrid hazelnuts to accumulate and store N as it slowly becomes available over many years. It demonstrates their “frugality”, in the words of Roversi and Ughini (2005).
DISCUSSION AND CONCLUSIONS ABOUT GROWTH OF ESTABLISHED PLANTINGS
At all three sites at which a growth response was observed, it did not occur until the second or third year after fertilization was initiated. This kind of delayed response to fertilization is common in woody plants and has three possible explanations. First, it takes time for N to be taken up, assimilated, used in photosynthesis to produce the carbohydrates needed for growth. Secondly, if soil-derived nutrients are limiting, photosynthates may be allocated to roots before shoots, which enables them to overcome the limitation by enhancing capacity for nutrient uptake (Millard and Neilson, 1989), as described for P above. Finally, N applications prior to the start of our research could have supplied enough N for the first year of the experiment, when plants were still small. It is likely that all of these explanations played a role. In conclusion, N fertilization may be effective in the long term even when results are not immediately observed.
Overall, we found that the N response of hybrid hazelnuts in the Upper Midwest was low relative to European hazelnuts in the Pacific Northwest. Whereas 8 oz N/plant are recommended for six- to seven- year old trees in Oregon (Olsen, 2001), our results from Fillmore, the only site at which N responses reached a plateau, suggest that 0.4 oz/plant is the maximum rate that we could recommend for eight-year-old hybrid hazelnuts. This discrepancy is likely due to the fact that the hazelnuts in our study were still relatively small and were not yet exporting significant quantities of N with nut harvests. In our tracer study (see below), we found that N uptake efficiency is only 5 to 9 %. Thus only 0.02 to 0.04 oz out of 0.4 oz N applied are likely to be absorbed in one year. Thus it is not surprising that it took hybrid hazelnut bushes at Fillmore eight years to accumulate 0.25 oz N in their above-ground biomass.
Another factor contributing to the low N requirements of hybrid hazelnuts may be the relatively high ability of the soils of the Upper Midwest to supply N to plants without supplemental N. The 0.1 oz N/plant we calculated as being available at Fillmore from non-fertilizer sources, is significant relative to the 0.25 oz N contained in 8-year-old bushes; soils at Chippewa and Rosemount provided even more N. To prevent overapplication of N, fertilizer recommendations should thus consider both the N that is already stored in the plants and the N that is potentially mineralized from soil organic matter.
LEAF N RESPONSES
Nitrogen recommendations for mature European hazelnuts in the Pacific Northwest are based on comparing the N content of leaf tissues to optimal levels (Olsen, 2001). Our objective in doing leaf analysis was to determine whether the standard leaf N ranges developed for hazelnuts in Oregon (Olsen, 2001) are applicable to hybrid hazelnuts in the Upper Midwest, and if not, to define alternative optimal levels. Our results, however, demonstrate the complexity of this approach. Krauss foliar vector diagnosis, as described by Black (1993), describes an array of different responses that can occur when a nutrient is applied: if the applied nutrient was limiting to growth, a growth response may occur with or without an increase in leaf concentration of that nutrient, depending on whether other factors then also become limiting. Growth with increased leaf concentration suggests that the applied nutrient was initially limiting, but that other nutrients became limiting, according to the law of the minimum, whereas increased concentration with no growth signifies that some factor other than the applied nutrient, such as other nutrients or moisture, is limiting to growth. Conversely, the concentration of the applied nutrient may decline by dilution if growth is vigorous. Induced deficiency, where application of one nutrient interferes with the uptake of another, further complicates the interpretation, as does toxicity. Ran et al. (1994) found that most woody plants maximize photosynthesis by using additional N to increase leaf surface area while keeping N per unit leaf area relatively constant. Conversely, in face of N limitation, leaf area is reduced before leaf N concentration falls.
