Hydrological process controls on nitrogen export during storm events in an agricultural watershed

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The dynamic characteristics of nitrogen (N) and suspended solids (SS) were investigated in stream water during four storm events in 2003 at the Shibetsu watershed, eastern Hokkaido, Japan. Analysis showed that total nitrogen (TN), nitrate-N (NO^[-]_[3] -N), dissolved organic nitrogen (DON), particulate nitrogen (PN), and SS concentrations all peaked sharply during the rising limb of the discharge hydrograph, but peaks in PN and SS were more significant than that of dissolved N. PN and SS consistently displayed clockwise hysteresis with higher concentrations during rising flows, whereas NO^[-]_[3]-N and DON showed different patterns among storms depending on the antecedent soil moisture. An M (V) curve, defined as the nutrient mass distribution vs. the volume of discharge, showed that a "first flush" of PN, NO^[-]_[3]-N, DON and SS was observed, however, the distribution of nutrient loads in the discharge was different. PN and SS had a shorter flushing characteristic time constant (t_[1/e], defined as the time interval required for a decline in nutrient concentrations in discharge water to e^[-1] (37%) of their initial concentrations) but contributed 80% of fluxes during the first 50% of the discharge, while longer flush time (t_[1/e]) of NO^[-]_[3]-N and DON with slowly decreased concentrations led to half loads during the recession of the discharge. These data indicate that the flush mechanisms might be distinguished between particulate nutrients and dissolved N. Analysis showed that the concentrations of PN and SS derived from soil erosion were related to surface runoff. In contrast, NO^[-]_[3]-N originated from the near-surface soil layer associated with the rising shallow ground water table and mainly flushed with subsurface runoff. Different flushing mechanisms implied that different watershed best management practices should be undertaken for effectively mitigating water quality degradation.
社団法人日本土壌肥料学会の論文

Hydrological process controls on nitrogen export during storm events in an agricultural watershed

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