Nitrogen Fertilization for Hybrid Hazelnuts in the Upper Midwest

N-RATE TRIALS

SOIL RESPONSES
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.

TRANSPLANT SURVIVAL
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.

PLANT GROWTH

NEW PLANTINGS
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.

ROOT RESPONSES
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.

NUT YIELDS
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.

LITERATURE CITED

Adiloglu, A. and S. Adiloglu. 2005 An investigation on nutritional problems of hazelnut grown on acid soils. Commun. in Soil Sci. and Plant Anal. 36:2219-2226.

Aguirre, P.B., Y.K.Al-Hinai, T.R. Roper, and A.R. Krueger. 2001. Apple tree rootstock and fertilizer application timing affect nitrogen uptake. Hort. Sci. 36:1202-1205.

Black, C.A. 1993. Soil fertility evaluation and control. CRC Press, Boca Raton, La.

Braun, L.C. 2008. Nitrogen fertilization of hybrid hazelnut in the Upper Midwest. PhD Diss., Dept. of Hort. Sci., Univ. Minn., St. Paul.

Campbell, C.D., L.A. Mackie-Dawson, E.J. Reid, S.M. Pratt, E.I. Duff, and S.T. Buckland. 1994. Manual recording of minirhizotron data and its application to study the effect of herbicide and nitrogen fertilizer on tree and pasture root growth in a silvipastoral system. Agrofor. Syst. 26:75-87.

Davis, M.A., K.J. Wrage, P.B. Reich, M.G. Tjoelker, T. Schaeffer, and C. Muermann. 1999. Survival, growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient. Plant Ecol. 145:341-350.

De Montard, F.X., H. Rapey, R. Delpy, and P. Massey. 1999. Competition for light, water and nitrogen in an association of hazel (Corylus avellana L.) and cocksfoot (Dactylis glomerata L.) Agrofor. Syst. 43:135-150.

Dong, S., C.F. Scagel, L. Cheng, L.H. Fuchigami, and P.T. Rygiewicz. 2001. Soil temperature and plant growth stage influence nitrogen uptake and amino acid concentration of apple during early spring growth. Tree Physiol. 21:541-547.

Ericsson, T. 1995. Growth and shoot:root ratio of seedlings in relation to nutrient availability. Plant and Soil. 168-169:205-214.

Grasmanis, V.O. and D.J.D. Nicholas. 1971. Annual uptake and distribution of N15-labelled ammonia and nitrate in young Jonathan/MM104 apple trees grown in solution cultures. Plant and Soil 35:95-112.

Gray, D. and H.E.G. Garrett. 1999. Nitrogen fertilization and aspects of fruit yield in a Missouri black walnut alley cropping practice. Agroforestry Systems 44:333-344.

Hangs, R.D., J.D. Knight, and K.C.J. VanRees. 2003. Nitrogen accumulation by conifer seedlings and competitor species from 15N-labelled controlled-release fertilizer. Soil Sci. Soc. Am. J. 67:300-308.

Josiah, S.J. 2001. Hybrid hazelnut: an agroforestry opportunity. Univ. of Neb. Lincoln School of Natural Resource Sci. and Coop Ext. http://www.unl.edu/nac/brochures/sfp/sfp4.pdf.

Kowalenko, C.G. 1996. Interpretation of autumn soil tests for hazelnut. Can. J. Soil Sci. 76:195-202.

Merwin, I.A. and J.A. Ray. 1997. Spatial and temporal factors in weed interference with newly planted apple trees. Hort. Sci. 32:633-637.

Millard, P. and G.H. Neilsen. 1989. The influence of nitrogen supply on the uptake and remobilization of stored N for the seasonal growth of apple trees. Ann. Bot. 63:301-309.

Millard, P. 1996. Ecophysiology of the internal cycling of nitrogen for tree growth. J. Plant Nutrition and Soil Science (Zeitschrift fur Pflanzenernahrung) 159:1-10.

Munoz, N., J. Guerri, F. Legaz, and E. Primo-Millo. 1993. Seasonal uptake of 15N-nitrate and distribution of absorbed nitrogen in peach trees. Plant and Soil 150: 263-269.

O’Conner, P. 2006. Oregon’s hazelnut harvest. Oregon Labor Market Information System, May 8, 2008 Available: http://www.qualityinfo.org/olmisj/ArticleReader?p_search=stagnant&searchtech=1&itemid=00005142

Olsen, J. 1997. Nitrogen management in Oregon hazelnut. p. 263-268 In A.I Koksal, Y. Okay, N.T. Bunes (eds). Proc. 4th Int. Congr. on hazelnut. Ordu, Turkey. 30 Jul.-2 Aug. 1996. Acta Hortic. 445:263-268.

Olsen, J.L., T.L Righetti, and E.I Sanchez. 2001. Absorption and distribution of isotopically labeled nitrogen in the hazelnut tree following ground and foliar applications. In S.A. Mehlenbacher (ed.) Proc. V Int. Congr. on Hazelnut. Corvallis, Oregon. Aug. 2000. Acta Hort. 556:437-444.

Olsen, J. 2001. Nutrient management guide--hazelnut. Ore. State Univ, Ext. Serv. Bul. EM8786. extension.oregonstate.edu/catalog/pdf/em/em8786-e.pdf

Pellett, H.M. and J.V. Carter. 1981. Effect of nutritional factors on cold hardiness of plants. Hort. Rev. 3:144-171.

