The limited effect of deforestation on stabilized subsoil organic carbon in a subtropical catchment

Abstract. Predicting the quantity of soil organic carbon (SOC) requires understanding about how different factors control the amount of SOC. Land use has a major influence on the function of the soil as a carbon sink, as shown by substantial organic carbon (OC) losses from the soil upon deforestation. Yet, predicting the degree to which land use change affects the SOC content, and the depth down to which this occurs, requires context-specific information related to, for example, climate, geochemistry, and land use history. In this study, soil samples collected down to 300 cm depth from forests and agricultural fields in a subtropical region (Arvorhezina, southern Brazil) were used to study the impact of land use on the amount of stabilized OC along the soil profile. We found that the stabilized SOC content was not affected by land use below a depth of 90 cm. Along the soil profile, the amount of stabilized OC was predominantly controlled by land use and depth, in addition to the silt and clay content, and aluminum ion concentrations. Below 100 cm, none of the soil profiles reached a concentration of stabilized SOC above 50 % of stabilized SOC saturation point (i.e., the maximum OC concentration that can physically be stabilized in these soils). Based on these results, we argue that it is unlikely that deeper soil layers can serve as an OC sink over a time scale relevant to global climate change, due to limited OC input in these depth layers. Furthermore, we found that soil weathering degree was not a relevant control on the amount of stabilized SOC in the profiles we investigated, because of the high weathering degree of the studied soils. It is therefore suggested that while the soil weathering degree might be an effective controlling factor of OC stabilization over large spatial scale, it is not an informative measure for this process at the scale of the soil profile in highly weathered soils.