LEAF N RESPONSES OF NEW PLANTINGS
Leaf N increased with applied N at Rosemount and Becker in both 2004 and 2005 (p < 0.006), and at Chanhassen in 2004 but not 2005, when leaf N was flat across N rates. Leaf N was also correlated with soil inorganic N at the same sites and years.
At Rosemount, leaf N was negatively correlated with plant growth in both 2004 and 2005. The high leaf N concentration without growth there suggests that something else besides N was limiting to growth there, or that perhaps growth was inhibited by high concentrations of leaf N.
At Becker, leaf N was negatively correlated with plant growth in 2004: a dip in leaf N for the low N rates corresponded to a growth response for these same rates, suggesting dilution of leaf N by growth. The relatively low concentrations of leaf N at Becker, even with the high N rates, suggest that something was interfering with N-uptake there. Perhaps the N was leached out of the sandy soils before substantial uptake.
At Chanhassen, leaf N concentration was not correlated with plant growth, suggesting that N was not limiting to growth. The decline in leaf N concentration from 2004 to 2005, in all treatments except for the control, suggests dilution by the growth that occurred in that time interval. The low N in the controls at Chanhassen abated from 2004 to 2005, shows that the controls were able to access additional non-fertilizer N, possibly by allocating photosynthate to root growth (Ericsson, 1995), while maintaining strong shoot growth.
LEAF N RESPONSES OF YOUNG ESTABLISHED PLANTINGS
The four established plantings showed four unique patterns of leaf N response.
At Chippewa, leaf N was not correlated with applied N. It averaged around 2.1 % across all treatments in 2003, 2.3 % in 2004, and 2.1% in 2005. Although 2.1% is slightly below the 2.2% threshold to sufficiency, we do not believe it indicates N deficiency because the leaves were dark green and leathery, and because plants were growing and yielding vigorously. Rather, this slightly low leaf N suggests dilution by growth. This is supported by the fact that leaf N was negatively correlated with growth, albeit weakly (p = 0.0714), in two of the years, and that leaf P and K were also low at Chippewa.
At Staples, there was a positive linear leaf N response to applied N the first year (p = 0.0026), but none the second or third years, when it averaged around 1.8% and 2.0%, respectively, for all treatments. This pattern is just the opposite of the growth pattern: there was no growth response the first year, but increasingly positive growth responses the second and third. This suggests that the N was taken up by the plants in the first year, resulting in increased leaf N that year, but that it took until the second year for it to be manifest as a growth response. Then, because growth resulted in more leaves between which to divide the additional N, the leaf N response disappeared in the second and third years. That soil N was not correlated with leaf N suggests that the plants took up the N fairly quickly after application, before it was leached out of the soil and before we sampled one month later.
At Rosemount leaf N increased with increasing N rates the first year, peaking at 2.0 %. In the second year, leaf N showed a curious pattern: it was about 2.3 % for both the controls and the three highest rates, but dipped to 2.0% for the three lowest rates. In the third year, leaf N in the control was 2.5%, which was higher than the 2.3% of the fertilized bushes. Again, this curious pattern is the inverse of the growth pattern. There was a leaf response the first year, but it took until the second for the additional N to be manifest as a growth response, but only for the lowest N rates. The dip in leaf N for the low N rates in the second year corresponded to a peak in growth response for the same rates. The graphs for the two were mirror images of each other. This suggests that the still relatively small plants were unable to take advantage of N rates higher than that, or perhaps the higher N rates exacerbated the K deficiency. But in the third year the growth response was positive linear, resulting in the dilution of leaf N except for in the controls, which maintained high leaf N by growing less.
