Catchment topography and the distribution of electron donors for denitrification control the concentration of nitrate in headwater streams of the Lake Hachiro watershed

Atsushi Hayakawa, Yu Funaki, Tatsuya Sudo, Ryoki Asano, Shintaro Watanabe, Yuichi Ishikawa, Shin Hidaka

Department of Biological Environment, Faculty of Bioresource Science, Akita Prefectural University, Akita, 010-0195, Japan, [email protected]

Abstract

We examined the linkages between topography and electron donors for denitrification in headwater streams in the Lake Hachiro watershed that has marine sedimentary rocks. In 33 headwater streams, we sampled water at the catchment nine times in 2 years. Stream sediment was sampled once for measurement of denitrification potential (DP), water-extractable soil organic carbon (WESOC), and easily oxidizable sulfide (EOS), which are considered the principal potential electron donors for denitrification. The topographical features of each catchment were calculated using a digital elevation model with 10-m grid cells. Stream NO3 concentrations displayed large spatial variation among catchments, ranging from 0.06 to 0.52 mg N L–1, and were significantly positively correlated with slope in the catchments (r = 0.663, P < 0.01, n = 33), indicating that NO3 was removed to a greater extent in gentle slope catchments. Generalized linear model showed slope, slope aspect, sediment DP, and EOS significantly affected in-stream NO3 concentration. Stream SO42– concentrations tended to increase as NO3 concentrations decreased and EOS contents increased, indirectly indicating sulfur-mediated denitrification. NO3 reduction with SO42– production, and sulfur-oxidizing bacteria was detected in the stream bank soil with high EOS content. We conclude that catchment topography and the distribution of electron donors in riverbed sediment explain the spatial variation in in-stream NO3 concentration and, by inference, catchment denitrification. These results indicate that more NO3 may be denitrified by sulfur-mediated denitrification owing to the abundance of sulfides in the catchment from marine sedimentary rocks.