Soil carbon stock changes in tropical croplands are mainly driven by carbon inputs : a synthesis

Soil organic carbon (SOC) balance is an important component of the terrestrial carbon (C) budget. However, effect of cropland management changes on SOC dynamics has not been recently assessed in the tropics. Studies were compiled in the tropics where SOC stocks were measured in the topsoil (0-20 or 0-30 cm depth) after the adoption of management practices that are expected to enhance SOC stocks, including tillage reduction, crop rotation, exogenous organic amendments, restitution of crop residues, mineral amendments, and combinations of these practices. Random forest regression was used to identify the determinants of SOC accumulation rates (ASOC) depending on the climate, soil characteristics and changes in management practices. 214 cases were identified in 48 studies in 13 different countries. The average ASOC was 0.41 +/- 0.03 Mg C ha(-1) yr(-1) (significantly greater than zero), for an average experiment duration of 13.6 +/- 0.6 years. Although a large part of the variability remained unexplained due to methodological bias in the studies or a lack of relevant predictors. The strongest predictors of ASOC were C inputs, duration of the experiments, and the management practices, whereas neither soil characteristics (soil type, clay content, and initial SOC stock) nor climate variables (mean annual temperature and rainfall, aridity index) affected ASOC. The SOC accumulation rates increased linearly with C inputs, and the conversion rate of C inputs to SOC was 8.2 +/- 0.8%. Given the competing uses of organic matter on many tropical farms, the benefits of using changes in management practices for climate change mitigation might be overrated. As ASOC decreased with the duration of the experiments, ASOC would probably be smaller if a period of 20 years were considered, as recommended by the IPCC guidelines. The management practice with the greatest ASOC was diversified crop rotation. Cropping systems where diverse practices were combined resulted in higher ASOC than individual practices such as reduced tillage and mineral fertilization on their own. The adoption of improved management practices that increase C inputs is still relevant for meeting the challenges of food security and adaptation to climate change