Spatial heterogeneity of denitrification in semi-arid floodplains

Riparian ecosystems are recognized as sinks for inorganic nitrogen (N). Denitrification, a heterotrophic microbial process, often accounts for a significant fraction of the N removed. Characteristics of both riparian soils and hydrologic vectors may constrain the locations where denitrification can occur within riparian ecosystems by influencing the distribution of substrates, water, and suitable redox conditions. We employed spatially explicit methods to quantify heterogeneity of soil characteristics and potential rate of denitrification in semi-arid riparian ecosystems. These results allow us to evaluate the relative contributions of hydrologic vectors (water courses that convey materials) and soil resources (materials required by biota) to spatial heterogeneity of denitrification. During dry and monsoon seasons we contrasted a mesic site, characterized by shallow groundwater and annual inundation by floods, with a xeric site that is inundated less often and has a deeper water table. Potential denitrification was detected throughout the mesic floodplain and the average rate of denitrification was greater at the mesic site than at the xeric site, indicating the influence of water availability on denitrification. At the xeric reach, sharp declines in pools of soil resources and rate of denitrification occurred away from the stream, demonstrating the importance of the stream in determining spatial patterns. Using geographically weighted regression analysis, we determined that soil organic matter and soil nitrate were significant predictors of denitrification at the xeric site, but that factors influencing denitrification varied spatially. Spatial heterogeneity of carbon (C) and N substrates in soils likely influenced spatial patterns of denitrification, but distribution of C and N substrates was ultimately organized by hydrologic vectors. Droughts will increase the abundance of reaches with hydrogeomorphic templates similar to the xeric reach studied here. Consequences of such a transition may include a reduced rate of denitrification and patchy distribution of denitrification in floodplain soils, which will decrease the contribution of riparian ecosystems to N removal.