Pilon, P. 2006. Perennial solutions, a grower’s guide to perennial production. Ball Pubs., Batavia, IL.

Proebsting, E.L. and E.F. Serr. 1954. Edible nuts. p. 477-494. In N.F. Childers (ed.) Mineral nutrition of fruit crops. Rutgers Univ., New Brunswick, N.J.

Ran, Y., R. Habib. B., Bar-Yosef, and A. Erez. 1994. Root volume effects on N uptake and partitioning in peach trees. Agron. J. 86:530-534.

Richardson, D.C. 1997. The health benefits of eating hazelnut: implications for blood lipid profiles, coronary heart disease, and cancer risks. p. 295-298 In: A.I Koksal, Y. Okay, N.T. Bunes (eds). Proc. 4th Int. Congr. on hazelnut. Ordu, Turkey. 30 Jul.- 2 Aug., 1996. Acta Hort. 445:295-298.

Rosen, C.J. and R. Eliason. 2005. Nutrient management for commercial fruit and vegetable crops in Minnesota. Univ. Minn. Extension Serv. BU-05886.

Roversi, A. and V. Ughini. 2005. Further investigations into the mineral uptake of hazelnut orchards. p. 285-289 In: Tous, J., M. Rovira, and A. Romero (eds.) Proceedings 6th Intl. Cong. Hazelnut. June 14-18, 2005 Tarragona-Reus, Spain. Acta Hort. 686:285-289.

Rosen, C.J. and R. Eliason. 2005. Nutrient management for commercial fruit and vegetable crops in Minnesota. Minn. Ext. Serv. Bul. 05886 www.extension.umn.edu/distribution/cropsystems/DC5886.html

Roversi, A., and V. Ughini. 2005. Further investigations into the mineral uptake of hazelnut orchards. p. 285-289 In: Tous, J., M. Rovira, and A. Romero (eds.) Proceedings 6th Intl. Cong. Hazelnut. June 14-18, 2005 Tarragona-Reus, Spain. Acta Hort. 686:285-289.

Rutter, P.A., and M.L. Shepard. 2002. Hybrid hazelnut handbook. 1st edition. Badgersett Research Corporation. RR 1, Box 141, Canton, MN 55922.

Sanchez, E.E., H. Khemira, D. Sugar, and T.L. Righetti. 1995. Nitrogen management in orchards. p. 327–380. In: P.E. Bacon (ed.) Nitrogen fertilization in the environment. Marcel Decker, New York.

Sicher, L., A. Doringoni, and G. Stringari. 1995. Soil management effects on nutritional status and grapevine performance. Conference proceedings Nutrition of Deciduous Fruit Plants. Acta Hort 383: 73-82.

Tagliavini, M., D. Scudellazi, B. Marangoni, and M. Toselli. 1996. Nitrogen fertilization management in orchards to reconcile productivity and environmental aspects. Fert. Res. 43:93-102.

Thevathasan, N.V., and A.M. Gordon. 2004. Ecology of tree intercropping systems in the North temperate region: Experiences from southern Ontario, Canada. Agroforestry Systems 61:257-268.

Titus, J.S. and S.M. Kang. 1982. Nitrogen metabolism, translocation, and recycling in apple trees. Hort. Reviews 4:204-246.

Tous, J., A. Romero, and J.Plana. 2005. Effect of N, B, and Fe fertilization on yield and nut quality of ‘Negret’ hazelnut trees. p. 277-280. In: Tous, J., M. Rovira, and A. Romero (eds.) Proceedings 6th Intl. Cong. Hazelnut. June 14-18, 2005 Tarragona-Reus, Spain. Acta Hort. 686:277-280.

Weinbaum, S.A., M.L Merwin, and T.T. Muraoka. 1978. Seasonal variation in nitrate uptake efficiency and distribution of absorbed N in non-bearing prune trees. J. Amer. Soc. Hort. Sci. 103:516-519.

Weinbaum, S.A., I. Klein, F.E. Broadbent, W.C. Micke, and T.T. Muraoka. 1984. Effects of time of nitrogen application and soil texture on the availability of isotopically labeled fertilizer nitrogen to reproductive and vegetative tissue of mature almond trees. J. Amer. Soc. Hort. Sci. 109:339-343.

Weinbaum, S.A., R.S. Johnson, and T.M. DeJong. 1992. Causes and consequences of overfertilization in orchards. HortTech 2:112-121.

Weinbaum, S.A. and C.Van Kessel. 1998. Quantitative estimates of uptake and internal cycling of 14N-labeled fertilizer in mature walnut trees. Tree Physiol. 18:795-801.

Willet, W.C. and M.J. Stampfer. 2003. Rebuilding the food pyramid. Scientific American. 288:64-71.

Worley, R.E. 1990. Long-term performance of pecan trees when nitrogen application is based on prescribed threshold concentrations in leaf tissue. J. Amer. Soc. Hort. Sci. 115:745-749.

Youssefi, F., S.A. Weinbaum, and P.H. Brown. 2000. Regulation of nitrogen partitioning in field-grown almond trees: effects of fruit load and foliar nitrogen applications. Plant and Soil 227:273-281.

Xu, Y.X., M.A. Hanna, and S.J. Josiah. 2007. Hybrid hazelnut oil characteristics and its potential oleochemical application. Industrial crops and products 26:69-76.