Soil redistribution by terracing alleviates soil organic carbon losses caused by forest conversion to rubber plantation

Abstract

Secondary forest-to-rubber (Hevea brasiliensis) plantation conversion is an important recent land-use change in the montane regions of mainland Southeast Asia. This land-use conversion caused a reduction of soil organic carbon (SOC) stocks by on average 19% down to 1.2 m over 46 years. Due to the mountainous topography of the region, most rubber plantations include narrow terraces parallel to contours. Manual terrace construction involves cutting of the soil from the upper slope and piling up the removed soil on the soil surface downslope. Soil redistribution by terrace construction may affect SOC dynamics through exposure of the subsurface soil at the terrace inner sides (cut section) and soil burial at the terrace outer edges (fill section).Our study, conducted in southern Yunnan province of China, aimed to quantify SOC stock changes induced by terrace construction. In three rubber plantations aged 5, 29 and 44 years, we systematically sampled the terraces according to soil redistribution zones, and the original sloping areas in between the terraces were used as reference.At the cut section of the terrace, topsoil removal caused a depletion of SOC stocks in the youngest plantation followed by SOC stock recovery in the two oldest plantations. The recovery of SOC stocks at the cut section in the two oldest plantations was attributed to the capacity of the exposed subsurface soil to store new organic carbon inputs from roots and litter, and to sedimentation of eroded topsoil materials from the upper slope. At the fill section of the terrace, soil deposition resulted in higher total SOC stocks compared to the reference position in all plantations. This was due to the deposition of redistributed soil material on top of the original soil surface combined with the partial preservation of carbon in the buried soil. Overall, the increase of SOC in the exposed subsurface soil at the cut sections, and the partial preservation of SOC in the buried soil at the fill sections resulted in higher SOC stocks down to 1.2 m at the terraces compared to the reference positions in the two oldest plantations. Our results imply that terracing may alleviate SOC losses caused by the conversion of secondary forest to terraced rubber plantation.