Hydrological and biogeochemical controls on the timing and magnitude of nitrous oxide flux across an agricultural landscape

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N(2)O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N(2)O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha-1 KNO(3)-N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N = 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N(2)O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N(2)O emissions (r2 = 0.47; P = 0.013). Within individual soil columns, N(2)O flux was a Gaussian function of water-filled pore space (WFPS) during drainage (mean r2 = 0.90). However, instantaneous maximum N(2)O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N(2)O flux rates occurred within a narrow range (-1.88 to -4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N(2)O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N(2)O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N(2)O fluxes across soils that differ in texture, structure and bulk density.