Seasonal and Interannual Variability of Soil Heterotrophic Respiration and Autotrophic Respiration in Typical Grassland of Inner Mongolia

Considerable differences exist between soil heterotrophic respiration and autotrophic respiration in terms of variation and regulating factors. However, there is still a lack of evaluation of the differences between the two components in grasslands of arid and semi-arid regions under a changing climate. We developed a semi-process model separately for heterotrophic respiration and autotrophic respiration for the Inner Mongolian grassland of China to estimate their rates over seasonal and interannual cycles from 2002 to 2021. The models were calibrated with site-specific parameters and validated against independent field measurements. Our model results show stronger seasonal and interannual variability in autotrophic respiration than in heterotrophic respiration over 2002–2021. There was a significant increasing trend in most seasons at all three sites for autotrophic respiration (with the greatest slope in summer, i.e., 2.12–3.97 g C m⁻² yr⁻²), but only in winter for heterotrophic respiration at the southern typical grassland site (0.27 g C m⁻² yr⁻²). Soil temperature was responsible for the seasonal and interannual variability in heterotrophic respiration but plant growth responsible for the temporal variations of autotrophic respiration. Our results highlight that distinguishing between soil heterotrophic respiration and autotrophic respiration is of importance not only for better understanding of the carbon cycle in the grasslands but also for predicting how soil respiration responds to climate change in the future.