I. Determine the effects of different groundcover management systems (mowed turfgrass, hardwood bark mulch, and pre- and post-emergence herbicides) on nitrogen (N) release, uptake, retention and recycling in a northeastern apple orchard.
II. Integrate and synchronize groundcover vegetation management in relation to critical periods of fruit-tree N demand, managing the groundcovers to prevent erosion and retain excess N during periods of low crop demand, so as to minimize N losses from orchards.
We studied the year-round partitioning, uptake, release, and losses of nitrogen (N) in a commercial New York apple orchard after 10 years under four different groundcover management system (GMS) treatments within the tree rows: mowed sod, wood-chip mulch, post-emergence herbicide (glyphosate), and pre-emergence herbicides (diuron, norflurazon, and glyphosate). A field-scale below-ground drainage and surface runoff collection system in each GMS treatment plot enabled us to sample and compare the N concentrations in subsoil leachate, surface runoff, and retained water in the soil root zone. The orchard received no N fertilizer applications from 1995 to 1998, and we applied no more than 0.5 grams of KNO3 per tree, enriched to 99% in the rare stable isotope of N (15N), as a tracer for N dynamics under low N supply during the three years of this study (1999-2001). Datalogged monitoring stations recorded water outflow rates, soil moisture content and temperature, and weather at the site. The atom percentage of 15N vs. 14N in biweekly plant and soil samples was analyzed during each growing season, by isotope ratio mass spectroscopy, to determine the relative uptake efficiency and retention of fertilizer N by soil, trees and groundcover vegetation in each GMS. Tree growth and yield were evaluated each year and compared among GMS treatments. Trends over time were statistically evaluated as a repeated measures model. A nitrogen nutrient budget is being developed for orchards under each GMS.
During the past three years we collected and analyzed the most comprehensive multi-year and multi-faceted data set available on N dynamics and partitioning in an apple orchard under different soil and groundcover management systems. To our knowledge, this is the only representative data set that includes continuously monitored soil physical conditions (moisture content and temperature from 5 to 20 cm depth), soil water drainage rates and N content, surface runoff N content, field-capacity root zone water N content, partitioning of N into above and below-ground portions of mature apple trees, and the relative N content of various groundcover vegetation types during three years in a mature orchard. The use of stable N isotopes as tracers enabled us to distinguish between tree and groundcover N uptake from soil reserves vs. fertilizer sources. This research has generated much interest in neighboring Ontario, Canada, where the Provincial government recently mandated that farms develop and adopt nutrient-budget based fertilizer programs, because there is a lack of research-based information on N dynamics or best management practices for conserving N in horticultural crop systems. It is likely that water and soil quality issues will lead to similar regulatory programs in the U.S., and eventually the data from this project could be vital for the Northeastern fruit industry, and beneficial for the environment.
This research demonstrated that fertilizer N inputs can be reduced substantially in mature irrigated apple orchards on silt-loam soils, without loss of tree vigor or fruit yields. Although there were significant differences in N leaching and runoff from alternative groundcover management systems, it should me noted that N losses to surface and groundwater even in the most “leaky” bare-soil residual herbicide system were much lower than values reported for other agronomic and horticultural crops systems where annual N fertilizer inputs are greater, and the soil surface is frequently disturbed by mechanical tillage. Nitrogen concentrations in drainage water from the test orchard were below 1 ppm on most occasions, and were comparable to values reported for undisturbed forests in sensitive watersheds such as the Catskill Mountains (Lovett et al., 2000). This bodes well for watersheds in regions like the Finger Lakes of central New York, where acreage of vineyards and orchards is increasing along the lake shores due to favorable climate effects and ecotourism.
Our research also demonstrated fundamental differences in nutrient release and retention from inorganic N fertilizers vs. organic N sources such as biomass mulch. Wood-chip mulch provided substantial N inputs to the orchard, with minimal leaching or runoff losses of N and dramatic increases in soil organic matter content. Fruit growers in regions at high risk for ground- or surface-water nutrient contamination could rely more upon organic sources of N to maintain tree vigor and yields.
