Agro-ecological nitrogen management in soils vulnerable to nitrate leaching: a case study in the Lower Suwannee Watershed

Abstract

Environmental benefits associated with reduced rates of nitrogen (N) application, while maintaining economically optimum yields have economic and social benefits. Although N is an indispensable plant nutrient, residual soil N could leach out to contaminate groundwater and surface water resources, particularly in sandy soils. A 2-year field study was conducted in an established bermudagrass (Cynodon dactylon) pasture in the Lower Suwannee Watershed, Florida, to evaluate N application rates on forage yield, forage quality, and nitrate (NO3-N) leaching in rapidly permeable upland sandy soils. Four N application rates (30, 50, 70, and 90 kg N ha−1 harvest−1) corresponding to 0.33, 0.55, 0.77 and IX, respectively, of recommended N rate (90 kg N ha−1 harvest−1) for bermudagrass hay production in Florida were evaluated vis-a-vis an unfertilized (0 N) control. Suction cups were installed near the center of each plot at two depths (30 and 100 cm) to monitor NO3-N leaching. The grass was harvested at 28 days intervals to determine dry matter yield, N uptake, and herbage nutritive value. Nitrogen application at the recommended rate produced the greatest total dry matter yield (~18.4 Mg ha−1 year−1), but a modeled economically optimum N rate of ~57 kg N ha−1 harvest−1 (~60% of the recommended N rate) projected an average dry matter yield of ~17.3 Mg ha−1 year−1, which represents >90% of the observed maximum yield. Nitrogen application increased nutritive quality of the grass, but increases in N application rate above 30 kg N ha−1 did not result in significant increases in in vitro digestible organic matter concentration, and tissue crude protein was not significant above 50 kg N ha−1. Across the sampling period, treatments with N rates ≤50 kg N ha−1 harvest−1 had leachate NO3-N concentration below the maximum contaminant limit of <10 mg l−1. Conversely, applying N at rates ≥70 kg N ha−1 harvest−1 resulted in leachate N concentration that exceeded the maximum contaminant limit, and suggest high risk of impacting groundwater quality, if such rates are applied to soils with coarse (sand) textures. The study demonstrates that recommendation of a single N application rate may not be appropriate under all agro-climatic conditions and, thus, a site-specific evaluation of best N management strategy is critical.