Modeling indirect N2O emissions along the N cascade from cropland soils to rivers

Abstract

The frequently observed discrepancy between estimations of N2O emissions at regional or global scale based either on field data or inventories (bottom-up) or on direct atmospheric observations (top-down) suggests that riparian areas and river surfaces play a significant role as hot spots of emission. We developed a modeling procedure to assess N2O emissions occurring during the transfer of water masses from the subroot water pool of the watershed to the outlet of the river drainage network, including their passage through riparian wetlands. The model was applied to three river basins of increasing size located in the sedimentary geological area of the Paris basin (France) and validated by its capability to predict river N2O concentrations and fluxes across the river–atmosphere interface. At the scale of the Seine watershed, indirect emissions, i.e. emissions linked to agricultural practices but occurring elsewhere than directly at the field plot, are estimated to represent approximately 20% of the direct emissions from the watershed soils, in good agreement with previous estimates based on empirical accounting approaches. Denitrification in riparian zones is responsible for the largest share of these indirect emissions. The model results are very sensitive to the value of the ratio of N2O versus (N2 + N2O), in the final products of denitrification in rivers and wetlands. By calibration on river N2O concentrations, a value of 0.015 ± 0.05 is proposed for this ratio, in agreement with recent studies. This represents the main uncertainty factor of the model. In basins with conditions prone to increasing the value of this ratio, higher proportions of indirect N2O emissions might possibly be observed.

Isotopic signals in an agricultural watershed suggest denitrification is locally intensive in riparian areas but extensive in upland soils

“…The isotopic composition of NO 3 − at these sites suggests that denitrification is occurring in both riparian corridors (which include soils, shallow subsurface flow and side channels, ponds and impoundments) and cultivated soils of our study area, a finding widely supported in the literature (Hedin et al 1998;Bouwman et al 2013;Billen et al 2020). Riparian areas had the strongest isotopic signals of denitrification, with some riparian groundwater hotspots suggesting near complete NO 3 − removal.…”

“…These findings support both calls to bolster strategic management of riparian corridors and wetlands (Cheng et al 2020) and to allocate more attention to N management in the soils of uplands and headwater catchments (Pinay and Haycock 2019). Loss of N r from cultivated soils via denitrification and leaching not only limits agricultural productivity but also has contributed to a ten-fold increase in N 2 O emissions from croplands since the 1860s (Stevens and Laughlin 1998;Tian et al 2019;Billen et al 2020). Results from our work suggest that fallow periods play a disproportionately important role in N r loss from cropland soils both directly to denitrification, and to leaching (Sigler et al 2020) of NO 3 − that is subject to denitrification down gradient.…”

“…Microbial denitrification is the primary means by which N r is returned to the non-reactive atmospheric N 2 pool, but this process can also produce the greenhouse gas N 2 O (ratio for major river basins in France = 0.015 ± 0.05; Billen et al 2020). Soils and freshwater environments contribute about equally to the 40% of global denitrification that is non-oceanic (groundwater, lakes, rivers; Seitzinger et al 2006).…”

Nitrogen biogeochemistry of water-agro-food systems: the example of the Seine land-to-sea continuum

“…First because N 2 O is a potent greenhouse gas so that the use of denitrification as a way to eliminate nitrate from seepage or wastewater is potentially harmful for the climate, second, because as N 2 O emission is more easy to measure than N 2 production, the former is often used as an indirect way to evaluate the latter. Rochester, 2003;Hénault et al, 2005Hénault et al, , 2020. The observed effect of these factors may simply reflect the transient perturbation linked to an enzymatic or population readjustment to new conditions imposed by the experimental protocol (Simek et al, 2002a,b).…”

“…The soil denitrification potential is expressed there by its capacity to reduce the nitrate loading of the water flow, before it reaches the river itself. Based on this conceptual scheme and on a basin-wide delimitation of riparian zones resting on land use and on topographic indexes (Berthier et al, 2014), Billen et al (2018bBillen et al ( , 2020 modeled nitrate and N 2 O dynamics associated with these riparian processes (Fig. 6), and provided an estimate of their significance at the local and basin scales (Fig.…”