Rainfall driven nitrate transport in agricultural karst surface river system: Insight from high resolution hydrochemistry and nitrate isotopes

High discharge associated with rainfall events causes rapid transport of nutrients to drainage systems in karst terrain. The objective of this study was to understand nitrate dynamics and transport mechanisms during rainfall events in a typical mixed land-use karst catchment in Southwest China. High frequency (hourly) water sampling, discharge and hydrochemistry sensor data at a 15-minute interval were collected from a karst agricultural surface river over four rainfall events, spanning 40 days at the commencement of the wet season. The results showed that high [NH4+-N] (up to 3.12 mg L-1) and [PO43--P] (up to 1037 mu g L-1) were observed in the first rainfall event of the wet season, after a long preceding dry period. Increased [NO3--N] also occurred during the high discharge associated with early event flows, but the highest [NO3--N] concentrations (up to 14.9 mg L-1) were observed later in the season during the highest discharge events. All minima for dual nitrate isotopes were found during high discharge conditions and increased through the event time series. This indicates that nitrogen accumulated in the catchment prior to the commencement of the wet season and was flushed by the early rainfall events into the drainage system. Analysis of [NO3--N], delta N-15 and delta O-18-NO3- values indicate that denitrification, not assimilation, resulted in the high nitrate isotope values before and after rainfall events. According to source apportionment, chemical fertilizer, soil organic nitrogen, and manure and sewage, which varied with discharge, were the main sources of nitrate during rainfall events. The high proportion of nitrate from soil organic nitrogen suggested that soil loss from peak clusters and lowland farmland during heavy rainfall events should be paid more attention in karst area.