A conceptual model explaining spatial variation in soil nitrous oxide emissions inagricultural fields

Soil emissions of nitrous oxide contribute substantially to global warming from agriculture. Our understanding of soil nitrous oxide (N₂O) emissions and its controlling factors in the environment is challenged by high temporal and spatial heterogeneity in emissions, which leads to uncertainty in the benefits of climate-smart agricultural practices. Here, we present a conceptual model explaining spatial variation in temporal patterns of soil nitrous oxide emissions developed from high spatial resolution measurements of soil nitrous oxide emissions, gross nitrous oxide fluxes, and soil physicochemical properties in two maize fields in Illinois, USA. In sub-field locations with consistently low nitrous oxide emissions, soil nitrate and dissolved organic carbon (DOC) constrained nitrous oxide production irrespective of changes in soil moisture. In sub-field locations where high emissions occurred episodically, soil nitrate and dissolved organic carbon availability were higher, and increases in soil moisture stimulated nitrous oxide production. These findings form the ‘cannon model’ which conceptualizes how sub-field scale variation in soil nitrate and DOC determines where increases in soil moisture can trigger high soil nitrous oxide emissions within agricultural fields. Understanding the drivers of spatial variation in soil nitrate and DOC is therefore the key to developing precision agricultural practices (e.g., variable fertilizer application rates within fields) that target reductions in N₂O emissions from hot spots that disproportionately contribute to field-scale N₂O budgets. This also suggests another way in which climate-smart agricultural practices aimed at increasing soil organic carbon may inadvertently increase soil N₂O emissions, by increasing DOC and soil nitrate derived from soil organic matter to turn cold spots into hot spots. Overall, this conceptual breakthrough in understanding controls on spatial variation in soil N₂O emissions holds promise for guiding future efforts to reduce uncertainty in and effectively mitigate agricultural soil N₂O emissions.