Vadose Zone Journal (May 2024)

Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate

  • Julie N. Weitzman,
  • J. Renée Brooks,
  • Jana E. Compton,
  • Barton R. Faulkner,
  • R. Edward Peachey,
  • William D. Rugh,
  • Robert A. Coulombe,
  • Blake Hatteberg,
  • Stephen R. Hutchins

DOI
https://doi.org/10.1002/vzj2.20324
Journal volume & issue
Vol. 23, no. 3
pp. n/a – n/a

Abstract

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Abstract A substantial fraction of nitrogen (N) fertilizer applied in agricultural systems is not incorporated into crops and moves below the rooting zone as nitrate (NO3−). Understanding mechanisms for soil N retention below the rooting zone and leaching to groundwater is essential for our ability to track the fate of added N. We used dual stable isotopes of nitrate (δ15N–NO3− and δ18O–NO3−) and water (δ18O–H2O and δ2H–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 2016 to 2020 in the southern Willamette Valley, Oregon, USA. Distinct periods of high nitrate concentrations with lower δ15N–NO3− values indicated that a portion of that nitrate was from recent fertilizer applications. We used a mixing model to quantify nitrate fluxes associated with recently added fertilizer N versus older, legacy soil N during these “fertilizer signal periods.” Nitrate leached below 3.0 m in these periods made up a larger proportion of the total N leached at that depth (∼52%) versus the two shallower depths (∼13%–16%), indicating preferential movement of recently applied fertilizer N through the deep soil into groundwater. Further, N associated with recent fertilizer additions leached more easily when compared to remobilized legacy N. A high volume of fall and winter precipitation may push residual fertilizer N to depth, potentially posing a larger threat to groundwater than legacy N. Optimizing fertilizer N additions could minimize fertilizer losses and reduce nitrate leaching to groundwater.