Soil CO₂ efflux and soil carbon balance of a tropical rubber plantation

Natural rubber is a valuable source of income in many tropical countries and rubber trees are increasingly planted in tropical areas, where they contribute to land-use changes that impact the global carbon cycle. However, little is known about the carbon balance of these plantations. We studied the soil carbon balance of a 15-year-old rubber plantation in Thailand and we specifically explored the seasonal dynamic of soil CO ₂ efflux ( F_S) in relation to seasonal changes in soil water content ( W_S) and soil temperature ( T_S), assessed the partitioning of F_S between autotrophic ( R_A) and heterotrophic ( R_H) sources in a root trenching experiment and estimated the contribution of aboveground and belowground carbon inputs to the soil carbon budget. A multiplicative model combining both T_S and W_S explained 58 % of the seasonal variation of F_S. Annual soil CO ₂ efflux averaged 1.88 kg C m ⁻² year ⁻¹ between May 2009 and April 2011 and R_A and R_H accounted for respectively 63 and 37 % of F_S, after corrections of F_S measured on trenched plots for root decomposition and for difference in soil water content. The 4-year average annual aboveground litterfall was 0.53 kg C m ⁻² year ⁻¹ while a conservative estimate of belowground carbon input into the soil was much lower (0.17 kg C m ⁻² year ⁻¹). Our results highlighted that belowground processes (root and rhizomicrobial respiration and the heterotrophic respiration related to belowground carbon input into the soil) have a larger contribution to soil CO ₂ efflux (72 %) than aboveground litter decomposition.