Nitrous oxide emissions from riparian forest buffers, warm-season and cool-season grass filters, and crop fields

Abstract. Denitrification within riparian buffers may trade reduced nonpoint source pollution of surface waters for increased greenhouse gas emissions resulting from denitrification-produced nitrous oxide (N₂O). However, little is known about the N₂O emission within conservation buffers established for water quality improvement or of the importance of short-term N₂O peak emission following rewetting dry soils and thawing frozen soils. Such estimates are important in reducing uncertainties in current Intergovernmental Panel on Climate Change (IPCC) methodologies estimating soil N₂O emission which are based on N inputs. This study contrasts N₂O emission from riparian buffer systems of three perennial vegetation types and an adjacent crop field, and compares measured N₂O emission with estimates based on the IPCC methodology. We measured soil properties, N inputs, weather conditions and N₂O fluxes from soils in forested riparian buffers, warm-season and cool-season grass filters, and a crop field located in the Bear Creek watershed in central Iowa, USA. Cumulative N₂O emissions from soils in all riparian buffers (5.8 kg N₂O-N ha⁻¹ in 2006–2007) were significantly less than those from crop field soils (24.0 kg N₂O-N ha⁻¹ in 2006–2007), with no difference among the buffer vegetation types. While N₂O peak emissions (up to 70-fold increase) following the rewetting of dry soils and thawing of frozen soils comprised 46–70% of the annual N₂O emissions from soils in the crop field, soils in the riparian buffers were less sensitive to such events (3 to 10-fold increase). The ratio of N₂O emission to N inputs within riparian buffers (0.02) was smaller than those of crop field (0.07). These results indicate that N₂O emission from soils within the riparian buffers established for water quality improvement should not be considered a major source of N₂O emission compared to crop field emission. The observed large difference between measured N₂O emissions and those estimated using the IPCC's recommended methodology (i.e., 87% underestimation) in the crop field suggests that the IPCC methodology may underestimate N₂O emission in the regions where soil rewetting and thawing are common, and that conditions predicted by future climate-change scenarios may increase N₂O emissions.