Seasonal and Landform Influence On Nitrous Oxide Fluxes In Riparian Buffers.

Despite the general recognition that riparian buffers are hotspots of nitrogen (N) transformations, their contribution to regional nitrous oxide (N₂O) inventory has not been quantified. In riparian ecosystems, N₂O fluxes are highly variable owing to the interacting effects of landscape heterogeneity, patchy distribution of nutrients, and hydro-climatic events. Here we present results of a study (bi-weekly sampling for 18 months) conducted at riparian sites representing three of the most common hydro-geomorphic (HGM) settings in Central Indiana: i) HGM-1 – till plain depression near headwater streams/ditches, tile drainage; ii) HGM-2 – incised valley in till plain with thin alluvium deposits above the till layer, no tile drains; and iii) HGM-3, alluvial deposits above thick outwash deposits adjacent to 3^rd-4^th order stream, no tile drains. Among the soil biochemical properties examined, N mineralization showed the best relationship (r²: 0.61, p<0.001) with the spatial distribution of N₂O emission at the study sites. Results showed that N₂O emission was on average 4-fold higher (7.02 g N ha^-1d^-1) at HGM-3 compared to the other units. Short-term validation measurements at other riparian buffers have also shown higher (3-fold) N₂O emission in HGM-3 sites than in HGM-1 sites during the wet season. These results are consistent with the higher groundwater NO₃^- concentration (0.2 - 13.7 mg N L^-1), periodic floods, and the more dynamic water table (mean: 42.9 ±86.1 cm) at HGM-3 sites than at other sites. The weak N₂O emission at HGM-1 units may be ascribed to tile drainage resulting in deep water table position (mean depth: 122.2 ±33.3 cm), and thus limited interaction between NO₃^--laden groundwater with the riparian buffer. These observations will be discussed to highlight relevant biogeochemical processes, and to demonstrate the potential of an HGM-based research approach to derive watershed-scale riparian N₂O budgets.