A synthesis of change in deep soil organic carbon stores with afforestation of agricultural soils

Abstract

Afforestation is known as an available mitigation activity to climate change because it causes sequestration of CO2 from the atmosphere and stores it as the living biomass and the dead organic matter. However, the response of soil organic carbon (SOC) to afforestation in deep soil layers is still poorly understood. We surveyed previously published literature for changes in deep SOC (defined as at least 10cm deeper than the 0–10cm layer) after afforestation of croplands and grasslands (total 63 sites from 56 literature), in order to examine changes in deep SOC and quantify the relationship between SOC change rates in topsoil and subsoil. The results of the meta analysis indicated that the responses of SOC to afforestation were opposite for cropland than grassland. The SOC in soil depth layers of 0–10, 10–20, 20–40, 40–60 and 60–80cm were reduced with afforestation of grassland but not significantly (p>0.05), while conversion of cropland to forests (trees or shrubs) increased SOC significantly for each soil depth layer up to 60cm depth (p<0.05). Significant relationships of SOC change rate were found between topsoil (0–20cm) and deeper soil layers (20–40 and 40–60cm). The linear regression showed that SOC change rate in 0–40cm, 0–60cm, and 0–100cm soil profiles was 1.33, 1.49, and 1.55 times greater, respectively than the change rates in the corresponding 0–20cm depth profile. Partial correlation analysis revealed that stand age and initial SOC content were determinants of deep soil SOC change after afforestation of agricultural soils. This study also showed that the O horizon can play an important role in carbon sequestration after afforestation of agricultural sites. We concluded that subsoil carbon must be taken into account when evaluating of SOC change with afforestation and, therefore, recommended that the soil sampling depth for afforested soils be set to at least 60cm in mineral soils and include the O horizon. However, due to poor study designs and lack of standardized sampling protocols in the literature, these results were high in uncertainty.