Effects of elevated CO₂ and warming on soil enzyme activity and temperature sensitivity of millet

Study on the response of soil enzyme activity and temperature sensitivity to elevated CO₂ concentration and warming at important growth stages of crops is of great significance to evaluate the impact of climate change on crop production and the functional stability of soil ecosystem. In view of this, the soil of millet (Setaria italica) at grain filling stage was selected as the research object for pot experiments, and three climate scenarios were designed using an artificial climate chamber as follows: elevated CO₂ concentration (700 μmol∙mol⁻¹ CO₂ concentration and 22 ℃ ambient temperature, EC), elevated CO₂ concentration and warming (700 μmol·mol⁻¹ CO₂ concentration and 26 ℃ ambient temperature, EC+T), and control (400 μmol·mol⁻¹ CO₂ concentration and 22 ℃ ambient temperature, CK). Two water conditions were set for each climate scenario as follows: adequate water supply (70% field capacity) and mild drought stress (50% field capacity). The responses of the activities and temperature sensitivity of β-glucosidase (βG), β-N-acetyl glucosidase (NAG), cellulase (CBH), and β-xylosidase (βX) to elevated CO₂ concentration and warming were analyzed. The results showed that the activities of βG, NAG, CBH, and βX in the control group (CK) first increased and then decreased with an increase in incubation temperature under the condition of sufficient water supply, and the temperature at which enzyme activities were the highest was 25 ℃. At the optimum temperature (25 ℃), elevated CO₂ concentration significantly decreased soil βG activity but had little inhibitory effect on the activities of soil NAG, CBH, and βX. The effect of the interaction between elevated CO₂ concentration and warming was related to water conditions; specifically, the enzyme activities were inhibited under mild drought condition but no significant difference was observed under adequate water supply. In addition, elevated CO₂ concentrations and warming significantly affected the temperature sensitivity (Q₁₀) of soil enzyme activity at millet grain filling stage. The elevated CO₂ concentration significantly increased Q₁₀, whereas warming decreased Q₁₀. Under sufficient water supply, warming counteracted the effect of elevated CO₂ concentration on Q₁₀, and the interaction between elevated CO₂ concentration and warming had no significant effect on Q₁₀. However, elevated CO₂ concentration and warming had significant effects on Q₁₀ under mild drought condition as follows: the interaction of CO₂ concentration, temperature, and water content on Q₁₀ was significant. In addition, the interaction between elevated CO₂ concentration and mild drought had a significant effect on Q₁₀, but there was no significant difference in the effect of elevated CO₂ concentration and the interaction of the three factors. Moreover, redundancy analysis showed that Q₁₀ was affected by environmental variables such as microbial biomass and soil nutrients. This study demonstrated that the effects of elevated CO₂ concentration, warming, drought, and their interactions on soil enzyme activities and temperature sensitivity were complex and particularly, the elevated CO₂ concentration inhibited the temperature sensitivity of soil enzymes and weakened the metabolic functions and stability of enzymes related to soil carbon and nitrogen cycling, which further affected the functional stability of the soil ecosystem.