At Fillmore there was no leaf response to applied N the first year, but there was a positive response in the second and third years, when it peaked at 2.2 %N. The amount of N required to reach the peak was higher in the third year than in the second. Again, this pattern can be explained by the growth pattern. Like at Staples and Rosemount, there was no growth response until a year after a leaf response, though at Fillmore the leaf response was delayed until the second year. Likely this is because the applied N was initially taken up by the weeds, but re-released to the hazelnuts in the second year, in time to stimulate growth in the third year. Nitrogen was clearly limiting to growth at Fillmore, as evidenced by the fact that leaf N in the controls and in the lowest N rate plots, in which growth was stunted, dipped well below the 1.8% threshold for severe deficiency by Oregon’s standards. Plants in these plots were so depleted of N by the third year that the plants could not maintain leaf N concentrations even by reducing leaf area; they became chlorotic as well. Low leaf N combined with no growth indicates a need for N. However, the fact that in the fertilized plots N was accumulating in leaves instead of being diluted by growth (that is, leaf N was positively correlated with growth at p = 0.0227), suggests that other factors were still limiting growth, which is why the growth response at Fillmore was not as strong as at other sites.
Putting results from all seven sites together shows that leaf N concentration should not be considered alone in making N recommendations. Plant vigor, leaf color, growth and yield all need to be considered together. These other factors suggest that the hazelnuts at Chippewa and Chanhassen do not need N fertilization in spite of borderline low leaf N concentration. Conversely, the high leaf N concentration in the established planting at Rosemount suggests that there may be no response to N fertilization until other limits on growth, such as K deficiency, are removed.
DEVELOPING STANDARD LEAF N RANGES FOR HYBRID HAZELNUTS IN THE UPPER MIDWEST
Once again, our objective in doing leaf analysis was to determine whether the standard leaf N ranges developed for hazelnuts in Oregon (Olsen, 2001) are applicable to hybrid hazelnuts in the Upper Midwest, and if not, to define alternative optimal levels.
Average leaf N concentration exceeded 2.5%, the threshold for toxicity by standards for Oregon, in plots fertilized with 0.4 oz/plant or more, in 2004 at Chanhassen and Rosemount. Most leaves in these plots were small, distorted, and very dark green. The highest concentrations observed at the two sites were 3.2 and 3.4% N in 0.8 oz/plant plots at Chanhassen and Rosemount respectively, both of which had 83% mortality. Thus 2.5% leaf N appears to be a valid threshold for toxicity, because plants with leaves containing > 2.5% N were clearly stunted and unhealthy.
The threshold between moderately deficient and severely deficient, 1.8% N, appears to be low. Plots with average leaf N less than 1.8% had a large proportion of highly chlorotic plants, so 1.8% is clearly deficient. Thus we argue that the threshold should be closer to 1.9%, if we accept the Piper-Steenbjerg effect (Black, 1993). This states that if the nutrient in question is strongly deficient in the control plants, the lowest levels of nutrient application will result in a decline in leaf concentration relative to the controls, while higher levels will result in increased concentrations. This was the case at Becker in 2004, where there was a statistically significant growth response to the lowest rates of N, and at the new planting at Rosemount in 2005, where although the overall growth response to applied N was negative, trends suggested that a growth response to low N rates may have developed in subsequent years. That the leaf N in the controls at both sites was about 1.9% N, suggests that 1.9% should be considered the threshold between moderately deficient and severely deficient.
A more important question is whether the 2.2% N threshold between N deficiency and sufficiency in European hazelnuts in Oregon is applicable to hybrid hazelnuts in the Upper Midwest. Our data from new plantings show that the N rates required to raise leaf N to 2.2% may be higher than the rates that produce the greatest growth response. In the established plantings, overall leaf N concentrations were low by the standards of Oregon hazelnuts. Even at Chippewa, where there was no reason to suspect N deficiency, leaf N averaged below the sufficiency range in two years out of three. The only established planting with leaf N consistently above the sufficiency threshold, Rosemount, appears to have had high leaf N because something else was limiting. These results are consistent with European hazelnuts in Oregon, where Olsen (1997) found that 44% of established hazelnuts orchards in Oregon tested below 2.2% N, and 5% tested below 1.8% N. They are also consistent with British Columbia, where Kowalenko (1996) stated that 2.2 % N should be considered a target to be attained rather than to be surpassed. Kowalenko further stated that the N rates required to increase leaf N above 2.2% may sometimes be higher than the rates that produce the greatest growth response and may lead to over application relative to environmental concerns.