Comparing two different herbicide systems revealed that soil-active residual herbicides that keep soil weed-free year round were actually less effective in promoting tree growth or fruit yields over 10 years, compared with post-emergence systemic herbicides that allowed weeds to regrow after each application. There was more soil erosion, N leaching and runoff from the bare-soil pre-emergence herbicide plots in comparison with sparsely weedy plots of the post-emergence herbicide treatment. Considering that “weed-free” residual herbicide systems are the most common in U.S. orchards, our findings suggest that growers should be encouraged to convert from conventional bare-soil systems to mulch based or post-emergence herbicides and transient instead of long-lasting weed suppression practices.
5. Potential Contributions:
In this long-term study of alternative groundcover management systems (GMSs) for orchards, there were major differences between the retention, release, uptake and transport of N by fruit trees, soil, groundcover vegetation and water in the four GMSs. Losses of N to drainage and runoff water, and N availability in retained soil water of the root zone, were greater in the two herbicide GMSs relative to mowed-sod or wood-chip mulch treatments. The greatest leaching losses of N occurred during the winter and spring months, suggesting that dormant-season groundcovers with high affinities for fertilizer N could help to retain N and reduces losses from perennial crop systems in cool humid regions. Long-term yields and tree growth were optimal in mulch plots and in post-emergence herbicide plots, where dormant- season weed regrowth improved N retention. In the bare-soil residual Pre-Herb treatments, N supply was excessive; and in mowed sod treatments N was marginally deficient for fruit trees. The mulch treatment provided a very large input of N in biomass form, but resulted in minimal N leaching or runoff because mulch also increased soil C, thereby retaining soil N in microbial biomass and other organic forms. Based on this study, we recommend that fruit growers consider utilizing organic forms of N when supplies of this nutrient are limiting. Where growers use herbicides for orchard weed control, non-residual post-emergence herbicides that permit weeds to reestablish during the dormant season would be preferable to residual soil-active herbicides that may result in excessive soil N supply and increased leaching and runoff losses of N. As regulatory agencies move toward closer accounting and regulation of N fertilizer usage in agriculture, and attempt to reduce non-point sources of water pollution, the data from this project could be of substantial value to farmers, and potentially benefit the soil and water environment.
6. Publications and Outreach:
Results of this study have been presented as invited talks to more than 1000 fruit growers in New York, New England, Michigan, and Ontario, Canada during the past two years. About 500 growers, students, and extension staff have toured the experiment, and five graduate students have been involved in research for this project. Five scientific reports have now been published for this study, and three more are in preparation at the time of this writing.
7. Future Recommendations:
We recommend that similar studies should be conducted in diverse soil types and horticultural crop systems, because N dynamics will vary considerably in other agro-ecosystems. At present there is insufficient scientific information on best management practices for soil and nutrient conservation in relation to water quality for perennial crop systems. This study represents a preliminary step toward providing such information and promoting more efficient and sustainable orchard soil and groundcover management systems.
Excessive and inefficient use of nitrogen (N) fertilizers is contaminating many surface and ground water resources, contributing to global warming, and causing widespread public and scientific concern (Vitousek et al., 1997). Agriculture and other human activities have doubled the transfer of N from Earth’s atmosphere into soil, crops and water, and much of that fixed N is concentrated within urban and agricultural regions (Schlesinger and Hartley, 1992). Recent increases in N fertilizer use have been dramatic. Industrially fixed N usage in the 1980s decade exceeded all previous inputs of synthetic N fertilizer in human history, and there is mounting evidence that present levels of N usage cannot be sustained without serious environmental degradation (Kates et al., 1990; Smil, 1997). This project examined N dynamics in a commercial orchard under different soil management systems, to understand better how fruit production can be improved by optimal utilization of N, minimizing losses of plant nutrients and water pollution.
