Landscape- and regional-scale quantification of nitrous oxide emission from a subhumid transitional grassland-forest region

This study was conducted to obtain landscape- and regional-scale estimates of N2O emissions for a representative part of the Black soil zone of Saskatchewan. A 4318-km2 study region was stratified based on soil texture and land use. At the regional scale, soil texture was the proxy variable used to represent the differences in soil moisture regimes and soil fertility, whereas land use was the surrogate variable used to reflect the differences in N and C cycling. Soil landscapes were selected to cover the range of soil texture and land use characteristics in the study region. At the landscape level, shoulder and footslope complexes were used as the spatial sampling units to cover the range of topographical and soil characteristics within the landscape. At the landform complex level, soil moisture (as assessed by volumetric moisture content and water-filled pore space) was the most important factor controlling N2O emission. At the landscape scale, soil moisture was, in turn, influenced by topography, and on the seasonal scale it was affected by climatic factor(s) (e.g., precipitation). The annual N2O emissions were calculated as the sum of the spring and the summer to fall fluxes. The spring emission was estimated by interpolating the N2O fluxes measured on discrete sampling days, whereas the summer to fall emission was estimated by establishing regression models that related N2O fluxes to water-filled pore space. Regional estimates of N2O emissions were obtained using the GIS database of soil texture and land use types. The average annual fluxes for fertilized cropped, fallow, pasture, and forest areas, weighted by their areal extent in the different textural areas of the study region, were 2.01, 0.12, 0.04, and 0.02 kg N2O-N ha-1 yr-1 respectively. The weighted-average annual fluxes for the medium- to fine-textured and sandy-textured areas were 1.31 and 0.04 kg N2O-N ha-1 yr$\sp{-1},$ respectively. For the study region, the weighted-average annual flux was 0.90 kg N2O-N ha-1 yr$\sp{-1}.