Benthic denitrification rates on heterogeneous substrata in streams draining forested and agricultural landscapes

C.P. Arango¹, J.L. Tank¹, M.J. Kemp¹, L.G. Wall¹, S.E. Eichler¹, and E.R. Hotchkiss². ¹Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, ²Emory University, Atlanta, GA, 30322

Denitrifying organisms are associated with most benthic substrata found in streams ranging from fine sand to coarse particulate organic material (CPOM). Each type of substratum may provide different resources for microbial colonization and subsequently may support different denitrification rates. Similarly, watershed land use may influence denitrification by altering nitrate and carbon availability. We collected samples of homogeneous substrata (CPOM, algae, vascular plants, iron-oxidizing bacterial mats, and main channel and backwater sediments) in forested and agricultural headwater streams and performed laboratory denitrification assays using the chloramphenicol-amended acetylene inhibition method. Generally, denitrification on substrata found in agricultural streams was an order of magnitude higher (14.52 to 92.90 μg N₂O g AFDM^-1 h^-1) than those same substrata in forested streams (1.01 to 13.96 μg N₂O g AFDM^-1 h^-1). Denitrification was highest on iron-oxidizing bacterial mats in agricultural streams and backwater fine benthic organic matter (FBOM) in forested streams. Denitrification rates were significantly correlated with stream nitrate concentration on CPOM (r² = 0.969, p = 0.002) and backwater FBOM (r² = 0.869, p = 0.068) suggesting denitrification was nitrate-limited on these substrata. In general, denitrification rates varied among benthic substrata, and watershed land use characteristics may mediate heterogeneity in benthic denitrification processes in streams.