Spatial variability of nitrous oxide fluxes and controlling soil and topographic properties

Spatial heterogeneity of nitrous oxide (N2O) nux was characterized along with various soil chemical, physical, and microtopographical properties to identify those determining flux in fertilized grassland in spring 1993 and in fertilized winter wheat in spring 1994. Measurements were made at random locations within regular grids, Nitrous oxide emission was measured using closed chambers at 84 locations in each grid, spread over 2 d. The ranges of emissions from the grassland and from the winter wheat were 0 to 134 and 0 to 26.4 g N2O-N ha(-1)d(-1). Variograms for N2O emission and for concentrations in the soil atmosphere at 100 mm depth indicated that spatial dependence was weak at both sites. Of the other properties, pH and nitrate showed weak autocorrelation but none of the soil physical properties showed any significant spatial dependence. The results of multiple Linear regression suggested that denitrification was the main N2O production process at the grassland site, but nitrification may have been equally important at the drier winter wheat site. At both sites, the highest N2O emissions were associated with areas of a few square centimeters to a few square meters, lying below the average slope. Use of a partial least squares regression technique to predict nitrous oxide flux revealed the contribution of air permeability in addition to nitrate, ammonium, and soil water contents. Our analyses suggested that nitrous oxide production, consumption, and transport processes varied markedly with depth (over a few centimeters) near the soil surface.