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

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

Summary

Summary
Nitrogen (N) fertilizers can increase tree fruit yields, but also may contaminate water resources. We are studying the mineralization, uptake, recycling and losses of N in a New York apple orchard, using stable 15-N isotopes to trace N dynamics year-round under different soil and groundcover management systems (GMSs) and identify factors that maximize N uptake and minimize N losses. Hardwood bark mulch and mowed turfgrass have reduced N losses and improved soil quality substantially. Early summer was the critical time for N uptake by trees, and N leaching was relatively low (less than 2 ppm) in all GMSs.

Objectives:
Determine the effects of different groundcover management systems (mowed turfgrass, hardwood bark mulch, and pre- and post-emergence herbicides) on N release, uptake, retention and recycling in a northeastern apple orchard.

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.

Approach
Many orchards are located on well drained sites near rivers and lakes where nitrate and phosphate contamination of surface and groundwater is a potential problem. Nitrogen (N) pollution of water resources can affect ecosystems and human health, but can be reduced by more efficient nutrient, crop and soil management. We are studying the impacts of alternative groundcover management systems (GMSs) on nutrient status of apple trees, and mineralization, uptake, retention and losses of N in an orchard. Four GMSs been maintained since 1992 within the tree-rows of a commercial orchard near Cayuga Lake in upstate New York. The GMSs are a mowed red fescue turfgrass (MwSod), a hardwood bark-chip mulch (ChpMulch), a conventional pre-emergence residual herbicides treatment (PreHrb) that keeps soil weed-free all year, and May and July post-emergence herbicide treatments (PostHrb) that permit weed re-growth and sparse soil cover from August to May.

We are using 15N enriched stable isotopic fertilizer to trace the movement of N throughout the test orchard. A small amount (0.25 g of 99 percent enriched K15NO3 per tree) of fertilizer was applied on May 10, 1999 beneath the drip lines of 24 trees (two trees per GMS plot). In 2000, using different trees for all treatments, we increased the amount of K15NO3 in the single May 3 application to 0.5 g per tree, and added a split application treatment consisting of 0.17 g in three doses on May 3, July 12, and Aug. 31, 2000. We are collecting biweekly samples of root-zone soil, drainage and suction lysimeter ground water, shoots, leaves and fruit from trees, and groundcover vegetation from each plot during each growing season. We are then analyzing their total N content, atom percent 15N proportion, and total carbon by mass spectroscopy. The 15N tracer enables us to determine the uptake efficiency and biological pathways of N under each GMS. Collectively, these data represent uniquely comprehensive information on the year-round dynamics of N in a representative commercial orchard under different soil management systems.

Results
We have collected 976 samples during the first two years of this study and to date have completed sample processing and analysis for all of 1999 and the first half 2000. Fruit yields per tree have been affected significantly by the GMSs, ranking PostHrb = ChpMulch > PreHrb > MwSod, and ranging from 55 to 40 kg per tree in 1999 and 2000. Nitrogen has been a major factor in these effects, with substantial differences in soil, leaf, shoot and groundwater N in both years. Soil N and carbon content have been twofold greater in ChpMulch than in the other GMSs. Leaf N content and fertilizer 15N uptake efficiency of trees have been consistently lowest in sod plots, and usually higher in the herbicide treatments. Tree uptake of soil N occurred mostly during the early summer (May and June), declining steadily later in the growing season. Despite the relatively small quantity of 15N applied to these trees, the tracer was rapidly taken up and mobilized throughout the trees following applications in May. This may be attributed to the low N status of trees at the outset of this experiment. Leaf N was remobilized into shoot tissues during Oct. and Nov. of 1999, then moved rapidly into other storage tissues after leaf drop in November, then increased in the shoots in early March 2000 before visible bud expansion. Similar trends have been observed for fruit N.

The N content and atom percent 15N of grass and herbaceous weed groundcover vegetation have differed strikingly from that in trees. The N content of grass and weeds averaged two to three times greater than in fruit tree tissues, and the efficiency of fertilizer 15N uptake has been similarly greater in groundcovers than in trees. These observations demonstrate the weak competitive ability of fruit trees relative to groundcovers for soil N, and the potential for retention and recycling of N in orchard groundcovers to reduce off-site N losses. Correspondingly, the observed losses of N in the drainage samples have been greater in the PostHrb and PreHrb than the other GMSs, but relatively low in all the treatments.

Averaged by season, concentrations of nitrate-N in drainage from this orchard ranged from 0.2 to 1.1 ppm during 1999 and the first half of 2000, with the highest spike of 6 ppm in the PreHrb plots during October of 1999. Surprisingly, nitrate-N losses during the irrigation season (May to October of 1999) were somewhat greater than during the dormant season. The past two growing seasons have been very different—unusually hot and dry during 1999, and wet and cool during 2000. Our data so far therefore cover the range of expected growing conditions in central New York, and suggest that nitrate leaching from orchards with grass or mulch groundcovers and sod drivelanes, receiving low inputs of N fertilizers, may be considerably less than reported for other crop systems.

Impacts and potential contributions
The results from this study are useful both as environmental monitoring data on N dynamics and losses from a representative commercial orchard in the northeastern USA—and as a practical comparison of soil and weed management systems impacts on the fate and efficiency of soil and fertilizer N sources. To our knowledge, there are no comparable test sites involving long-term experiments using stable isotope tracers in alternative groundcover management systems. With a comprehensive three-year data set at the conclusion of this study, we will also be able to evaluate by computer simulation and multiple regression the influence of GMSs and the environmental variables we have continuously monitored—such as soil water, soil temperature, soil carbon, and soil physical conditions related to long-term management practices—on N dynamics in orchards. These outcomes could have a substantial impact on orchard management practices.

Reported November of 2000