Dissimilatory nitrate reduction to ammonium and N2O flux: effect of soil redox potential and N fertilization in loblolly pine forests

Nitrogen (N) fertilization and soil redox potential influence N cycling processes in forested ecosystems. Gross N transformations are indicators of NH4 (+) and NO3 (-) production and consumption within soil. Furthermore, dissimilatory nitrate reduction to ammonium (DNRA), a typically overlooked process in terrestrial N cycling, can conserve N within soil by reducing losses of soil N via NO3 (-) leaching and denitrification. We tested the effects of urea fertilization and soil redox on microbial N cycling processes and N2O fluxes using a N-15 tracer experiment in soils from loblolly pine plantations located in different physiographical regions (i.e., Coastal Plain of North Carolina and Piedmont of Virginia). Mineral soils (0-15 cm) from fertilized and unfertilized plots were incubated at high (Eh, 200 to 400 mV) and low redox potential (Eh, -100 to 100 mV). Site differences were limited primarily to edaphic factors, although gross N mineralization was higher in NC. Gross nitrification, DNRA, and NO3-(-)-N concentrations were higher in soils from fertilized plots. DNRA was higher at high compared to low redox potential, while N2O fluxes were higher at low redox potential. Fluxes of N2O were further enhanced in fertilized treatments incubated at low redox potential. DNRA was positively correlated with NO3 (-) availability, but not to soil C pools. Furthermore, DNRA was negatively correlated with C/NO3 (-) ratio, implying that NO3 (-) pool size was the primary factor influencing DNRA. These results suggest N fertilization has alleviated limitations to nitrification, DNRA, and N2O production processes and that gaseous losses of N will prevail over N conservation pathways at low soil redox potentials.