Nitrate and dissolved nitrous oxide in groundwater within cropped fields and riparian buffers

Abstract. Transport and fate of dissolved nitrous oxide (N₂O) in groundwater and its significance to nitrogen dynamics within agro-ecosystems are poorly known in spite of significant potential of N₂O to global warming and ozone depletion. Increasing denitrification in riparian buffers may trade a reduction in nitrate (NO₃⁻) transport to surface waters for increased N₂O emissions resulting from denitrification-produced N₂O dissolved in groundwater being emitted into the air when groundwater flows into a stream or a river. This study quantifies the transport and fate of NO₃⁻ and dissolved N₂O moving from crop fields through riparian buffers, assesses whether groundwater exported from crop fields and riparian buffers is a significant source of dissolved N₂O emissions, and evaluates the Intergovernmental Panel on Climate Change (IPCC) methodology to estimate dissolved N₂O emission. We measured concentrations of NO₃⁻; chloride (Cl⁻); pH; dissolved N₂O, dissolved oxygen (DO), and organic carbon (DOC) in groundwater under a multi-species riparian buffer, a cool-season grass filter, and adjacent crop fields located in the Bear Creek watershed in central Iowa, USA. In both the multi-species riparian buffer and the cool-season grass filter, concentrations of dissolved N₂O in the groundwater did not change as it passed through the sites, even when the concentrations of groundwater NO₃⁻ were decreased by 50% and 59%, respectively, over the same periods. The fraction of N lost to leaching and runoff (0.05) and the modified N₂O emission factor, [ratio of dissolved N₂O flux to N input (0.00002)] determined for the cropped fields indicate that the current IPCC methodology overestimates dissolved N₂O flux in the sites. A low ratio between dissolved N₂O flux and soil N₂O emission (0.0003) was estimated in the cropped fields. These results suggest that the riparian buffers established adjacent to crop fields for water quality functions (enhanced denitrification) decreased NO₃⁻ and were not a source of dissolved N₂O. Also, the flux of dissolved N₂O from the cropped field was negligible in comparison to soil N₂O emission in the crop fields.