Left in the cold: Changes in snow cover alter nitrogen cycling in agricultural soils
Agricultural ecosystems contribute the majority of global anthropogenic emissions of the greenhouse gas nitrous oxide (N2O). Up to 80% of annual N2O fluxes occur during the non-growing season, often as pulses during midwinter and spring thaws. In the northeastern United States and eastern Canada, climate change may increase average winter temperatures and annual precipitation, with effects on agricultural N2O fluxes. Warmer temperatures may reduce snow cover, exposing soils to freezing air, whereas increased snowfall would maintain soils closer to 0°C. Although warmer soils might cause greater microbial activity and fluxes during winter, colder soils may have high fluxes during thawing, as frost disrupts soil particles and releases carbon that can fuel denitrification and thus N2O production. Deeper snow also increases water input to soils during thawing, facilitating denitrification via soil anoxia. To better assess these effects in humid temperate agricultural ecosystems, we conducted a two-year field study with snow removal (Removal), passive accumulation (Accumulation) and ambient (Ambient) treatments. To examine in situ dynamics, we measured greenhouse gas (N2O and CO2) fluxes using static chambers, and belowground gas accumulation using soil gas wells. These measurements were coupled with assays of denitrification, potential nitrification, and soil nutrient concentrations (NO3-, NH4+ and NO2-).
Results from the first winter indicate that snow cover affected the timing and magnitude of N2O production and fluxes. In December and January, N2O fluxes ceased from Removal plots but were consistent in snow-covered plots, likely due to warmer soils. Potential nitrification rates were lowest in these months, but showed no treatment effect. In February and March, gaseous fluxes were negligible in all treatments, as emissions were blocked by ice on the soil surface and in the snowpack. During this time, denitrification rates in surface soil (0-7.5 cm) were higher than in fall or spring, and N2O accumulated at 15 and 30 cm depth in all treatments, with greater concentrations in Removal than Ambient plots. As snow melted and soils warmed in April, potential nitrification increased, and N2O fluxes occurred from all plots. However, fluxes from Removal plots were up to an order of magnitude greater, and began earlier, than snow-covered plots. Taken together, results indicate that snow removal increased N2O production from midwinter onward, possibly because of soil freezing to lower temperatures, which may have increased soil carbon availability. In future winters with decreased snow, colder soils could cause a positive climate change feedback via greater N2O production.