Orchards are often located on well-drained soils near major aquifers and large bodies of fresh water. Agrichemical contamination of surface and groundwater is a potential problem in these sites. Nitrogen pollution of water resources has proven to be a serious problem in many fruit- growing regions, and could be reduced by more efficient N management (Weinbaum et al., 1992). Various soil and groundcover management systems (GMSs) are used in orchards, and because they affect the amounts of soil water and nutrient uptake, retention and loss, these systems can be adapted to help prevent N pollution (Hogue and Neilsen, 1987; Merwin et al., 1994a, 1994b, 1996). This project investigated N dynamics in the major components of an apple orchard where mowed red fescue turfgrass, hardwood bark-chip mulch, pre-emergence herbicides, and post-emergence herbicide GMSs have been maintained since 1992. Field-scale leaching and runoff monitoring systems and extensive sampling of soil, root, shoot, and leaf tissue in apple trees and groundcover vegetation provided information on the movement and cycling of N within this orchard.
Our first objective was to develop a comprehensive, mechanistic year-round record for N release and uptake, transient and long-term reserves, and the relative availability and amounts of N recycled or lost from the orchard under each GMS. Our second objective was to develop N budgets for each GMS, enabling fruit growers to select and adapt soil management systems to minimize N losses while maintaining tree health and yields. The intended outcome of this project was to integrate and synchronize GMS and other management practices such as N fertilizer timing, establishing relay groundcovers or mulches, and deferring weed suppression during critical periods when fruit tree uptake is not sufficient to retain N within the orchard. The results of our study will be made available to growers, students, extension staff and crop consultants, and disseminated in commercial fruit production recommendations and the referred research literature.
This project was part of a long-term study on impacts of alternative GMSs on tree growth and yield, nutrient status, and the fate of agrichemicals in a commercial orchard, described at length in a previous report (Merwin et al., 1996). In 1991, we installed a replicated grid of perforated drainage lines under a gently sloping 0.8 ha site on the east shore of Cayuga Lake, near Ithaca N.Y. The soil was a relatively uniform silty clay loam with pH averaging 6.8 and organic matter content of 4.7% when this study began. In April 1992, 334 apple trees (‘Empire’ grafted on M.9/MM.111 rootstocks) were planted at 3 by 6 m spacing, and three replications of four GMS treatments were randomly assigned to 12 independently drained plots, each containing 20 trees. The GMS plots were 9 m wide (across-slope) and 25 m long (down-slope), in two blocks each containing four tree rows separated by 4 m-wide grass drive lanes between the 2 m-wide GMS-treated tree rows (Fig. 1, in Appendices).
The 12 subsurface drainage systems each captured leachate from four parallel tree rows and three intervening grass drive lanes that comprised a GMS plot (Fig. 1). A single perforated PVC line was buried at 0.7 m depth down the center of every GMS plot, draining at the down-slope edge of treatment areas to a below-ground access station where the subsurface leachate from each plot could be sampled and outflow rates measured with datalogged tipping buckets. Surface runoff was collected in grade-level sample bottles behind 1.5 m-wide steel weirs driven into the soil at the lower edges of central rows in each plot. A micro-sprinkler system was operated to uniformly irrigate the surface of all tree-row treatment areas as necessary from June to Sept., maintaining soil water potential in the optimal range for apple growth (between -10 and –150 kiloPascals).
The four GMS treatments established in 1992 have been maintained continuously since then in 2 m-wide strips within tree rows:
1. Mowed-Sod: A red fescue (Festuca rubra) turfgrass seeded in 1991, including some white clover (Trifolium repens) that self-established; mowed monthly during the growing season.
2. Post-Herb: Post-emergence applications of glyphosate (RoundUp) herbicide at a rate of 2 kg a.i./treated ha in early May and July each year. This treatment allows weed regrowth about 6 weeks after treatments, providing sparse groundcover during the dormant season.
3. Pre-Herb: Pre-emergence applications of glyphosate as above, tank mixed with norflurazon (Solicam) and diuron (Karmex) herbicides, at 2, 3.0 and 2.5 kg a.i./ha, respectively, each year in early May. This treatment keeps the tree-row soil surface virtually weed free year-round, and is a conventional GMS in commercial orchards.
