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) (<i>P</i> P 3 ^– 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.

本研究针对日本八郎湖流域（发育海相沉积岩）的源头集水区，探究了地形与反硝化作用电子供体对溪流硝态氮（NO₃⁻）浓度的关联机制。本研究选取35个面积介于0.07~16.9 km²的源头集水区，在两年内按季度采集溪流水样。对采集的水样进行了硝态氮（NO₃⁻）、溶解态一氧化二氮（dissolved nitrous oxide, dN₂O）以及硫酸根离子（SO₄²⁻）的浓度测定。单次采集溪流沉积物样品，用于测定反硝化潜势（denitrification potential, DP）。同时测定了沉积物中的水浸提态土壤有机碳（Water-extractable soil organic carbon, WESOC）与易氧化硫化物（easily oxidizable sulfide, EOS）——二者被视为反硝化作用的主要潜在电子供体。利用10米网格分辨率的数字高程模型（digital elevation model）计算各集水区的地形特征。各集水区的溪流硝态氮浓度存在显著空间异质性，范围为0.06~0.52 mg N L⁻¹，且与地形湿度指数（topographic wetness index, TWI）呈显著负相关（P<0.05，r=-0.47）。在湿度更高、坡度更缓的集水区中，溪流硝态氮浓度更低。沉积物反硝化潜势与沉积物中的水浸提态土壤有机碳含量均与地形湿度指数呈显著正相关。上述结果表明，地形湿度指数更高的集水区更易发生反硝化作用。广义线性模型（generalized linear model）分析结果显示，地形湿度指数、坡向以及沉积物反硝化潜势对溪流硝态氮浓度存在显著影响，而水浸提态土壤有机碳是沉积物反硝化潜势的显著解释变量。河床沉积物中的易氧化硫化物含量未被选为溪流硝态氮浓度或沉积物反硝化潜势的显著解释变量。但在河床沉积物易氧化硫化物含量较高的集水区中，其河岸下层土壤的反硝化潜势与易氧化硫化物含量均较高，这表明河床沉积物中的易氧化硫化物可携带由硫化物驱动的反硝化热点的位点特异性信息。本研究结论为：集水区地形与河床沉积物中电子供体的分布，是解释溪流硝态氮浓度与沉积物反硝化潜势空间异质性的关键因素。