The optimum temperature of soil microbial respiration: Patterns and controls

The temperature response of soil microbial respiration (Rh) is of significance, with the optimum temperature of Rh being the key parameter for accurately modeling how it responds to temperature change under climate warming scenarios. However, knowledge about Topt in natural ecosystems remains limited, especially at large scales, which increases the uncertainty of climate projections. Here, we collected 25 soils from tropical to cold-temperate forests in the northern hemisphere to quantify regional variation in Topt and the controls underlying this variation. Rh was measured at high frequency using a novel system under the mode, with temperature gradually increasing from 5 to 50 °C. The results showed that Topt ranged from 38.5 to 46.0 °C (mean: 42.4 °C). Of note, this study is the first to demonstrate that Topt is far higher than the assumed value used in models (35 °C), varying greatly across different climatic zones and increasing with latitude from tropical to cold-temperate forest soils. To some extent, our results supported the substrate supply hypothesis, and contrast with the climate adaption hypothesis. In addition, climate, nutrient, and soil microorganisms jointly regulate regional variation in Topt, together explaining 53% of variation in Topt. The higher Topt in northern regions indicated that these regions have a greater potential to release more CO2 from soil, which might lead to a positive feedback to global warming. In conclusion, process-based models should incorporate the high variability of Topt across regions to improve predictions of the carbon dynamics of terrestrial ecosystems under climate warming scenarios.