Competition for Nitrogen and Groundwater Nitrate Levels in Temperate Alley Cropping Systems

Competition for Nitrogen and Groundwater Nitrate Levels in Temperate Alley Cropping Systems

Summary

A three-year research project is examining how trees and crops compete for nitrogen within a temperate alley cropping system with pecan and cotton, and how groundwater nitrate levels are affected. Nutrient competition was quantified via 15N enriched fertilizer movement and by variations in crop yields. Monthly changes groundwater nutrient levels were also documented using lysimetry. Early findings indicate significant interspecific competition for nitrogen and water, with reduced groundwater nitrate levels when trees are nearby.

Objectives/Performance Targets

1. 1. To quantify competition for nitrogen between pecan and cotton using 15N labeled fertilizer
   2. To determine the effect of tree-crop competion on ammonfication, nitrification, and mineralization; and
   3. To determine the degree to which nutrient uptake in trees affects groundwater ammoinum and nitrate levels in this system.

Accomplishments/Milestones

A mature pecan-cotton alley cropping system was established in Spring 2001, as a principal research focus. The system is a RCBD consisting of 10 plots, of which 5 are “trenched” and 5 are “no trench” (i.e., control). Trenched plots have a 3 ft deep plastic liner installed at a distance of 5 ft from the center of tree, for the length of the plot, to prevent tree roots from interacting with the adjacent cotton. To quantify nutrient competition, 15N fertilizer (ammonium sulfate, 5% enrichment) was applied to selected microplots at the same time, rate and formulation as the regular fertilizer being applied to the field. Six plants (above-ground portions only) from the center of each microplot were harvested at physiological maturity (early October), and were separated into stem, leaf, square, and boll components. Foliar samples from trees were also collected. Nitrogen mineralization was quantified by the use of in-situ soil mineralization bags. Grounwater nitrate and ammonium levels were monitored using lysimeters at 30 cm and 90 cm depths in both the treatments. Initial findings indicate that competition for nitrogen is present in the system and trenching has the potential to allevioate the intensity of competition. Mature pecan trees are able to reduce the amount of nitrate in in-situ groundwater. Yields were also lower in untrenched plots, indicating competition for nitrogen and water from tree roots.

The agroforestry project has enjoyed important exposure in the scientific community, being the subject of a conference presentation by Samuel C. Allen, graduate researcher, who attended the ASA/CSA/SSSA meeting in Indianapolis in November 2002. In addition, the research site was observed by local farmers and landowners who attended a Field Day in JUne 2002 at the Jay, FL, experiment station, where the research is conducted.

The agroforestry project (as carried out by Mr. Allen) is in its’ last year of data gathering and analysis. Soil and groundwater samples will continue to be collected through early 2003. Remaining activities will involve analysis and write-up of research findings, with a target completion date of December 2003.

Impacts and Contributions/Outcomes

Outcome from the agroforestry project will be used to provide a profile of the interaction effects of trees and crops on the status of nitrogen and groundwater nitrate levels in temperate alley cropping systems. Findings from crop yields, soil moisture measurements, and water, soil and plant tissue analyses, will shed light on crop and tree root activity, and their effects on site environmental conditions. This knowledge will help to improve our basic understanding of temperate alley cropping systems, so that better systems can be created, with tighter nutrient cycling and reduced groundwater pollution.

Ultimately, it is hoped that this research will encourage farmers and landowners to adopt agroforestry practices. The researchers believe that such systems can help to diversify and strengthen the family farm, by providing alternate forms of income at various times of the year, while utilizing land that would otherwise remain unused. We also hope that landowners would be encouraged to plant longleaf pines on their property, given the possibility of incorporating this species with other more immediate cash crops. The prospect of reducing nitrate levels in groundwater is also an exciting possibility for these types of systems, which is a vision that we hope landowners are able to catch.

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