Soil pH Modulates the Global Stabilized Soil Organic Carbon

Soil mineral-associated organic carbon (MAOC) constitutes over 50% of total soil organic carbon (SOC) and serves as a key determinant of its long-term stabilization. Although soil pH is recognized as a "master variable" in biogeochemical cycles, it remains unclear how soil pH influences MAOC and SOC at large spatial scales. To address this knowledge gap, we conducted a continental-scale survey in China and synthesized global observations from literature, totaling 1300 independent observations. We analyzed the relationships between SOC and MAOC across pH gradients and assessed the contributions of other environmental factors to MAOC variability. Our results showed that higher SOC content is generally positively correlated with higher MAOC. However, this relationship is strongly influenced by soil mineralogy, texture, and climatic conditions. We identified distinct pH-dependent pathways controlling MAOC formation. In acidic soils (pH ≤ 5.5), Fe/Al (hydr)oxides explain most of the variation in MAOC content and in MAOC fraction (the ratio of MAOC to SOC). In neutral-to-alkaline soils (pH > 6.5), however, clay content and mean annual temperature emerge as the primary regulators. Net primary productivity (NPP) augments the particulate organic carbon (POC) pool, thereby diluting the MAOC fraction. This divergence challenges the universal applicability of clay-centric SOC models in acidic soils. Our findings provide a conceptual framework for biogeochemical models, urging the integration of pH-dependent metal oxide dynamics to improve predictions of SOC persistence under diverse future scenarios.