Long-term soil carbon loss and accumulation in a catchment following the conversion of forest to arable land in northern Laos

Quantifying the magnitude of soil organic carbon loss linked to the conversion of forest to arable land in tropical environments requires long-term estimates of soil redistribution trends within the catchment. In order to assess the cumulative soil carbon loss that occurred since deforestation and cropping started in 1967, a multidisciplinary study was carried out in a small catchment (67 ha) with a swamp area in its middle part, characterized by steep slopes under shifting cultivation of upland rice, typical for northern Laos's landscapes. Using 137Cs and total organic carbon (TOC) inventories, it was possible to estimate a soil TOC depletion rate of 0.039 kgC m−2 yr−1 in 42 years (1963–2005), corresponding to ca. 21% of its initial content in the top 10 cm. Because farmers preferred clearing more fertile soils located downhill and leave forest on poorer soils on the crests, the redistribution of 137Cs and TOC along slopes of the catchment was mainly controlled by the topographic position of soils and the cultivation frequency. Estimates of soil TOC accumulation in wetlands with δ13C measurements and sediment volumes by electrical resistivity tomography showed that TOC accumulation rates were linked to land use change since the 1960s with three successive periods corresponding to undisturbed forest, deforestation-first rice crop and rice-fallow rotations. Forest clearing triggered a higher soil TOC delivery to the swamp (19.6 ± 5.5 kgC ha−1 yr−1) over 8 years (1967–1975) than the former undisturbed forest (8.5 ± 1.8 kgC ha−1 yr−1) and crop-fallow rotations over 30 years (1975–2005, 6.4 ± 4.8 kgC ha−1 yr−1). Due to the small size of the area covered by the swamp, only a limited fraction of eroded soil organic carbon (ca. 7 wt.%) was stored in wetlands of the catchment. Mineralization apparently played a key role for soil TOC depletion since first forest clearing.