4. Wood-Chip Mulch: A 10-cm layer of shredded mixed hardwoods bark mulch renewed biennially since 1992. The N content of this bark mulch averaged 0.68% (dry wt basis), equivalent to 0.093 kg N/m2 of tree row, or 970 kg N/ha-yr. In the second year after each Chip-Mulch renewal, weeds were spot-sprayed with glyphosate as needed.
For the first four years after planting this orchard, ammonium nitrate fertilizer was applied to all GMS treatments at the rate of 30 to 65 kg N per ha, while trees were establishing. By 1996, trees were fully grown and annual foliar nutrient analyses indicated no need for additional N fertilizers to maintain photosynthetic capacity and yields, so N fertilizer applications were omitted from 1996 to 1998. During each year of the present study we applied a very small amount (0.5 gram of KNO3 beneath two trees in each GMS plot) of a rare but naturally occurring N isotope (15N) as a biological tracer. Preliminary soil and plant tissue samples determined that the natural abundance ratio (atom-% 15N) of these two stable N isotopes was 0.3667% (15N/14N) in our test orchard at the outset of this study. Thus atom %15N values above that ratio represented fertilizer-derived nitrogen.
Small amounts of K15NO3 fertilizer enriched to 99% 15N served as tracers to quantify and compare the partitioning, uptake and losses of N among trees, soil, and water in the four GMS systems. We chose to use negligible quantities of N tracer to assess the base level N dynamics in this orchard without disruption by higher fertilizer inputs. Previous N tracer studies in orchards have used 10 to 100 fold greater quantities of isotopic N applied to trees in pots, or bare-soil plots without competing surface vegetation. Our approach was therefore innovative and unique in studying N dynamics under natural, low levels of N supply.
In the first year of this study (1999), 0.25 g of 99% enriched K15NO3 fertilizer was applied on May 10 (bloom-time) beneath the drip-line of 24 trees (2 trees per GMS plot). To increase the tracer signal in the second year, we used different trees for all treatments and increased the amount of 15N in the single May application to 0.5 g K15NO3, also adding 3-way split applications of 0.17 g K15NO3 on May 3, July 12, and Aug. 31, to two other trees. In the third year, we repeated 3-way split applications at the same rates and approximate timings, using two additional trees in each plot. In other words, the single application of 025 or 0.5 g K15NO3 per tree was repeated (to different trees) in 1999 and 2000, while the 3-way split application of 0.17 g K15NO3 (for a cumulative total of 0.5 g K15NO3 per tree) was repeated in 2000 and 2001, on different trees each year.
Soil temperature and volumetric water content from 2 to 20 cm depth were monitored continuously from May 1999 to Dec. 2001, with automated dataloggers (Campbell Scientific CR10X, Logan UT) scanning TDR (Time Domain Reflectometry) probes (Campbell CS615) and constantin-coupled thermistors in the center row of each GMS plot. Datalogged tipping buckets in one replicate of each GMS recorded rates of subsoil drainage water flow, and an automated weather station monitored ambient rainfall and temperature at the orchard. Water samples retained in the small pore soil matrix were extracted with suction lysimeters in the center of each plot. Saturated leachate water samples were taken at daily to weekly intervals whenever there were outflow events throughout the year, depending upon rainfall and irrigation during the growing season, and freezing/thawing events during the dormant season.
Tree leaf, shoot, spur, and fruit were sampled from 15N-treated trees approximately biweekly before, during and after each growing season (Table 1). Surface vegetation coverage (weeds and grass) was visually estimated and harvested for biomass and nutrient analysis during the second growing season. In April 2001, one tree in each plot where 15N had been applied in three split doses the previous year was excavated with a backhoe, sectioned into different age-classes of roots, shoots, and trunk tissue, and dry weights of each tree tissue sub-sample were determined. The total N and atom-%15N of each soil, tree, leaf, fruit and groundcover sample were determined by isotope-ratio mass spectrometry at Isotope Services Lab (Los Alamos, NM). The N content of most water samples was too low for isotope ratio analyses and their ammonia-N content was negligible, so we determined total NO3-N content of all water samples with an autoanalyzer Flow Solution Model IV (O.I. Analytical, College Station, TX).
