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

We examined the linkages between topography and electron donors for denitrification on in-stream NO₃ ⁻ concentration in headwater catchments in the Lake Hachiro watershed having marine sedimentary rock, Japan. In 35 headwater catchments (0.07–16.9 km²), we sampled stream water every season in 2 years. The water samples were analyzed for NO₃ –, dissolved nitrous oxide (dN₂O), and SO₄ ² – concentrations. Stream sediment was sampled once for the measurement of denitrification potential (DP). Water-extractable soil organic carbon (WESOC) and easily oxidizable sulfide (EOS) in the sediment, which can be considered the principal potential electron donors for denitrification, were measured. 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 negatively correlated with topographic wetness index (TWI) (P < 0.01) and were positively correlated with catchment slope (P < 0.01), indicating that NO₃ – concentrations decreased in wetter and gentle slope catchments. Sediment DP and the WESOC content in sediments were positively correlated with TWI, significantly. These results suggested denitrification was likely to occur in higher TWI catchments. Generalized linear model showed that TWI, slope aspect, and sediment DP significantly affected in-stream NO₃ – concentration and WESOC was a significant explanatory variable for sediment DP. EOS content in riverbed sediments was not selected as a significant explanatory variable for either in-stream NO₃ ⁻ concentrations or sediment DP. But higher soil DP with higher EOS was detected in the stream bank subsoil at the catchment where the higher EOS content in the riverbed sediment was observed, which suggested EOS in riverbed sediments can contain site-specific information about denitrification hotspot driven by sulfides. We conclude that catchment topography and the distribution of electron donors in riverbed sediment can be important factors to explain the spatial variation in in-stream NO₃ – concentration and sediment DP.