We thus conclude that leaf N concentrations below 2.2% N should not be a cause for concern as long as new shoot growth is vigorous. Thus the relatively low leaf N at Chanhassen and Chippewa in 2005, 2.0% and 2.1% respectively, may be acceptable, considering that these plants were growing vigorously and did not respond to fertilizer N. On the other hand, leaf N concentrations in the same range at Becker possibly do represent deficiency because growth was slow at Becker. However, applying more N is not necessarily the solution on sandy irrigated soils with a high leaching potential such as at Becker.
Putting these considerations together, we suggest that for hybrid hazelnuts less than 1.9% leaf N should be considered deficient, 1.9 to 2.0% should be considered borderline, 2.0 to 2.5% should be considered sufficient, and above 2.5% should be considered excessive. Because leaf nutrient concentrations are only one factor amongst several to be considered when making nutrient recommendations, these should be viewed as guidelines rather than rigid thresholds. Olsen’s work suggests the same is true in Oregon.
The strongest predictor of nut yield was bush size (p < .0001): larger bushes produced larger yields. This suggests that soil conditions and management practices that favor bush growth should also favor early nut bearing. Nitrogen fertilization, however, did not affect yield or nut size in the two years in which we collected yield data at the three sites that were planted in 2000. This was most likely because these bushes were just coming into nut bearing, and thus still allocating most of their resources to vegetative growth. Likewise, when nuts started to be produced at the new planting at Chanhassen in 2007, plants in control plots had as many nut clusters as plants in fertilized plots.
At Fillmore, however, where bushes were three years older than at other sites and had started yielding before our study began, there were significant, though inconsistent, yield responses to N rate: in 2004 the 0.4 oz N rate produced higher yields than the control (p < 0.0001). This is the same relatively low rate that produced the highest growth, which is contrary to the observation that there is sometimes a trade-off between vegetative growth and reproduction (Weinbaum et al., 1992). However, in some cases we found a negative relationship between N rate and nut size, which is consistent with Sparks (1987) who observed that high N rates can stimulate pecans to set more nuts than they are able to fill, leading to smaller nuts and more blanks.
We expect that N rate effects are likely to develop at other sites as they mature. Until further research can be done on mature plantings, we recommend that N fertilization rates be based on estimates of N removal with harvest, as recommended by Tagliavini et al. (1996) and Tous et al. (2005).
RESPONSES TO SOIL INORGANIC N
Our results show highly inconsistent responses to soil inorganic N. This is in keeping with the comments of Sparks (1977, p. 26) that “tree performance is neither consistently nor highly correlated with soil analysis,” and that “soil analysis is not highly correlated with leaf analysis”. N dynamics in woody crops are more complicated than in annuals for two reasons: first, their roots are present in the soil all year, and can absorb soil N whenever temperatures favor uptake (Dong et al., 2001), and second, they store N over the winter and reuse it the following season (Titus and Kang, 1982; Weinbaum et al., 1992). Soil inorganic N usually is measured at one instant in time. In contrast, woody plants respond to the N that is available over the course of many seasons. Because of the transient nature of inorganic N in the soil, in humid regions of the United States soil N recommendations for annual crops are based on soil organic matter or on crop productivity instead of on inorganic N. The rationale for this is even more valid for perennial crops. Measuring soil inorganic N in the control plots provided an estimate of N available from mineralization of soil organic matter. As expected, the two sites with rapid growth, Chippewa and Rosemount, had high organic matter soils and large pools of inorganic N in control plots relative to the two sites with slower growth, Fillmore and Staples. However, soil analysis is still useful, as in the cases of Rosemount and Fillmore, where correlations between growth and soil inorganic N were observed only in plots with high levels of K and P and helped our diagnosis that K and P were limiting to growth at these sites.
