Freeze‐Thaw Events and Nitric Oxide, Nitrous Oxide, and Carbon Dioxide Fluxes in a U.S. Corn Belt Soil

Fluxes of carbon dioxide (CO 2 ), nitrous oxide (N 2 O) and nitric oxide (NO) from agricultural soils to the atmosphere drive climate warming and affect atmospheric chemistry. We conducted laboratory experiments to assess the effects of varied intensity and frequency of freezing on these fluxes from soils cropped with maize in Nebraska, U.S. We evaluated the effects of variations in freezing temperature, freezing degree days (FDDs: the sum of the number of degrees below zero Celsius for each respective day below zero, for example, 12 days at −2°C is equal to 24 FDDs), and freeze‐thaw cycling. Soil cores were incubated in gas‐tight chambers, and fluxes of N 2 O and CO 2 were directly measured by gas chromatography, while NO fluxes were measured with a chemiluminescence detector. We made concurrent measurements of potential net N mineralization, net nitrification, microbial biomass, and denitrification enzyme activity. NO was the most responsive to freezing treatments, with some treatments markedly stimulating emission (75FDD, 15FDD) and one treatment (15FDD Fast) significantly reducing emission compared to the control. Effects on N 2 O and CO 2 were less marked than effects on NO, but there was significant stimulation of both these fluxes in the most extreme freezing treatment (75FDD) and significant reductions in flux in the 15FDD Fast treatment at several sampling dates. Our results suggest that severe freezing events influence the flux of important trace gases. These events, and how changes in winter climate affect them, may need to be integrated into climate models given their implications for atmospheric chemistry.