Tree growth and yield data were evaluated by harvesting fruit from eight trees in the center of each GMS plot, and measuring trunk circumference during mid winter each year. Soil physical conditions, pore size distribution and hydraulic conductivity were evaluated during July 2000 (Oliveira and Merwin, 2001). Intact soil sample cores 5 cm wide to 20 cm depth were extracted beneath trees in each plot and sent to the lab of Dr. G.W. Bird at Michigan State University for extraction and identification of plant parasitic, and non-parasitic (predators, detritovores, fungivores and bacterivores) nematodes in each GMS (manuscript in preparation).
For statistical purposes, this experiment was a fully randomized repeated measures model with three replications of each GMS treatment. Obtaining soil water samples under field conditions continuously over three years inevitably involved some missing data points, especially in the Pre-Herb treatment where the soil surface was slaked and degraded, reducing water infiltration and leaching in that treatment relative to the other three GMSs (Merwin et al., 1996). Leachate, suction lysimeter, and runoff N concentrations were averaged over quarterly intervals, and treatment means separated using Fisher’s Protected LSD for pooled standard errors.
During the past three years we collected and analyzed almost 12,000 soil, water and vegetation samples for this study—continuing to the end of Dec. 2001. Our observations represent the most comprehensive and extensive data of this kind, and we believe they are the only data representing N dynamics under different soil/groundcover management systems in an orchard where soil physical and edaphic conditions were monitored continuously along with N partitioning into agroecosystem components. At the time of this report, about 200 fruit N samples have yet to be analyzed, due to the huge number of samples to be processed, and mechanical problems with a freeze-drying unit necessary to prepare some samples for 15N analysis. We anticipate that full integration, modeling and interpretation of this extensive and complicated data set will require at least another year, while the nutrient budget based on N data from each GMS will be completed during the coming months once all fruit samples are processed. For this final project report we therefore concentrate on the important trends observed from 1999 to 2001. We describe and discuss significant observations, and the practical implications of this study for fruit growers, water resources, and soil conservation. Reference is made to extensive figures and tables in the Appendices, documenting N dynamics in soil, water, and crop system components.
Fruit yields and tree growth.
Cumulative trends in tree growth, fruit yields, and yield efficiency (kg fruit per unit tree size) are shown in Fig. 2 (Appendices). Trees were larger and yields were greater in the Chip-Mulch and Post-Herb, and smaller in the Pre-Herb and Mowed-Sod GMS. Yields averaged about 20 to 25 MT per ha from 1995-2000—above the New York state average yield for Empire apples—even without N fertilizer applications since 1995. In 2001, yields were almost double the state-wide average, but fruit size was reduced because of inadequate thinning. Cumulative yield efficiency was greatest for trees in the Post-Herb treatment, equivalent in Pre-Herb and Mulch, and lowest in the Sod treatment. These observations demonstrate that despite elimination of N fertilizers for the past six years, tree growth and yield were sustained at high levels in this orchard. The greater growth and yield in the Post-Herb and Mulch GMSs relative to the bare-soil Pre-Herb was especially interesting, because Post-Herb and Mulch plots were relatively “weedy” from Aug. to April each year (Figs. 3A-B). It may be unnecessary and undesirable to eliminate weeds year-round in irrigated orchards with similar soil types (Merwin and Ray, 1997).
Nitrogen Losses in Runoff and Drainage.
The automated tipping buckets proved to be somewhat unreliable for continuous measurements of outflow rates from the drainage lines, but they did provide a clear indication of the major pulses and seasonality of drainage outflows from this orchard (Fig. 4). Peak drainage outflow rates occurred during late winter and early spring each year, with other pulses from Hurracane Floyd in Sept. 1999, and during unusually wet May and June of 2000. Summer irrigation and typical rainfall patterns produced relatively brief outflow events in comparison with winter and spring outflows. To correct for atmospheric deposition and irrigation N inputs, we monitored the nitrate-N content and amount of precipitation and irrigation at the test site (Fig. 5). Rainfall and the Cayuga Lake irrigation water sources contained very low nitrate-N concentrations throughout this study, with only a few occasions when nitrate-N exceeded 2 parts-per-million (ppm). Ammonium-N was negligible or undetectable in almost all our water samples, and will not be presented or discussed further in this report.
