Rice agriculture increases base flow contribution to catchment nitrate loading in subtropical central China

• Rice agriculture increases N leaching into groundwater system.
• Rice agriculture increases NO3−–N concentrations in base flow.
• Rice agriculture increases base flow contribution to catchment NO3−–N loading.
• Fallow season has greater base flow contribution to catchment NO3−–N loading than rice-growing season.

Base flow is recognized as an important hydrological pathway for NO3−–N export, however, the base flow contribution to NO3−–N loading in rice agriculture catchments remains unknown. In this study, stream discharge and NO3−–N concentration were observed in two contrasting rice agriculture catchments (named Tuojia and Jianshan) in subtropical central China between November 2010 and December 2013, to quantify the base flow contribution to NO3−–N loading and determine its relationship with rice agriculture. The results suggested that Tuojia produced more base flow (727.0 vs. 426.5 mm) and had higher base flow contribution to stream discharge (41.9% vs. 28.4%) than Jianshan did during the observation period, due to the more groundwater recharge associated with the higher areal proportion of rice agriculture in Tuojia. The average flow-weighted NO3−–N concentration in the base flow was higher in Tuojia than in Jianshan (1.43 vs. 1.07 mg N L−1), because rice agriculture could result in obvious N leaching into groundwater system. The NO3−–N loading via the base flow reached 0.27 kg N ha−1 month−1 in Tuojia, which contributed 36.5% of the NO3−–N loading via the stream discharge. These values were much greater than 0.12 kg N ha−1 month−1 and 27.3% in Jianshan. The more NO3−–N loading and greater base flow contribution in Tuojia were attributed to the more base flow and higher NO3−–N concentration in base flow associated with the intensive rice cropping. Specifically, the base flow contribution to the NO3−–N loading was greater during the fallow seasons than during the rice-growing seasons, likely due to the NO3−–N “landscape memory” effects from previous rice cropping seasons. Therefore, NO3−–N reduction practices in the rice agriculture catchments should be applied to mitigate the base flow contribution to NO3−–N loading in subtropical central China.