Stability of soil organic carbon during forest conversion is more sensitive in deep soil than in topsoil in subtropical forests

Despite much research, a lot of uncertainty remains regarding the effects of forest conversion to plantation on soil organic carbon (SOC) stabilization, particularly in deep soils. After comparing the SOC content and its distribution in over 200 years old natural broadleaved of forest of Castanopsis carlesii to that in an adjacent 38 years old C. carlesii plantation, we evaluated the effect of land use intensification on soil carbon (C) storage indicators - soil aggregates, density fractions and SOC mineralization rate. The conversion of natural forest to plantation caused divergent, but seemingly progressive responses in the topsoil and deep soil. In the topsoil, SOC stocks were up to 32 % lower following the forest conversion, with a lower labile C pool, whereas the recalcitrance indices in the topsoil of the plantation forest was similar to that in the topsoil of the natural forest. In contrast, in the deep soil, SOC stocks were unaltered, but the recalcitrance indices of SOC decreased by 64 % after forest conversion. The decreased stability of deep SOC was confirmed by the observed decrease in biochemically protected C and increase in specific C mineralization (normalized for soil C content). The decline in biochemically protected C may attribute to greater physical accessibility of organic C to microbes due to soil disturbance induced by forests conversion practices. Consequently, our results supported the view that despite the variable processing rates of C in differently protected pools, all SOC pools are potentially decomposable and dynamic. Physical disturbance appears to be the key factor modifying SOC protection status and long-term stabilization.