Nitrate-N concentrations in drainage water were surprisingly low in all GMS during this study, rarely exceeding 1 ppm (Fig. 6), and remained much lower than observed during from 1992-94 when moderate rates of N fertilizer were applied at this orchard (Merwin et al., 1996). The greatest N leaching losses were observed in the two herbicide GMSs, especially the bare-soil Pre-Herb treatment. When irrigation rates were increased during summer of 2001, nitrate-N content spiked to a seasonal average of 6 ppm and 2 ppm in the Pre-Herb and Post-Herb treatments, even after five years without fertilizer N applications. The N content in water samples was so low that we were not able to determine the atom %15N content in these samples, as it was below accurate detection levels for mass spectroscopy. These observations indicate clearly that the potential for leaching losses of N are considerably greater in herbicide GMSs relative to sod or mulch systems. They also demonstrate that apple orchards on silt-loam soils can retain and recycle soil N very efficiently when they are not saturated with excess fertilizer N applications. The consistently low N content of drainage from mulch plots was remarkable considering that the N content of bark mulch applied at this thickness was substantial—equivalent to 970 kg N/ha-yr!
Concentrations of nitrate-N in runoff water were greater than in drainage water, but there were fewer runoff events and the trends among GMSs were complex. During spring and summer, N concentrations were usually higher in the herbicide plots (Fig. 7), while during rainy weather in Fall of 1999 and 2001, pulses of N runoff were observed after mowing Sod plots, and from saturated Mulch plots. Despite different trends over time among GMSs, the N losses in runoff from this orchard were very low in all treatments compared with runoff N loads reported for agronomic crop systems (e.g., Steinheimer et al., 1998). It appears that orchards with mowed sod drive lanes between tree rows, and mature bearing trees are nutritionally “tight” agroecosystems that do not “leak” much N when fertilizer inputs are held to a need-based minimum.
The suction lysimeter soil water samples represented retained soil water in the root zone, presumably the most important source of N for immediate plant uptake (Fig. 8). Nitrogen concentrations were relatively higher in this pool than in runoff and leachate water, and the trend of greater N content under the two herbicide GMSs than under mulch and sod was very consistent over time. Frost penetrated deep into the soil in the winter of 2000-01, and we could not extract samples then. Since there is little tree-root uptake of N during the dormant season, the relatively higher concentrations of soil-water N during mid-winter and early spring months represent an increased potential for losses of N from orchards. Increased irrigation rates during Summer and Fall of 2001 also boosted soil N availability, and leaching losses in the herbicide plots. Soil N availability was usually lowest in mulch plots during this study, despite the substantial N content of bark mulch—illustrating the interaction of carbon and nitrogen immobilization and release in biomass sources of nitrogen.
Soil Nitrogen and Carbon Content.
We determined both total soil N content (dry weight basis) and the atom %15N content in soil cores at approximately biweekly intervals during all three growing seasons (Figs. 9 and 10). The main trend was that soil N content was consistently greater under the mulch treatment, although the magnitude of this difference was only 0.1% in most samples. The variation over time in soil N samples was attributed to random inclusions of root pieces or partially decomposed mulch humic material in some cores. There was no upward or downward trend in soil N content for the GMSs, indicating that each system had attained a stable equilibrium for soil N losses and inputs during 10 years of GMS treatments at this orchard. The 15N tracer signal was apparent in soil from all treatments (Fig.10). From the pre-application baseline of 0.367 atom %15N, tracer concentrations in soil cores quickly increased following K15NO3 applications each year; that increase was usually greater in the herbicide GMSs, and lesser in the mulch plots where the greater total soil N content diluted the tracer pulse. Providing three split applications of 0.17 g in May, July and Sept. instead of a single May application of 0.5 g K15NO3 did appear to prolong the tracer signal in soil, suggesting that split application of N fertilizer at low rates is an effective way to improve soil N availability throughout the growing season. The extreme mobility of fertilizer N was demonstrated in the movement of 15N tracer from one plot to another in July 2000, when our second split application leached within days from one row of the orchard to the adjacent row, producing a greater spike in soil 15N in the downhill single application plot than at the uphill tree row where it was applied (Fig. 10 B vs. 10C)!
