Adaptive Nitrogen Management in Orchards: Developing Soil - Ground Cover Management Systems that Optimize Nitrogen Uptake, Retention - Recycling

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.