Nitric and nitrous oxide emissions following fertilizer application to agricultural soil: Biotic and abiotic mechanisms and kinetics

Emissions of nitric and nitrous oxide (NO and N2O) from agricultural soils may have several consequences, including impacts on local tropospheric and global stratospheric chemistry. Elevated NO and N2O emissions following application of anhydrous ammonia to an agricultural field in California were driven by the biological generation of nitrite (NO2.) and subsequent abiotic decomposition of nitrous acid (HNO2). Maximum fluxes of > 1000 ng NO-N cm(-2) h(-1) and > 400 ng N2O-N cm(-2) h(-1) were observed, and emissions of > 1000 ng NO-N cm(-2) h(-1) and > 50 ng N2O-N cm(-2) h(-1) persisted for > 4 weeks. Laboratory experiments were performed to determine race coefficients and activation energies for HNO2-mediated NO and N2O production. Kinetic parameters describing the conversion of NO to N2O were measured and were found to vary with water-filled pore space (WFPS). Regression models incorporating HNO2, WFPS, and temperature accounted for 75-77 % of the variability in field fluxes. A previously developed NO emissions model was modified to incorporate a kinetic expression for HNO2- and temperature-dependent production. The model tended to underestimate fluxes under low-flux conditions and overestimate fluxes under high-flux conditions, These data indicate that (1) control of acidity may be an effective means for minimizing gaseous N losses from fertilized soils and possibly for improving air quality in rural areas, (2) the transformation of HNO2-derived NO may be an important mechanism of N2O production even under relatively aerobic conditions, and (3) mechanistic models which account for spatial heterogeneity and transient conditions may be required to better predict field NO fluxes.