Loss of soil organic carbon following natural forest conversion to Chinese fir plantation

Abstract

China manages the largest area of forest plantations on the globe, with most of them converted from natural forests. Establishment and management of forest plantations often involves severe human disturbances such as slash burning and intense site preparation which could lead to rapid and substantial soil organic carbon (SOC) losses and affect long-term SOC recovery. In this study we examined SOC dynamics and soil microbial respiration following the conversion of natural secondary forest to a Chinese fir plantation with slash burning in southern China. SOC decreased by 28% in the first year after forest conversion, with more than 40% of the decrease due to volatilization from slash burning. Slash burning also increased soil microbial respiration in the first five months following forest conversion. SOC of the Chinese fir plantation did not recover to the pre-burning level in 40-years, indicating that the loss of SOC is a long-term phenomenon in forest conversion. We found that soil microbial respiration was largely controlled by photosynthesis in the natural secondary forest; however, both newly formed photosynthates and SOC were important C sources for soil microorganisms in the young Chinese fir plantation. The intensive burning of harvest residue not only induced direct SOC losses through volatilization, but may also have accelerated the decomposition of SOC in the first few years after forest conversion. We conclude that slash burning is the primary, initial pathway by which SOC is lost in these subtropical forest plantations, and that the recovery of SOC in this subtropical forest system will likely be a slow process, requiring centuries or more. Replacing natural forest with Chinese fir plantations using slash burning is likely to substantially deplete C stored in the soil for many years and may negatively affect long-term SOC sequestration potential.