Soil autotrophic-to-heterotrophic-respiration ratio and its controlling factors across several terrestrial biomes: A global synthesis

The ratio of soil autotrophic-to-heterotrophic respiration (or R a /R h ) varies as a function of trade-offs between the production (release) of carbon dioxide (CO 2 ) by both (i) plant roots and their symbionts, and (ii) microbeinduced decomposition of litter and soil organic matter. Comprehensive knowledge of this ratio 's global distribution and its biome-specific drivers is notably limited. Based on the newest Global Soil Respiration Database, we conducted a systematic analysis of R a / R h in a global assessment of 404 independent studies, considering both direct and indirect effects of climatic, edaphic and vegetation features on R a / R h . The study proceeded by using machine-learning and conventional techniques, leveraging extreme gradient boosting ( xgboost ) in the isolation of crucial biophysical variables in the calculation of R a / R h and structural equation modeling (SEM) in discerning potential causal relationships among retained variables. Our results showed that the global average R a / R h was 0.66 +/- 0.21 (mean +/- standard deviation), and strongly regulated by mean annual temperature, mean annual precipitation, and soil properties. Variation in soil organic-carbon-to-nitrogen ratios (i.e., C:N) and soil bulk density had distinct influence on root respiration and microbe-induced decomposition. Specifically, higher soil C: N signaled nitrogen scarcity, which suppressed plant growth and, consequently, root respiration ( R a ). Meanwhile, microbes continued to maintain near-unchanged heterotrophic respiration rates ( R h ) by accessing soil organic matter as food substrate. This differential response contributed to the global variations in R a / R h . Vegetation-related variables, such as leaf area index and net primary production, exhibited a biphasic influence on R a / R h , with R a / R h initially declining due to higher aboveground biomass allocation, and subsequently increasing as plants shifted biomass accumulation belowground. Our findings underscored the complex interplay among climatic, soil and vegetation factors in shaping root and microbial activity, providing valuable insights into belowground C dynamics. However, uncertainties remain regarding the differentiation across various regions and seasons.