Soil denitrification and nitrous oxide losses under corn irrigated with high-nitrate groundwater

Inappropriate management of irrigation water and fertilizer N in irrigated corn (Zea mays L.) has resulted in excess N leaching from the rooting zone and NO3- contamination of groundwater. A better understanding of microbially mediated conversion of plant available N to gaseous N forms is needed for more efficient N use, to evaluate the impact of radiatively important gas production such as nitrous oxide (N2O) on air quality, and to determine the potential of microbial denitrification for remediating high-NO3 irrigation water. Soil denitrification and N2O losses from pivot irrigated corn were measured during the 1991 and 1992 growing seasons using surface gas chambers at the Management System Evaluation Area (MSEA) site for water quality research in the Central Platte Valley near Shelton, NE. Denitrification and N2O losses remained low throughout most of the growing season, generally ranging from 10 to 50 g N ha(-1) d(-1), when soil water contents were generally below 60% water-filled pore space (WFPS). Nitrous oxide was the major gaseous N product under these conditions. When soils were wetted by irrigation or rainfall to >70% WFPS, denitrification losses ranged from 0.2 to 1.4 kg N ha(-1) d(-1) with N-2 comprising 80 to 98% of the denitrification gases measured. Monthly average denitrification losses across years were highly correlated (r = 0.693) with time of growing season, suggesting a possible link between plant growth and denitrification. Seasonal gaseous N losses via denitrification and N2O emission from irrigated corn represented 1 to 5% of the N applied as fertilizer or in irrigation water during a relatively dry 1991 season and the 1992 season with near normal precipitation. Results of our 2-yr study indicate that under good irrigation and N management practices gaseous N losses from denitrification and N2O production pose little additional threat to atmospheric quality or N-use efficiency but also provide little hope for bioremediation of high-NO3- groundwater.