Soil pH and precipitation controls on organic carbon retention from organic amendments across soil orders: A meta-analysis

One central goal of global change research is to explore the potential to mitigate rising atmospheric CO₂ by promoting carbon (C) sequestration in terrestrial ecosystems, particularly in low-C agricultural soils in tropical and subtropical regions. Existing evidence suggests that the application of organic amendments is not effective in promoting accrual of soil organic carbon (SOC) in weathered tropical soils like Acrisols, but the specific causes that constrain SOC sequestration are not exactly clear. Here, we synthesized data from 224 publications to assess changes in SOC stocks in response to organic amendments across Acrisols and five other soil orders (Anthrosols, Cambisols, Fluvisols, Luvisols, and Phaeozems). We found that Acrisols, characterized by the lowest soil pH, exhibited the lowest C retention efficiency of organic amendments among the six soil orders, whereas no significant differences were observed among the other five soil orders. Initial soil pH and mean annual precipitation (MAP) were key predictors of SOC retention efficiency, which increased with initial soil pH and decreased with MAP. In addition, low soil pH and high MAP also suppressed microbial growth in response to organic amendments, limiting the retention of mineral-associated organic C (MAOC), which was strongly linked to SOC retention efficiency. Together, these findings suggest that limited SOC accumulation in Acrisols likely results from rapid decomposition and inefficient microbial transformation under acidic and humid conditions. Developing viable practices to improve SOC retention in weathered tropical soils with high acidity should focus on both enhancing the microbial pathway of SOC formation (e.g., through liming) and reducing C decomposition (e.g., through reduced tillage or deep residue incorporation).