Field Soil Properties Influencing the Variability of Denitrification Gas Fluxes

The spatial variability of field denitrification gas fluxes was investigated in relation to water content, soil-gas diffusivity, nitrate concentration, and water soluble organic C. Water was applied in a periodic fashion along a strip of Yolo soil (Typic Xerorthents) amended with chopped alfalfa hay and nitrate. Data were analyzed statistically using simple correlations and spectral and coherency analysis. Log spectrums showed that nitrate cycled at the frequency at which water content cycled and was negatively related to water content due to apparent leaching of nitrate from the high water application areas. Denitrification gas flux cycled at twice the frequency of water content with maximum fluxes occurring at the sides of the water peaks. Spectrums for water soluble organic C and gas diffusivity indicated no significant spatial cycling. Although denitrification gas flux was more highly correlated to soil-water content than to any other variable, coherency analyses revealed no significant relationships between denitrification gas flux and water or nitrate at specific frequencies due to opposing effects related to nitrate leaching and small gas diffusivities at the soil surface. The spatial pattern of denitrification calculated from a simple equation was examined using spectral analysis and was found to be not representing measured denitrification gas flux adequately at the frequency at which water content cycled. The discrepancy between measured denitrification gas flux and calculated denitrification rates was attributed to very small gas diffusivities preventing gas transport to the surface and inadequate characterization of actual C and nitrate concentrations at microsites. Contribution from the Agric. Exp. Stn. and the Dep. of Land, Air and Water Resources, Univ. of California, Davis. Research supported partially by a scholarship from the Ministry of Industry and Research, Government of France and Charbonnage de France Chimie-Azote et Fertilisants, France.