Nitrous oxide production in riparian zones and its importance to national emission inventories

Riparian zones, which sit at the interface between terrestrial and aquatic components of the landscape, often receive and process large amounts of excess nitrogen (N) that moves out of agricultural fields towards streams. These areas thus have the potential to be “hotspots” of nitrous oxide (N₂O) production in the landscape. However, current Intergovernmental Program on Climate Change (IPCC) methodologies for calculating national N₂O emission inventories do not explicitly account for riparian N₂O production. In this paper, we examine the nature and extent of N₂O production in riparian zones, present some new data on N₂O production in these areas, and propose a modification to the current IPCC methodology for quantifying N₂O emissions from agriculture. We also present an example of how large-scale riparian restoration efforts to achieve agricultural water quality objectives could cause significant changes in regional N₂O budgets. Although current data are inadequate to propose a quantitative emission factor for riparian N₂O emissions, they suggest that these emissions are likely to be significant in many regions. Specific data on riparian N₂O emissions should be collected in association with detailed watershed mass balance studies that allow for evaluation of several aspects of the IPCC methodology at once and provide constraints on the magnitude of fluxes that are difficult to measure, e.g. N₂O flux, N₂O:N₂ ratio. Riparian and wetland restoration projects to reduce NO₃⁻ delivery to coastal waters are being considered in many areas of the world. These projects may affect regional and global N₂O budgets, but only if they alter the N₂O:N₂ ratio during denitrification.