Whole-Profile Soil Carbon Responses to Concurrent Warming and Precipitation Changes Across Global Biomes

The joint effects of simultaneous warming and precipitation shifts on soil organic carbon (SOC)-the largest terrestrial carbon pool-remain poorly understood across large spatial extents. By evaluating a global dataset of SOC measurements in the top meter of soil through a space-for-change substitution approach, we show that, averaging across the globe, increased precipitation compensates for warming-induced SOC reductions regardless of soil depth and vice versa. Although additive effects between these two factors are predominant, negative interactive effects, which exacerbate SOC losses, are also common, particularly in tropical and subtropical grasslands/savannas and Mediterranean/montane shrublands. SOC responses vary widely across the globe, primarily correlated to baseline SOC content and local climatic conditions. Notably, SOC responses in tundra systems are opposite the responses in other ecosystems, showing positive and negative responses to warming and precipitation increases, respectively. Under a scenario of 2°C air warming with projected precipitation changes, global SOC stocks in the 0-1 m depth are projected to decrease by 13.1% ± 6.6% (mean ± 95% confidence interval, or 351 ± 100 Pg C). These results demonstrate that accurately predicting SOC dynamics under climate change necessitates explicit consideration of local climatic conditions and existing SOC content in relation to concurrent precipitation shifts and warming.