Experimental Warming Effects on Soil Respiration, Microbial Abundance, and Extracellular Enzyme Activities in a Switchgrass Cropland in Middle Tennessee

Global warming is projected to accelerate soil carbon (C) loss to the atmosphere. However, soil CO₂ emissions under warming and the underlying microbial processes are not adequately studied in bioenergy croplands. To address this issue, a soil warming experiment was established in a switchgrass cropland at Tennessee State University in May 2021. Four paired plots with infrared and dummy heaters (i.e., warming vs. control plots) were randomly installed in four blocks. Collections of hourly soil heterotrophic respiration (R_s), temperature, and moisture at surface soil (0–10 cm), as well as biweekly soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon, and nitrogen (MBC and MBN), and extracellular enzyme activities (EEAs) were conducted consecutively for 2 years. Warming elevated soil temperature by 2.2°C, reduced volumetric water content by 17.5%, and significantly increased hourly R_s but had no significant effects on the contents of SOC, TN, MBC, MBN, and soil EEAs. Despite the insensitive responses of soil microbial, enzymatic, and bulk features, the elevated R_s was closely associated with warming-caused changes in soil temperature and moisture. Overall, the elevated R_s in response to 2-year experimental warming informed a likely positive response of switchgrass soil CO₂ emission to a warmer future and a shift toward increased autotrophic respiration. The current study implied the importance of long-term experimental observations to accurately predict soil respiratory responses in switchgrass croplands.