Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate
Citation:
Weitzman, J., Jacqueline Brooks, J. Compton, B. Faulkner, R. Peachey, W. Rugh, R. Coulombe, B. Hatteberg, AND S. Hutchins. Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate. Vadose Zone Journal. Soil Science Society of America, Madison, WI, 23(3):e20324, (2024). https://doi.org/10.1002/vzj2.20324
Impact/Purpose:
Nitrate contamination of groundwater is an important issue in many agricultural areas across the country. Agricultural practices aiming to improve nitrogen management and reduce nitrate leaching to groundwater have received a great deal of attention in crop and soil science. However, better understanding of the controls on leaching rates and nitrogen dynamics within deeper soils are needed to determine potential management practices that could help mitigate groundwater contamination. We used stable isotopes of water and nitrate to understand how fertilizer N mixes within an agricultural field in the southern Willamette Valley. This work represents a collaboration between EPA researchers from the PESD and GCRD divisions, in addition to Oregon State University researchers.
Description:
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.
