Simultaneous field measurements of biogenic emissions of nitric oxide and nitrous oxide

Seasonal and diurnal emissions of nitric oxide (NO) and nitrous oxide (N 2 O) from agricultural sites in Virginia and Colorado were simultaneously determined as a function of soil temperature, percent moisture, and exchangeable nitrate, nitrite, and ammonium concentrations. Nitric oxide fluxes at the Virginia site were significantly correlated ( P < 0.01) with nitrate concentration, temperature, and percent moisture. At the Colorado site, NO fluxes were both positively and significantly correlated ( P < 0.01) with temperature and moisture. Nitrous oxide emissions were only observed when percent moisture approached or exceeded field capacity of the soil. Nitric oxide emissions at the Virginia site were observed throughout the entire year with 76% of the annual flux produced between May and October and 24% between November and April. Wintertime fluxes of NO have not previously been reported in the literature. Annual NO emissions at the Virginia site ranged from 0.53 kg(N) ha −1 yr −1 from unfertilized land to 2.08 kg(N) ha −1 yr −1 from fertilized land. Of the 196.4 kg ha −1 of fertilizer added to the soil site being studied, 0.79% was lost as NO(N), and 1.2% was lost as N 2 O(N). A series of diurnal studies demonstrated that variations in NO flux throughout the day were correlated with changes in soil temperature. Nitric oxide was emitted over a broad range of soil moisture conditions, provided that percent moisture did not exceed field capacity of the soil. When field capacity was exceeded, NO fluxes declined whereas N 2 O emissions increased. Rewetting of dry soils at the Colorado site resulted in dramatic increases in emissions of both NO and N 2 O. Our data suggest that NO is produced primarily by nitrification in aerobic soils whereas N 2 O is formed by denitrification in an aerobic soils.