CONCLUSIONS FROM N-RATE TRIALS
Our plant mortality data support the recommendation from Oregon to postpone N fertilization until the second or third year after transplanting to avoid damage to the plants. Although applying N in the second year may not be harmful, there are no clear benefits to it. Our plant growth data suggest that the N requirements of established hybrid hazelnuts that are not yet in full nut production are very low in the Upper Midwest, relative to those of European hazelnuts in the Pacific Northwest. Thus we recommend only 0.4 oz N/plant/year starting in the third year, gradually increasing to 0.8 oz in the seventh year. After that, N fertilization should be based on leaf analysis until nut bearing is reached, after which N rates should replace N removed with harvest, in addition to leaf analysis. Our leaf N data suggest that for hybrid hazelnuts less than 1.9% leaf N should be considered deficient, 1.9 to 2.0 % should be considered borderline, 2.0 to 2.5% should be considered sufficient, and above 2.5% should be considered excessive. Factors such as deficiencies of other nutrients, soil moisture, weed management system, and yield must also be considered in making N recommendations.
NITROGEN TIMING STUDIES
SAMPLES COLLECTED IN 2005
There were no differences in N concentration in leaves, bark, nuts or catkins due to N fertilization, not even between fertilized and unfertilized plants.
SHOOTS AND LEAVES: Very little fertilizer N appeared in leaves until late May, suggesting that N uptake and assimilation from April applications is low. Even as late as early July, the proportion of leaf N that came from fertilizer was only 9% at Fillmore and 5% at Amherst; the rest was most likely derived from N reserves within the plants. In general, the highest proportion of fertilizer N measured in leaves was measured on the first sampling date following an application date; after that fertilizer N declined as a proportion of total N.
BARK: As expected, fertilizer N allocation to bark was very low in the spring, but increased through the season, reaching its highest levels in October. Not only did fertilizer N applied in the early spring appear in bark at increasingly higher levels as the season progressed, but proportionately more was allocated to the bark the later the N was applied in the season. The highest proportion of N to come from fertilizer that we measured in bark was 6%, in October at Fillmore, in plants that had been fertilized in September. This is consistent with Weinbaum et al. (1978) and Olsen et al. (2001b), who found that fall-applied N is stored in the bark or roots for use the following season.
NUTS: A much higher proportion of the N in nuts harvested in September came from fertilizer applied in early August (12%) than from fertilizer applied in late May (3%). This suggests that between August and September a high proportion of newly-taken up N is allocated to developing nuts.
CATKINS: The proportion of N derived from fertilizer in catkins was highest for the August and September N application dates. This is consistent with a strong demand for N in developing catkins in the late summer and fall.
SAMPLES COLLECTED IN 2006
STUMP SPROUTS AND EARLY SPRING SHOOTS: The later in the season that N was applied in 2005, the more fertilizer N that appeared in the stump sprouts that emerged from where the bushes had been coppiced at Amhert in spring 2006. The proportion of fertilizer N in the stump sprouts which had been fertilized in September (3%) was higher than the proportion of fertilizer N in the leaves of these same plants the previous autumn (1%). We speculate that this fertilizer N had been stored in the roots over the winter. Alternatively, it may have remained in the soil over the winter, and have been taken up by the plants that spring, but we consider this unlikely, because N left in the soil over the winter would probably have been leached out of the soil by spring.
We observed similar patterns at Fillmore in 2006, where we sampled new spring shoots from standing bushes at Fillmore instead of stump sprouts. Over 10% of the N in spring shoots, and 9% of the N in spring twigs, came from fertilizer in plants that had been fertilized in August and September. This was a much higher proportion of fertilizer N than we observed at any time during the year of application. This N was likely in transit to expanding shoots from storage tissues. For reasons mentioned previously, we consider it less likely that N was newly absorbed from the soil.
