Changes in soil carbon stocks across the Forest-Agroforest-Agriculture/Pasture continuum in various agroecological regions: A meta-analysis

The contribution of agroforestry systems (AFS) to enhance soil organic carbon (SOC) storage in soil layers due to the presence of deep tree roots are of interest in the context of promoting carbon sinks and greenhouse gas mitigation. To quantify the relative soil C contribution from trees in agroforestry systems (AFS), this study assessed the reported differences in SOC stocks under agroforestry systems in comparison with other land-use systems (Agriculture, Forestry, Pasture, or Uncultivated Land) in various soil-depth classes in four agroecological regions (arid and semiarid, ASA; lowland humid tropics, LHT; Mediterranean, MED; and temperate, TEM) around the world. Using mixed-effect models and a meta-analytical approach, we synthesized data from 78 peer-reviewed studies that generated 858 data points (sites) on SOC stock under various AFS practiced globally. Comparing Agroforest vs. Agriculture or Agroforest vs. Pasture, SOC stocks under AFS were higher by + 27% in the ASA region, + 26% in LHT, and + 5.8% in TEM, but -5.3% in the TEM in the 0-100 cm soil depth. The Agroforest aged between 10-20 years had higher SOC stock than newly established, as well as < 10-year-old systems across all soil-depth classes and agroecological regions. Overall, Agroforest vs. Agriculture land management practices resulted in positive SOC stock changes within AFS up to 100 cm soil depth, whereas SOC stock under AFS was lower than under Forest. The results suggest that a general pattern of Forest - Agroforest - Agriculture - Pasture continuum could be expected in SOC stock decline during land-use changes. Improvement of SOC stocks under AFS varied across agroecological regions, the highest being under arid and semiarid region, closely followed by the low humid tropics. The important outcome of this meta-analysis is worthy of emphasizing of the role of AFS in climate change adaptation and greenhouse gas mitigation strategies by promoting carbon sinks.