Grasslands are important for C storage and supporting livestock production, yet little is understood about how warming interacts with defoliation to alter soil C and N dynamics. We examined how increased growing season temperature and defoliation influenced soil microbial biomass C (MBC) and N (MBN), N mineral dynamics, and greenhouse gas (GHG) flux within the top 15 cm of mineral soil in a northern temperate grassland of Alberta, Canada. We performed a 2-yr experiment with open-top chambers to increase soil temperatures, with warming assessed alone and combined with a single intense midsummer defoliation treatment. Soil MBC and MBN did not respond to warming or defoliation (P > 0.05). Warming increased soil nitrification and net N mineralization in the spring of the second growing season (P < 0.05) but did not persist beyond May. Although warming and defoliation both increased CO2 efflux separately during the first growing season (P < 0.05), the combination of these treatments reduced CO2 efflux to levels similar to the controls, with no lasting effects into the following growing season. Warming also interacted with defoliation to increase N2O flux in the second growing season relative to plots under either warming or defoliation treatment, whereas CH4 uptake was not influenced by our treatments. These results show the complex but transient impacts of warming and defoliation on soil C and N, particularly GHGs. Further research is needed to understand how these northern temperate grasslands respond to long-term factorial experiments involving warming, defoliation, and variation in other factors (e.g., precipitation).
