Associate Professor University of Illinois at Urbana-Champaign, Illinois, United States
Abstract: Nitrous oxide emissions are the single largest source of greenhouse gas (GHG) emissions from U.S. agricultural activities. Yet, field-scale GHG emissions from agriculture are poorly constrained due to the high spatial and temporal variability of N2O emissions. We generated an unprecedented high spatiotemporal resolution dataset of nitrous oxide (N2O) and carbon dioxide (CO2) fluxes, with potential environmental and edaphic drivers, to further our understanding of the spatiotemporal variation and control of field-scale GHG emissions. In June 2021, we installed 20 soil flux autochambers paired with soil sensors for moisture, temperature, and oxygen in a 4 ha section of 32 ha corn-soybean rotation system planted in corn in central, IL, USA. Soil CO2 and N2O fluxes along with soil sensors were measured hourly from June 2021 until April 2022.
Soil CO2 fluxes were less spatiotemporally variable and could be better predicted than soil N2O fluxes. Soil CO2 flux varied by 2x among chambers and showed no distinct hot moment events. In contrast, soil N2O flux varied by 5x among chambers, with half of cumulative N2O emissions across the field from the “hottest” 25% of chambers. The three largest hot moments accounted for 58% of the cumulative N2O flux. Path analyses using environmental and edaphic predictors explained much of the variation in soil CO2 flux (R2=0.843), with soil temperature as the most important predictor. The drivers measured were only weak predictors of N2O flux over the 10-month measurement period (R2=0.069), likely resulting from seasonally distinct drivers, and lag and threshold effects within drivers (e.g. N2O flux increases on a lag only after rain events large enough to saturate microsites). We propose a conceptual model for denitrification-driven N2O fluxes that considers the threshold drivers (i.e. nitrate, dissolved organic carbon, and anoxic microsites) of hot spots and hot moments, and we apply this framework in order to better understand seasonal triggers of N2O hot moments. For example, the mid-summer hot moment (13% of cumulative flux) occurred after a large rain event. The post-harvest hot moment occurred (24% of cumulative flux) as fall fertilization replenished soil nitrate. The winter hot moment (20% of cumulative flux) resulted from a large thaw event. Flux variation among chambers within each hot moment occurred due to spatial variation in the other drivers controlling N2O flux. In short, this research provides critical insight into the multiple, seasonally distinct threshold controls of N2O emissions using an unprecedented high spatiotemporal resolution data.