Soil carbon (C) content was two to three times greater beneath Mulch than other treatments, and slightly greater beneath Sod compared with herbicide plots (Fig.11). As with nitrogen, the variation in soil C over time within GMSs represented random variation in humic fractions and root pieces in cores. Although both N and C content increased in the Mulch plot soil relative to other treatments, C increased relatively more than N, so the soil C-to-N ratio was substantially higher under Mulch than other treatments (Fig. 12). This important trend probably explains the strong retention of N by mulch plots despite the very high N inputs added with this groundcover material. High C/N ratios apparently immobilized most of the mulch N in microbial biomass, maintaining a stable soil reserve of N with minimal losses to groundwater. Wood-chip or bark mulches are thus an excellent source of N for perennial crop systems, with sustained N reserves and releases over many years.
Whole-Tree Nitrogen Uptake and Partitioning.
Total N content of excavated trees was similar among GMSs except for fine and secondary roots, which contained more N in the Mulch and Post-Herb treatments (Fig.13). However, the atom %15N content was consistently greater in above and below-ground portions of trees in Pre-Herb, and usually greater in Post-Herb trees relative to trees in sod or mulch plots (Fig.14). This indicated greater availability of N fertilizer in plots without sodgrass, and/or dilution of the tracer by a greater total N pool in the mulch plots and trees. In terms of fertilizer use efficiency, we surmise that fertilizer N applied at low rates was more efficiently taken up by trees previously acclimated to low N supply. Total biomass was slightly greater in the Post-Herb and Mulch trees, but this trend was not significantly different among the four GMSs, despite the differences in trunk cross sectional area and cumulative yields. We also measured biomass allocations to leaves and prunings of trees, and determined its relative N content; these data will be used in developing comprehensive N budgets from this study.
Efficiency of N uptake by Trees and Groundcover vegetation.
To assess the relative efficiency of fertilizer N uptake by apple trees in comparison with surface vegetation, we sampled leaves of trees, weeds, and turfgrass at regular intervals each year. The total N content of shoots and leaves on trees was within optimal ranges in all treatments throughout the study, but usually remained lower in Sod treatment trees. In previous studies when large amounts of N tracers were applied in spring to fruit trees acclimated to high N supply, researchers observed greater delays before the tracer was detected in shoots and leaves (Tagliavanni et al, 1997; Toselli et al, 2000). In our trees, the 15N appeared within days at elevated levels in actively growing parts of the trees (Fig. 15 A and B), and the uptake of 15N was greatest in the Pre-Herb plots, and least in the Sod plots. The increased N from early summer uptake persisted in spurs and fruit of herbicide plot trees throughout the growing season, with potentially negative effects on fruit quality at harvest.
The atom %15N of grass and weed leaves beneath trees also spiked immediately after fertilizer applications, was greatest for weeds in the herbicide plots, and atom %15N values were two to five times greater in groundcover vegetation than in tree leaves (Fig.16). These observations have interesting implications; since groundcover “weeds” and sodgrass have a greater affinity for fertilizer N than fruit trees and grow earlier and later in the year than trees, these groundcovers can be managed as “relay crops” to retain and store N within the crop system during times of low tree N demand. This adaptive groundcover management could (and did in our study) decrease leaching losses of N during the dormant season, and restrict N supply to trees during late summer and fall when excessive N supply can be detrimental to fruit quality and tree cold acclimation. Timing of weed suppression or grass mowing can be synchronized to release groundcover nutrient reserves at optimal times for tree assimilation and/or groundwater protection—specifically the early summer months when tree N uptake is rapid and beneficial.
