Land use effects on nitrification in Michigan headwater streams

E.R. Hotchkiss¹, M.J. Kemp², J.L. Tank², S.E. Eichler², L.G. Wall², and C.P. Arango². ¹Emory University, Department of Environmental Studies, Atlanta, GA 30322, ²University of Notre Dame, Department of Biological Sciences, Notre Dame, IN 46556

Increases in the nitrogen concentrations of surface waters in the agricultural Midwest are a large concern due to adverse effects such as eutrophication and hypoxia in the Gulf of Mexico. Studies have shown that smaller order streams can play a key role in retaining nitrogen. Thus, understanding nitrogen cycling in headwater streams is imperative. We compared nitrification rates in forested, agricultural and urban-influenced headwater streams to determine if land-use influences nitrification rates. Using the nitrapyrin inhibition method, we measured nitrification rates associated with sediment collected from several Michigan streams. Nitrification rates were compared to several other stream variables including denitrification rates, discharge, land use, and concentrations of ammonium, nitrate, and dissolved organic carbon. Nitrification rates in urban streams were an order of magnitude higher than nitrification in urban and forested headwater streams. Agricultural streams (which had the highest overall denitrification rates) had only slightly higher nitrification rates compared to forested streams. There was a significant relationship between nitrification and denitrification rates (r²=0.695, P=0.0001, N=10). Nitrification rates were also influenced by water column dissolved oxygen concentration, with a weak negative relationship (r²=-0.906, P=0.002, N=8). Further studies examining more streams throughout the year are planned.