WHOLE BUSHES: No growth response to applied N was observed at either site, regardless of application date, nor was winter damage visible that could be attributed to August or September applications. This is consistent with results from another study in which we observed no increase in winter twig death with early September N applications. Nitrogen application date also did not affect whole bush N concentration. August and September N applications resulted in the highest proportions of fertilizer N in the above-ground woody biomass at Fillmore, but at Amherst there were no significant application date effects on proportion of fertilizer N in the above-ground woody biomass.
NUTS: As observed in woody biomass, August and September N applications resulted in the highest proportions of fertilizer N in nuts collected at Fillmore the second year after N application.
These results are consistent with the work of Olsen (1997) and Olsen et al. (2001b) in Oregon, which showed that most of the N for spring leafout of European hazelnuts was derived from stored N reserves, not from newly-applied fertilizer, and that significant amounts of N are retained in roots even one year after application. Millard (1996) reported that from 18 to 93% of N used during bud break and leaf expansion was derived from stored reserves in bark and roots, depending on the age of the plant and prior N status. Nitrogen uptake from the soil increases as plant reserves are depleted and remains high through the rest of the growing season up until leaf senescence (Weinbaum and Van Kessel, 1998). These researchers concluded that although the N applied in the spring contributes to growth in the current season, its main value may be for building reserves for long-term crop health and longevity.
Our data also are consistent with the “source-to-sink” concept described by Weinbaum et al. (1984): newly absorbed N is allocated preferentially to the plant part with the greatest demand. Thus, in our experiment, fertilizer N applied in the spring appeared in leaves relatively quickly, fertilizer N applied in early August appeared in nuts harvested a month later, and fertilizer N applied in September appeared in new catkins in October. If there was no developing plant part with demand for growth, as in the fall, fertilizer N was stored in bark. Our most surprising result was the extent to which September-applied N remained below ground and appeared in above-ground tissues the following year.
NITROGEN UPTAKE EFFICIENCY (NUE)
NUE and bush size were highly correlated at both sites (p < 0.001). This would be expected because larger plants have larger root systems with which to intercept fertilizer (Ran et al., 1994). Also, larger plants have larger photosynthetic capacity to supply energy for N uptake and constitute a larger sink for N.
Application date effects on NUE were highly significant (p = 0.0009 at Amherst; p < 0.0001 at Fillmore), but not consistently between sites. Early August was the most efficient date at both sites, but early April was nearly as efficient at Amherst, whereas September was nearly as efficient at Fillmore. Early April was the third most efficient time at Fillmore. Early April may have been less efficient at Fillmore relative to Amherst because an inch of rain fell at Fillmore, but not at Amherst, during the two days immediately following that application, and may have leached the N away. Considering the two sites together, NUE was significantly higher for August, September and early April than for late April or late May (p < 0.005).
Late April and May were the least efficient N application times at both sites. This was contrary to our hypothesis that late May applications would be optimal. We had reasoned that photosynthesis in May would supply sufficient photosynthate to the roots for N uptake, and that soil conditions in May should be optimal for rapid N uptake. A likely explanation for the reduced NUE in late April and late May could be competition from the cool season weeds, which are most active at that time. We did not think to sample the grasses for 15N until after they had begun to senesce, so we could not accurately quantify their impact on NUE. But in another experiment we determined that N uptake by the grass was linearly related to applied N. Thus, although the grasses may have contributed to a reduction in NUE in the short term, theoretically the N they took up should become available to the hazelnuts in later years as the hazelnuts shade them out, which would enhance long-term NUE. In conclusion, although our results show that late spring was not a good time to apply N under these condition, it may be a good time to apply it at sites without cool season weeds.
The efficiencies we measured did not include N that was lost with nut harvest and leaf drop, or that was N retained in roots at the time of coppicing, which could have been a significant. By adding the N we estimated was lost with nuts and leaves, we increased our estimate of NUE for the best treatment at Fillmore from 5.6% to 9.0%. N still retained in roots was not included in this accounting, which did not change the relative efficiencies of the different application dates.
