Nitrate–nitrogen and oxygen isotope ratios for identification of nitrate sources and dominant nitrogen cycle processes in a tile-drained dryland agricultural field

Agricultural systems are a leading source of reactive nitrogen to aquatic and atmospheric ecosystems. In this study environmental δ15Nnitrate and δ18Onitrate are used to identify the dominant nitrogen cycle processes and sources of NO3− leached from a tile-drained, dryland agricultural field. Tile-drain water discharge δ18Onitrate values suggest nitrification is the dominant soil nitrogen cycle process throughout the 5-year study period, because the expected δ18Onitrate from nitrification is indistinguishable from the measured value of −1.3 ± 1.5‰. Given this there is no evidence that denitrification was occurring at a large enough scale to influence [NO3−]. Values for δ15Nnitrate varied seasonally during the high-discharge season (January through May) and low-discharge season (June through December) with weighted means of 1.0 ± 1.0‰ and 4.7 ± 2.3‰ respectively. This suggests that during the high-discharge season NO3− originates from nitrification of NH4+ fertilizer, and during the low-discharge season NO3− originates from mineralized soil organic nitrogen. The estimated travel time through the soil for nitrified NH4+ fertilizer leached during the high-discharge season is less than 6 months, from fall fertilizer application to leaching through the tile-drain. This study suggests that understanding the hydrology of a region is necessary before dominant nitrogen cycling processes can be reliably determined.