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Abstract Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO 3 − ) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N 2 ) or the greenhouse gas nitrous oxide (N 2 O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO 3 − available for downstream denitrification. Here we use patterns in the isotopic composition of NO 3 − observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ 15 N and δ 18 O values of NO 3 − , as well as increasing δ 15 N values with decreasing NO 3 − concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO 3 − from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO 3 − mass balance with δ 15 N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N 2 O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase.
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Simulating agroecosystem soil inorganic nitrogen dynamics under long-term management with an improved SWAT-C model
Despite the extensive application of the Soil and Water Assessment Tool (SWAT) for water quality modeling, its ability to simulate soil inorganic nitrogen (SIN) dynamics in agricultural landscapes has not been directly verified. Here, we improved and evaluated the SWAT–Carbon (SWAT-C) model for simulating long-term (1984–2020) dynamics of SIN for 40 cropping system treatments in the U.S. Midwest. We added one new nitrification and two new denitrification algorithms to the default SWAT version, resulting in six combinations of nitrification and denitrification options with varying performance in simulating SIN. The combination of the existing nitrification method in SWAT and the second newly added denitrification method performed the best, achieving R, NSE, PBIAS, and RMSE of 0.63, 0.29, −4.7 %, and 16.0 kg N ha−1, respectively. This represents a significant improvement compared to the existing methods. In general, the revised SWAT-C model's performance was comparable to or better than other agroecosystem models tested in previous studies for assessing the availability of SIN for plant growth in different cropping systems. Sensitivity analysis showed that parameters controlling soil organic matter decomposition, nitrification, and denitrification were most sensitive for SIN simulation. Using SWAT-C for improved prediction of plant-available SIN is expected to better inform agroecosystem management decisions to ensure crop productivity while minimizing the negative environmental impacts caused by fertilizer application.
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- PAR ID:
- 10483327
- Publisher / Repository:
- Science of The Total Environment
- Date Published:
- Journal Name:
- Science of The Total Environment
- Volume:
- 879
- Issue:
- C
- ISSN:
- 0048-9697
- Page Range / eLocation ID:
- 162906
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.scitotenv.2023.162906
