Soil respiration in sloping rubber plantations and tropical natural forests in Xishuangbanna, China

Soil respiration (SR) plays an important role in the global carbon cycle. The widespread and continued conversion of tropical forests to plantations is expected to drastically alter CO2 production in soil, with significant consequences for atmospheric concentrations of this crucial greenhouse gas. In Southeast Asia, rubber plantations are among the most widespread monoculture tree plantations. However, knowledge of how SR differs in rubber plantations compared to natural forests is scarce. In this study, surface CO2 fluxes and soil CO2 concentrations (at 5 cm, 10 cm, 30 cm and 70 cm depths) were measured at regular intervals over a one-year period along slopes at three sites in paired natural tropical forests and mature rubber plantations. Annual surface soil CO2 fluxes were 15% lower in the rubber plantations than in natural forest. This difference was due to substantially lower SR during the dry season in rubber plantations compared to natural forest. During the wet season, SR did not differ significantly between rubber plantations and natural forests. In rubber plantations, soil moisture increased from lower and middle to upper slope positions, but this did not significantly impact SR. Throughout the year, net CO2 production per unit volume in the topsoil (2.5-7.5 cm) exceeded by 2-3 orders of magnitude net CO2 production in the subsoil (7.5-50 cm). However, CO2 originating from 5 cm depth and below in both land cover types could only explain up to 30% of the aboveground measured CO2 flux, indicating that >70% of the total CO2 respired and emitted to the atmosphere originated from the uppermost few cm of the soil. Net CO2 production at different soil depths did not differ significantly between rubber plantations and natural forests. Our results indicate that SR characteristics in mature rubber plantation and natural forest were broadly similar, although dry season soil surface CO2 fluxes were lower in rubber plantations. However, further information on the drivers of CO2 production in the uppermost topsoil layers which are responsible for most CO2 emissions is needed to understand the extent to which these results are generalisable.