Soil erosion impact on soil organic carbon spatial variability on steep tropical slopes

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The main objectives of this study were to evaluate soil organic C (SOC) variability in a representative hillslope of Laos and to quantify the impact of some environmental factors. We collected 2348 soil samples from 581 georeferenced soil pits within a hillslope of northern Laos under traditional shifting cultivation at 0- to 0.05-m depth and then every 0.1 m to 0.35-m depth. The SOC stocks at 0- to 0.05-m depth varied between 0.4 kg C m–2 (standard error of ±0.046 kg C m–2) and 1.9 (±0.22) kg C m–2 and stocks in the 0- to 0.35-m depth were between 2.6 (±0.29) and 11.4 (±1.31) kg C m–2. About 85% of SOC spatial variability occurred at a distance less than 20 m. As expected, SOC content and stocks at 0- to 0.05-m depth were significantly greater with higher soil clay content and shorter durations of cultivation (P < 0.001). But at 0- to 0.35-m depth, the significance of clay content was only P = 0.04 and stocks surprisingly increased with increasing slope gradient (P < 0.001). Thus, it seems that sloping lands under shifting cultivation act as a conveyor that stores atmospheric inorganic C in soils during the regeneration of natural fallows and ultimately transfers it by water erosion to the steepest areas of hillslopes, where it accumulates, probably due to greater infiltration by water. These results on SOC spatial variations under steep slope conditions of the tropics give a better picture of SOC dynamics that may allow development of optimal strategies of land management to foster main soil functions and offset the current rise in atmospheric CO2.

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