Microbe-mediated attenuation of soil respiration in response to soil warming in a temperate oak forest

Soil respiration (Rs) in response to climate warming received a wide concern due to its important role in terrestrial ecosystem carbon (C) cycling, but the warming-induced effects of soil microbes on soil respiration are still less understood, especially over time. Our study aims to understand the long-term warming induced effects of soil microbes on Rs. A field soil warming experiment using a completely randomized design was conducted in a naturally regenerated oak forest (Quercus aliena) in central China. Soil warming was executed by infrared heater throughout the period from 2011 to 2015. Our results showed that soil temperature was a main factor in regulating Rs in a temperate oak forest throughout the experiment, while soil water content determined Rs only when a naturally dry year occurred. The positive effect of soil warming on Rs that was observed (i.e., 37.5 to 42.0% in the first two years) gradually diminished in the following three years (i.e., 0.9 to 15.4%). Significant positive warming effects on the temperature sensitivity of Rs (Q₁₀) only occurred in the second year. Continuous soil warming caused the decline in nitrogen (N) availability, with a significant increase in microbial biomass-specific enzyme activities for N-acquisition. The attenuation of microbial biomass increment and the decreased ratio of enzymatic C:N acquisition contributed to the diminished warming effect on Rs over time. Our study suggests that microbe-mediated attenuation of Rs, accompanied by the concomitant decline in soil N availability in response to warming, should be taken into consideration in global C cycle modeling.