Effects of forest conversion to rubber plantation and of replanting rubber trees on soil organic carbon pools in a tropical moist climate zone

In the past two decades, conversion of forests to rubber (Hevea brasiliensis) plantations has spread widely in continental Southeast Asia. In addition to the effect of the establishment of new rubber plantations, the impact of the replanting of rubber trees, i.e., second-rotation plantation establishment, on the soil organic carbon (SOC) pool is unknown because of the short history of land management at such sites. We examined whether the first-rotation rubber plantation (RUB_1st) soils have more of a legacy effect from previous natural forests than the second-rotation plantation soils in Cambodia, so as to improve SOC stock estimates for a series of land use changes in a tropical moist climate zone, e.g., the lowlands of continental Southeast Asia. The total SOC stock, estimated for soil masses equivalent to 0–30 cm depth, was larger in natural forest (FOR) plots (52.7 MgC ha−1) than in RUB_1st (38.4 MgC ha−1), early-stage second-rotation plantation (RUB_2nd_A1–6; tree age ≤6 years; 37.2 MgC ha−1), and middle-stage second-rotation plantation (tree age >6 years; 33.9 MgC ha−1) plots. Values for factors related to SOC dynamics, such as the mass and carbon/nitrogen (C/N) ratio of leaf litter, soil C/N ratio, soil aluminum content, and bulk density of coarse plant residue (>2 mm), were also higher in FORs than in the rubber plantations. The SOC pools in both high- and low-density fractions of topsoil were lost largely within the first 10 years of forest–rubber plantation conversion. The loss of SOC in the low-density fraction in the RUB_1st might have been compensated to some extent by the remaining plant residue in these former forests. Although weeding resulted in the mixing of temporarily growing C4 grasses with soil in RUB_2nd_A1–6 plots, these grasses might have contributed minimally to the SOC pool in the long term, as reflected by C stable isotope ratios. Accordingly, we presented a conversion factor of SOC stock as 0.66 ± 0.18 for the 9-year transition period from forests to rubber plantations on a flat topography in a tropical moist climate zone of continental Southeast Asia, and concluded that the replanting of old rubber plantations should take priority over the forest-rubber plantation conversion as possible to reduce the emission of greenhouse gas under the land use change.