Intensive agriculture in the Midwestern United States contributes to excess nitrogen in surface water and groundwater, negatively affecting human health and aquatic ecosystems. Complete denitrification removes reactive nitrogen from aquatic environments and releases inert dinitrogen gas. We examined denitrification rates and the abundances of denitrifying genes and total bacteria at three sites in an agricultural watershed and in an experimental stream in Minnesota. Sampling was conducted along transects with a gradient from always inundated (in-channel), to periodically inundated, to noninundated conditions to determine how denitrification rates and gene abundances varied from channels to riparian areas with different inundation histories. Results indicate a coupling between environmental parameters, gene abundances, and denitrification rates at the in-channel locations, and limited to no coupling at the periodically inundated and noninundated locations, respectively. Nutrient-amended potential denitrification rates for the in-channel locations were significantly correlated (α = 0.05) with five of six measured denitrifying gene abundances, whereas the periodically inundated and noninundated locations were each only significantly correlated with the abundance of one denitrifying gene. These results suggest that DNA-based analysis of denitrifying gene abundances alone cannot predict functional responses (denitrification potential), especially in studies with varying hydrologic regimes. A scaling analysis was performed to develop a predictive functional relationship relating environmental parameters to denitrification rates for in-channel locations. This method could be applied to other geographic and climatic regions to predict the occurrence of denitrification hot spots.
Bibliographical noteFunding Information:
We acknowledge the assistance of Jacques Finlay, Martin du Saire, and Kurt Spokas for laboratory use in denitrification assays, along with the technical staff and student help at the St. Anthony Falls Laboratory. Funding was provided in part by the Clean Water Research Program through the Minnesota Department of Agriculture with funding from the Minnesota Clean Water, Land, and Legacy Amendment. This project was also supported by Agriculture and Food Research Initiative Competitive grant 2015-06019-23600 from the USDA National Institute of Food and Agriculture. We thank the three anonymous reviewers for their helpful comments on this manuscript. Data described in this paper are available upon request by emailing the corresponding author.
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- gene abundances