Even when we add estimates for N that was taken up but not measured, 9% NUE, the best we estimated, is low compared with other woody crops, which average about 20% N recovery (Weinbaum et al., 1992). By comparison, Olsen et al. (2001b) recovered 28% of N that had been applied at a rate of 4 oz/plant to 11-year-old European hazelnuts in Oregon. One possible explanation for this is that our N application rate in this experiment, 0.2 oz/plant, may have exceeded demand. NUE declines the more N that is applied when N exceeds plant demand (Weinbaum et al., 1992). We estimated that the 8-year-old hybrid hazelnut bushes at Fillmore contained a total of only about 0.25 oz/plant in their above-ground biomass, whereas the 13-year-old bushes at Amherst contained 0.67 oz/plant. This means that these plants were accumulating N at a rate of about 0.03 to 0.05 oz/plant/year. Assuming a rooting depth of 12 inches and a rooting width equal to canopy width, soil tests showed that at Fillmore 0.11 oz of inorganic N was available to each plant from the soil, which is plenty to supply a demand of 0.03 to 0.05 oz/plant/year. These results, together with the results from the N-rate study, suggest that the requirements of hybrid hazelnuts for supplemental N in Minnesota soils are low. They are, in the words of Roversi and Ughini (2005), referring to European hazelnuts, a “frugal” species, given their efficient reuse of endogenous N.
RECOMMENDATIONS TO IMPROVE NITROGEN UPTAKE EFFICIENCY
If NUE is only 9%, it means that over over 90% of applied N is not absorbed by plants, and that applying higher N rates is not the solution to lack of response to N. Instead, growers must strive to improve NUE with an integrated approach. Methods may include applying slow-release forms of N, as recommended by Gray and Garrett (1999), or foliar N applications (Olsen et al., 2001b). Eliminating weeds that may compete for N should also improve NUE, at least in the short run. Organic approaches, such as fertilizing with manure or compost, or using leguminous cover crops, which add N in stable slow-release forms, should also be investigated.
Most importantly, growers need to more closely match applied N with plant demand (Sanchez et al., 1995). This is what is recommended in Oregon, where N application rates are based on age of plant for immature hazelnut plants, or on leaf analysis for mature plants (Olsen, 2001a). The high correlation we found between above-ground biomass and fertilizer N uptake supports this recommendation. Approaches to matching applications with demand include applying only as much N as is removed in the crop and prunings, minus estimates of inorganic N in the soil, as proposed by Tagliavini et al. (1996), or applying N only when leaf N falls below the threshold of sufficiency, as proposed by Worley (1990) for pecans. Based on the leaf analyses from our N rate trials, we propose 2.1 % as the threshold of sufficiency (Braun, 2008).
In summary, these results support the conclusions from our N rate trials (Braun, 2008), which suggest that the N demand of hybrid hazelnuts is very low, as is common for woody crops, at least until they start bearing nuts. This probably is due to their efficiency at extracting native N from the soil and at recycling N internally (Millard and Neilson, 1989). These are traits that help woody crops, such as hazelnuts, be such a valuable part of a sustainable agricultural system.
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Educational & Outreach Activities
The abstract to the dissertation pertaining to this research is available at http://conservancy.umn.edu/handle/46236
Braun was also asked to speak informally at meetings of Third Crop winter seminars and summer field days sponsored by Rural Advantage of Fairmont Minnesota, including, most recently, one at the Erickson farm near Lake City Minnesota in September 2008. Together with Jeff Jenson of Rural Advantage, her conclusions have been formulated into fertilizer recommendations in a Production Guide. The production guide for hybrid hazelnuts has been posted on the internet. You can find it at
In her new job as coordinator of a program to improve hybrid hazelnut germplasm, Braun has plenty of opportunity to visit with growers and to share her recommendations.
