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

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 (NO 3 -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-à-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 NO 3 -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 NO 3 -N concentration below the maximum contaminant limit of