Winter nitrification contributes to excess NO3− in groundwater of an agricultural region: A dual-isotope study

[1] Conventional thinking is that bacterial nitrification leading to labile nitrate in fertilized agricultural soils of northern regions greatly diminishes during winter. We have carried out seasonal water sampling over 2 years to understand the fate of nitrate present in a rapidly responding groundwater/surface water system. Nitrate results show no seasonal δ15N trend. Significant δ18O downward shifts were observed between the spring-summer and autumn-winter periods of 2003–2004 (10.0‰) and 2004–2005 (1.3‰). Using mass-balance mixing calculations of soil leachate with groundwater and assuming seasonal nitrification, we reproduce the observed water and nitrate oxygen-isotope trends. These calculations suggest that nitrification takes place throughout all seasons. We also use a hydroclimatic index to establish a relationship between δ18O values in nitrate and recharge weighted by temperature. Our findings imply that nitrifying activities occur all year long and that winter nitrate production is high. This conclusion has important implications for modeling the nitrogen cycle of regions where seasonal changes in soil water mark the oxygen isotopes of nitrate.