Biogeochemical processes of nitrate removal by prairie buffers in Iowa agricultural watersheds

Background/Question/Methods

Integration of perennial vegetation into agricultural watersheds has been promoted to reduce nitrate loss. The effectiveness of this practice depends on the biogeochemical processes by which perennial vegetation and associated microbes and soils remove nitrate from subsurface flow. The principal nitrate removal processes are denitrification and assimilation into soil organic matter (SOM). While denitrification can act as a long-term sink for nitrate inputs without decreasing over time, the long-term sink for nitrate in SOM eventually saturates. We investigated nitrate removal mechanisms in buffers of perennial prairie vegetation integrated into row crop watersheds in southern Iowa. These buffers have been shown to reduce watershed nitrate losses by about 60%. The magnitude of this reduction has not exceeded that attributed to denitrification in other sites. However, preliminary data show accumulation of nitrogen in the buffer soils in excess of atmospheric deposition and biological fixation. Denitrification enzyme assays were used to compare potential denitrification rates between row crop and prairie buffer soils in equivalent topographic positions. Additionally, isotopically-labeled nitrate was used to estimate the importance of SOM as a nitrate sink in the buffers.

Results/Conclusions

Denitrification potential was significantly greater in prairie compared to row crop soils (p = 0.0494), despite significantly greater soil nitrate concentrations in row crop soils (p < 0.0001). Mineralizable carbon was correspondingly greater in prairie compared to row crop soils during incubation (p = 0.0039). These results indicate that increased rhizosphere carbon inputs enhance denitrification in the prairie buffers, providing a long-term sink for nitrate inputs in subsurface flow. Forthcoming results from the isotope labeling experiment will clarify the importance of SOM as an additional sink for nitrate inputs. The combined results will demonstrate whether or not perennial vegetation buffers in row crop watersheds can be expected to continue to reduce nitrate loss in the long term, with substantial implications for watershed management and water quality.