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Tracking fertilizer flushing through the vadose zone using the dual stable isotopes of water and nitrate
Citation:
Weitzman, J., J. Renee Brooks, J. Compton, B. Faulkner, R. Peachey, W. Rugh, R. Coulombe, B. Hatteberg, AND S. Hutchins. Tracking fertilizer flushing through the vadose zone using the dual stable isotopes of water and nitrate. AGU 2022 Fall Meeting, Chicago (& Virtual), IL, December 12 - 16, 2022.
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:
Nitrogen (N) fertilizer is important for agricultural yield, yet a substantial fraction of applied N is not incorporated into the crop and moves below the rooting zone as nitrate (NO3-) in agricultural systems. Our understanding of soil-water interactions below the rooting zone is limited, including the mechanisms of N retention in soil and leaching to groundwater. The dynamics of legacy N accumulation throughout the soil further complicates our ability to accurately predict the fate of added N. Stable isotopes are powerful tools for identifying sources, processes, and movement of N and water through the vadose zone. We sought to better understand the mechanisms driving NO3- leaching within and across three depths (0.8, 1.5, and 3.0 m) of an intensively sampled (every two weeks over four years) irrigated corn field located in the southern Willamette Valley, Oregon, USA, by analyzing pore water samples collected from lysimeters for the natural abundance dual stable isotopes of water (δ18O-H2O and δ2H-H2O) and NO3- (δ15N-NO3 and δ18O-NO3). Based on isotopic mixing models, we differentiated periods when contributions to NO3- fluxes were likely associated with recently added fertilizer N versus older, processed legacy N. We saw distinct annual pulses of NO3- with lower δ15N-NO3 values indicating that a portion of that NO3- was from recent fertilizer applications. While the absolute amount of leached NO3- associated with recent fertilizer applications was similar across all three depths (average range 14-28 kg N ha-1 yr-1), these newer inputs made up the largest proportion (~54%) of leached N at the deepest depth compared to the two shallower depths (~15% each), indicating a role of preferential flow of recently applied fertilizer N from the surface to depth in soils. Further, N that was more associated with recent fertilizer additions appeared to have a lower propensity for retention when compared to remobilized legacy N. Thus, fall and winter precipitation may cause residual fertilizer-associated N within the vadose zone to be pushed to depth, ultimately posing a more immediate threat to groundwater. Optimizing the amount and timing of fertilizer N additions has the potential to improve crop N uptake and reduce NO3- leaching to groundwater.