Soil organic carbon losses and its consequence for land use change in southwestern China

Background/Question/Methods Soils are a major pool of global carbon which constitutes more than twice as much stored carbon as that of the earth’s vegetation. Increasing soil carbon level is believed an option to reduce the rise of atmospheric CO₂ concentration that contributes to global warming. Soil organic carbon (SOC) losses due to water erosion in different land use patterns were investigated in Xiejiawan watershed, Sichuan province, China. The purpose of the project was to determine the effects of the soil-management and cultivation practices on SOC loss and carbon sequestration capacity in a serious soil and water erosion region. The relationships between soil erosion and rainfall event, land slope, and vegetation coverage were also examined. In this study, we conducted the investigations in three land use patterns, representing the common cultivated practices in southwestern China: bare soil, downslop cultivated cropland, and horizontal terraced cropland. Experimental runoff plots were established on the three land use types in the study watershed. Surface runoff was monitored using the water-level recorders from the triangle and rectangular weirs installed on the lowest side of each plot. Water samples were taken during the rainfall events and the earth content and SOC concentration in water samples were estimated by chemical analysis.

Results/Conclusions Our results showed that it was the total rainfall amount instead of rainfall intensity that made the major contribution to SOC loss in the study area. Soil erosion and SOC loss were positively correlated to land gradient in all land use patterns. On average, land gradient increased 1.75%, annual soil erosion and SOC loss increased 39.0 and 0.3t/km², respectively. The loss of SOC was negatively related to the increase of vegetation cover (VC) and can be mathematically described using the l model: SOC = 8.1532/(1+e^{-77.8629+118.0716VC}). Such relationship was much significantly when vegetation coverage was high. Under the similar land slope and rainfall event, bare soil had the highest SOC loss, the next was downslop cultivated cropland, and the least SOC loss was found in horizontal terraced cropland. For example, SOC losses were 4.1, 2.3 and 0.4t/km² in bare soil, downslop cultivated cropland and horizontal terraced cropland during a 55 mm rainfall event, respectively. In other words, if the bare soils were converted to horizontal terraced cropland the SOC loss would be reduced about 90%. Our study suggested that appropriate soil-management practices have great potential to protect SOC loss through water erosion.