The ecosystem carbon accumulation after conversion of grasslands to pine plantations in subtropical red soil of South China

Since 1980s, afforestation in China has led to the establishment of over 0.53 × 10⁸ ha of new plantation forests. While this leads to rapid accumulation of carbon (C) in vegetation, the effects of afforestation on soil C are poorly understood. In this study, a newversion of the Atmosphere-Vegetation Interaction Model (AVIM2) was used to examine howchanges in plant C inputs following afforestation might lead to changes in soil C at one of the Chinaflux sites and to estimate the effect of afforestation on ex-grassland. The potential total C accumulation of tree plantation was also predicted. The model was calibrated by net ecosystem exchange (NEE), ecosystem respiration (RE) and gross primary production (GPP) based on eddy-covariance measurements. The simulated vegetation C and soil C stocks were compared with the filed observations.

The simulates indicate that after 22 yr of conversion of grassland to needle leaf forests (Pinus massoniana and Pinus elliottii), the net carbon accumulation in tree ecosystem was 1.96 times more than that in grassland. The soil C in the initial 7 yr of planting decreased at a rate of 0.1871 kg C m^-2 yr^-1, and after that it increased at a rate of 0.090 kg C m^-2 yr^-1. The C accumulation in the studied plantation ecosystem is estimated to be 76–81% of that value in equilibrium state (the net ecosystem productivity approaches to zero).

Sensitivity analyses show that conversion from grassland to plantation caused an initial (7 or 8 yr) periods of decrease in soil C stocks in wider red soil area of southern China. The soil C stocks were reduced between 19.2 and 20.4% in the initial decreasing period. After 7 or 8 yr C loss, the increased in soil C stocks was predicted to be between 0.073 and 0.074 kg C m^-2 yr^-1.