Results of this study have been presented as invited talks to more than 1000 fruit growers in New York, New England, Michigan, and Ontario, Canada during the past two years. About 500 growers, students, and extension staff have toured the experiment, and five graduate students have been involved in research for this project. The following scientific reports have been published from research in this study, and three more are in preparation:
1. Oliveira, M.T., and Merwin, I.A. 2001. Soil physical conditions in a New York orchard after eight years under different groundcover management systems. Plant and Soil 234:233-237.
2. Hopkins, M.A. 2002. Nitrogen-15 labeled fertilizer face in groundcover management systems in apple (Malus domestica) orchards. M.S. Thesis, Dept. of Horticulture, Cornell Univ., Ithaca, NY
3. Merwin, I.A., Hopkins, M.A. and Byard, R.R. 2002. (Abs.) Groundcover management influences nitrogen release, retention and recycling in a N.Y. apple orchard. HortScience 36(3):451.
4. Merwin, I.A. (in press). Orchard Floor Management Systems. In: Apples: Botany, Production and Uses (D.C. Ferree, ed.) CABI Publ., Wallingford, England.
5. Merwin, I.A. (in press). Orchard Floor Management. In: Concise Encyclopedia of Temperate Tree Fruits (T Baugher and S. Singha, Eds.) Haworth Press, Binghampton, New York.
6. Merwin, I.A, and Stiles, W.C. 1998. Integrated weed and soil management in fruit plantings. Cornell Coop. Ext. Inf. Bulletin No. 242. Ithaca, NY, 16 pp.
Impacts of Results/Outcomes
There were fundamental differences between the retention, release, uptake and transport of N by fruit trees, soil, groundcover vegetation and water in alternative GMSs. Losses of N to drainage and runoff water, and N availability in retained soil water of the root zone, were all greater in the two herbicide GMSs relative to mowed-sod or wood-chip mulch treatments for most of the year. The greatest leaching losses of N occurred during the winter and spring months, suggesting that groundcover vegetation with high affinities for fertilizer N could help to retain N and reduce nutrient losses from perennial crop systems in cool humid regions. Long-term yields and tree growth were better in wood-chip mulch plots, and in post-emergence herbicide plots where weed regrowth during late summer improved N retention, in comparison to bare-soil residual Pre-Herb treatments where N supply was excessive, or mowed sod treatments where N was marginally deficient for fruit trees. The mulch treatment provided a very large input of N in biomass form, but resulted in little N leaching or runoff, because it also increased soil carbon and organic matter, thereby retaining soil N in microbial biomass and other organic forms. Based on this study, it appears that fruit growers should utilize organic forms of N when supplies of this nutrient are limiting in orchards. Where growers use herbicides for orchard weed control, non-residual post-emergence herbicides that permit weeds to reestablish during the dormant season are clearly preferable to residual soil-active herbicides that result in excessive soil N supply and can increase leaching and runoff losses of N from orchards.
We published an economic analysis and comparison of alternative orchard GMSs as part of a multi-state USDA-SARE apple production systems project (Merwin et al., 1995). A truly meaningful comparison of the economic impacts of the four GMSs in this study would require quantification and economic valuation of the indirect impacts or externalities of relative soil and water quality impacts of each GMS, which is beyond the scope of this project.
Areas needing additional study
An economic analysis that included externalities or indirect economic costs and impacts of different soil management systems for orchards would be timely and useful. Reganold et al. (2001) recently published a study assessing the sustainability of various orchard systems including groundcover alternatives in Washington State, but the appropriate systems and their impacts are likely to differ substantially in cool humid regions of the eastern U.S. This would be an important topic for further research. It would also be worthwhile to conduct N dynamics soil management studies in orchards with diverse fruit-crop and soil types. Ultimately, sustainable fertilizer programs should be based upon research-based nutrient budgets, and we will complete such a budget for this study during the next year.