Lois C. Braun1 and Jeffrey H. Gillman. “Fertilizer Nitrogen Timing and Uptake Efficiency of Hybrid Hazelnuts in the Upper Midwest, USA.” HortScience 44: 1688-1693 (2009). Available online at: http://hortsci.ashspublications.org/cgi/content/abstract/44/6/1688?ct=ct
Summary of Recommendations
1. Before planting hazelnuts, do soil tests for P, K, pH, and micronutrients, and amend deficiencies according to recommendations for other woody crops.
2. Do not apply N to hybrid hazelnut transplants until an entire year after transplanting.
3. If N deficiency is suspected in the second season (soils have low organic matter and leaves are yellow in mid-summer) use only very low rates (<0.1 oz/plant) of very low salt index N fertilizers, such as slow release fertilizers or mature compost. These are good fertilizers to use for mature plants as well.
4. On low organic matter soils, you may start fertilizing in the third season, starting with about 0.1 oz/plant and gradually increasing to 0.4 oz/plant in the eighth year.
5. When bushes reach maturity (start bearing nuts) you may base N applications on either of two methods:
a. Collect leaves for analysis in early August. If the N concentration is below 2.1%, apply N, or increase rates if you have already been fertilizing. If the N concentration is above 2.1%, do not fertilize, but test again in a year.
b. Calculate average kernel yield (out of shell) per plant, multiply that by the N concentration of kernels (3%) to get N removed in harvest, and apply double that amount to each plant.
6. Apply N during the growing season, any time between when leaves are starting to expand to early September. Although other times of year may be okay, we did not test them, and theory supports applying N when plants are actively growing.
7. Control weeds and supply adequate water.
The hazelnut industry in this region is still not very well developed, and thus there are still not very many growers. Moreover, we do not have a good established mechanism for communicating between us, so I do not know how well my recommendations have been getting out to growers. On a number of occasions I have had growers send me their soil and leaf analysis results and I have formulated specific recommendations for them. Although I would like to be able to give growers simple formulas with which to calculate N rates, there are so many interacting factors to consider, such as soil quality, soil moisture, growth rates, yield, etc that I have learned that such a formula would be an oversimplification. So it might be best for me to continue to give growers personalized recommendations, as long as there aren’t too many of them.
Areas needing additional study
Economic parameters were not included in this research, but in future research it would be recommended. For example, we made a decision to use ammonium nitrate instead of coated N pellets or other slow release fertilizers based on the assumption that farmers would be unlikely to use these fertilizers due to cost. However, after discovering how low the N requirements of hazelnuts are, we believe that slow-release fertilizers may actually be more cost effective, due to their greater potential uptake efficiency, and due to the lowered risk of damaging seedlings with them. Another economic analysis needed is the trade-off between expense of various methods of weed control and the improved growth with weed control investment.
Growing systems research is also needed. For example, research is needed to evaluate the trade-offs between practices that enhance the growth of hazelnuts, such as intense weed control, especially in the establishment phase, and practices that enhance the environmental benefits of growing them. Grasses and legumes allowed to grow in the alleyways between hazel rows enhance these benefits, while enhancing wildlife habitat and ecosystem diversity, which may contribute to better pest control and reduce the need for pesticides. However, other plants growing close to hazelnut seedlings when they are young may compete with them excessively, retarding their maturation and reducing their economic profitability.
Our research has shown that the N requirements of hybrid hazelnuts are low, especially during the establishment phase. This enhances their value as an ecologically sustainable crop, both because of the high energetic costs of production of N fertilizers, and because of the high potential for N fertilizers to become pollutants. Further research is needed, however, to see if high yields can be sustained with low N fertilization, and if so, for how long. Research is also needed to evaluate the potential to maintain productivity using only organic sources of N, such as with compost or manure, or with legume intercrops planted in the alleyways. If intercropping systems can be developed to completely supply the N needed, then hybrid hazelnuts will have proven themselves as a truly sustainable crop.