Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate
A substantial fraction of nitrogen (N) fertilizer applied in agricultural systems is notincorporated into crops and moves below the rooting zone as nitrate (NO3−). Under-standing mechanisms for soil N retention below the rooting zone and leaching togroundwater is essential for our ability to track the fate of added N. We used dualstable isotopes of nitrate (δ15N–NO3−andδ18O–NO3−) and water (δ18O–H2O andδ2H–H2O) to understand the mechanisms driving nitrate leaching at three depths(0.8, 1.5, and 3.0 m) of an irrigated corn field sampled every 2 weeks from 2016to 2020 in the southern Willamette Valley, Oregon, USA. Distinct periods of highnitrate concentrations with lowerδ15N–NO3−values indicated that a portion of thatnitrate was from recent fertilizer applications. We used a mixing model to quantifynitrate fluxes associated with recently added fertilizer N versus older, legacy soil Nduring these “fertilizer signal periods.” Nitrate leached below 3.0 m in these peri-ods made up a larger proportion of the total N leached at that depth (∼52%) versusthe two shallower depths (∼13%–16%), indicating preferential movement of recentlyapplied fertilizer N through the deep soil into groundwater. Further, N associated withrecent fertilizer additions leached more easily when compared to remobilized legacyN. A high volume of fall and winter precipitation may push residual fertilizer N todepth, potentially posing a larger threat to groundwater than legacy N. Optimizingfertilizer N additions could minimize fertilizer losses and reduce nitrate leaching togroundwater.