Land use influences nitrogen metabolism of headwater streams

C.P. Arango, J.L. Tank, J.J. Beaulieu, and L.T. Johnson.Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556

Nitrogen (N) metabolism in streams may be grouped into dissimilatory and assimilatory pathways. Dissimilatory pathways include nitrification, the microbial conversion of ammonium (NH₄⁺) to nitrate (NO₃^-), and denitrification, the microbial conversion of NO₃^- to nitrogenous gases whereas assimilatory pathways include N uptake into biomass. Only denitrification permanently removes N from ecosystems. We investigated how land use alters these processes in 12 streams that differed in light availability and nutrient concentrations by measuring nitrification and denitrification monthly and whole-stream N uptake seasonally. Nitrification rates were positively related to benthic organic matter (simple linear regression; p=0.005, R²=0.06), and denitrification rates were positively related to NO₃^- concentration and benthic organic matter (multiple linear regression; p<0.001, R²=0.76). Biological demand for NH₄⁺ was positively related to whole-stream chlorophyll a (simple linear regression; p=0.038, R²=0.13), suggesting that assimilatory uptake by primary producers influences whole-stream NH₄⁺ demand. Although seasonality and land-use did not influence whole-stream NO₃^- uptake, the proportion of whole-stream NO₃^- uptake as denitrification increased with organic matter standing stocks and proportion of fine benthic organic matter (multiple linear regression; p=0.004, R²=0.32). Land-use activities that increase light availability and decrease organic matter may shift stream N retention from permanent N removal pathways to transient assimilatory pathways.