Global patterns and drivers of soil dissolved organic carbon concentrations

Dissolved organic carbon (DOC) constitutes the most active carbon pool in soils and plays critical roles in soil carbon cycling, plant productivity, and global climate change. Accurately assessing soil DOC quantity is essential to elucidate ecosystem functions and services. However, global driving factors and the spatial distribution of soil DOC remain poorly quantified, largely due to limited large-scale data. Here, we compile a comprehensive global database of soil DOC concentrations, encompassing 12,807 observations extracted from 975 scientific publications published between 1984 and 2020. We also record detailed geographic locations, climatic variables, and soil properties as predictors. Machine learning techniques were employed, including 10-fold cross-validation and evaluating model performance by R-squared and root-mean-square error, to predict the relative importance of various predictors and the global distribution of soil DOC concentrations. Worldwide soil DOC concentrations ranged from 0.04 to 7859 mg kg-1, averaging 222.78 mg kg-1. The 14 selected predictors, including elevation, soil properties, and climate, explained 63 percent of the variance in soil DOC concentrations. Elevation played the most important predictor for soil DOC prediction, followed by soil organic carbon, seasonal variability of temperature, and soil clay. Soil DOC decreases initially but increases when soil clay exceeds 20% and seasonal variability of temperature exceeds 0.7. Using these findings, a global map of predicted soil DOC concentrations was produced at a 0.05° by 0.05° resolution. Global soil DOC concentrations generally increased from the equator to the poles, and the topsoil layer (0-30 cm) holed 13.47 Pg of soil DOC, with substantial variations across continents. These results informed soil management practices strategies, ecosystem services evaluations, and climate change mitigation efforts. Furthermore, we envisioned integrating our database with other carbon pools to advance understanding of total soil carbon turnover and to refine Earth system models.