Effects of vegetation restoration and slope positions on soil aggregation and soil carbon accumulation on heavily eroded tropical land of Southern China

Soil organic carbon (SOC) accumulation is strongly affected by soil erosion and deposition that differ at slope positions of a watershed. However, studies on the effects of topography on soil aggregation and SOC dynamics, especially after the implementation of vegetation restoration, are rare. Poorly understood mechanisms and a lack of quantification for the suite of ecological benefits brought by the impacts of topography after planting further obstructed our understanding of terrestrial ecosystem carbon (C) sequestration. The purposes of this study are to (1) quantify the impacts of vegetation restoration on size and stability of soil aggregates and the sequestration of C in soil and (2) to address the impacts of various slope locations on aggregates and SOC distribution. The experimental sites were set up in 1959 on a highly disturbed barren land in a tropical and coastal area of Guangdong province in South China. One site received human-induced vegetation restoration (the restored site), while the other received no planting and has remained as barren land (the barren site). The soil in the study sites was a latosol developed from granite. Soil samples were taken from 0 to 20 and 20 to 40 cm soil layer at shoulder and toe slope positions at both sites for comparisons. Soils were analyzed for proportion of soil macroaggregates (> 0.25 mm), the SOC in soil layers, and the aggregate soil organic carbon (AOC) at different aggregate sizes. Measurements in 2007 showed that fractions of water stable macroaggregates in 0-40 cm at shoulder and toe slope ranged from 28% to 45%, about one third to one half of those of dry macroaggregates (91-95%) at the restored site. Soil macroaggregates were not detected at barren site in 2007. Average SOC storage in 0-40 cm soil layer of shoulder and toe slope positions at the restored site was 56.5 +/- 10.9 Mg C ha(-1), about 2.4 times of that (23.4 +/- 4.6 Mg C ha(-1)) at barren site in 2007. Since 1959, the soil aggregation and SOC storage are significantly improved at the restored site; opposite to that, soil physical and chemical quality has remained low on the barren land without planting. SOC storage in 0-40 cm at toe slope was 15.9 +/- 1.8 Mg C ha(-1), which is only half of that (30.9 +/- 9 Mg C ha(-1)) at shoulder slope of the barren site; this is opposite to the pattern found at restored site. The ratios of AOC in 0-20 cm to AOC in 20-40 cm at toe slope were lower than those at shoulder slope of the restored site. The comparison of organic carbon sequestered in soils at different slope positions suggest that soil aggregates played a role in sequestering C based upon landscape positions and soil profile depth as a consequence of soil erosion and deposition. Results indicate that vegetation restoration and SOC accumulation significantly enhance soil aggregation, which in turn promotes further organic C accumulation in the aggregates via physical protection. Soil aggregation and soil C accumulation differed between slope positions. Soil aggregation was significantly enhanced in 0-20 cm layer and aggregates absorb C into deep layers in depositional environment (toe slope) under protection from human disturbances. The interactions of erosion-deposition, soil aggregates, and vegetation restoration play important roles on SOC accumulation and redistribution on land. The positive feedback between SOC and soil aggregates should be evaluated for improving the quantification of the impacts of land use change, erosion, and deposition on the dynamics of SOC and soil structure under the global climate change.