Abstract
Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO3−) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N2) or the greenhouse gas nitrous oxide (N2O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO3− available for downstream denitrification. Here we use patterns in the isotopic composition of NO3− 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 δ15N and δ18O values of NO3−, as well as increasing δ15N values with decreasing NO3− concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO3− from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO3− mass balance with δ15N 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 N2O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase.
Highlights
Over the last ca. 150 years, anthropogenic fixation of dinitrogen (N2) gas into reactive N (Nr) has increased by an order of magnitude
While the need to reduce NO3− losses is well recognized, human activity in many systems has resulted in large surplus N storage in soils (Sebilo, et al 2013; Van Meter et al 2016), vadose zones (Ascott et al 2017), and groundwater (Vautier et al 2021), suggesting water quality response will likely lag behind any management to reduce N inputs (Ehrhardt et al 2019)
Values for 15NNO3, 18ONO3, and NO3− concentration in high dissolved oxygen (DO) groundwater, sampled in terrace aquifer wells and spring outlets isolated from riparian vegetation, cluster tightly near the center of values for soil, reinforcing findings by Sigler et al (2018) that chemical composition of the Moccasin terrace aquifer is primarily determined by the chemistry of overlying cultivated soils
Summary
Over the last ca. 150 years, anthropogenic fixation of dinitrogen (N2) gas into reactive N (Nr) has increased by an order of magnitude (annual total mass estimates of 15 Tg N in 1860 to 165 Tg N in 2000; Galloway et al 2003). Global-scale fertilization has resulted in a cascade of disruptive side effects on planetary systems (Galloway et al 2003; Erisman et al 2008; Rockstrom et al 2009). Nitrogen fertilizer is typically applied to agricultural soils in a reduced form such as ammonium or urea, which is subject to biogeochemical transformations and loss from soil that compromise the goal of Nr delivery to plants. While the need to reduce NO3− losses is well recognized, human activity in many systems has resulted in large surplus N storage in soils (Sebilo, et al 2013; Van Meter et al 2016), vadose zones (Ascott et al 2017), and groundwater (Vautier et al 2021), suggesting water quality response will likely lag behind any management to reduce N inputs (Ehrhardt et al 2019)
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