Benefiting from southeast monsoon, the red soil hilly region in southeast China has excellent water and heat conditions, and land use here changed rapidly these years. Unreasonable land use, however, resulted in serious ecosystem degradation. Before 1980, most of the zonal vegetation in the study area, mainly consisting of evergreen broad-leaved forest, had been seriously destroyed, and degraded to grassland dominated by perennial grasses with scattered trees and shrubs. At the same time, severe soil degradation also occurred in some places of this region. The ecosystem degradation not only reduces the land productivity, but also has a deep impact on global change. So the restoration and reconstruction of the degraded ecosystems are not only important to the sustainable development of agriculture, but also helpful in understanding the role of this region playing in global change. In this paper, soil nutrient changes in surface horizon (0-20cm) of masson pine woods, wetland pine woods and Chinese fir woods planted in 1984, which are widely planted in the red soil hilly area, were studied. The results showed that Soil Organic Carbon decreased dramatically in the first 10 years, then increased gradually in the following 6 years, and reached to a relative steady status in the latest 5 years, while total nitrogen remained relatively steady, and soil acidification is visible.
Soil erosion is a widespread geological process with strong capabilities to entrap, transport, and redistribute surface soil. It is important to grade erosion intensity and estimate eroded soil mass. On the other hand, soil organic carbon pool (SOC), as the largest terrestrial carbon content, is prone to be affected by soil erosion. Soil erosion redistributes surface soil over landscape, and changes the physical environmental conditions. The consequences are depletion of SOC in eroded regions, emission of carbon dioxide during transportation and input of extra SOC in deposited sites. This paper quantified the effects of soil erosion on soil organic carbon through following three steps: firstly, soil erosion type and soil erosion intensity was determined by interpreting Landsat TM images. Secondly, through field investigation, soil type and soil profile data were collected, and SOC contents in the 0-20cm layers were constructed. Finally, supported by map algebra in Geographic Information System (GIS), soil organic carbon loss induced by soil erosion was calculated based on the national soil erosion modulus standard. Total annual SOC loss in China was about 1.595×10<sup>8</sup> ton−C yr−<sup>1</sup>. Assuming 20% of SOC was oxidized, erosion induced CO<sub>2</sub> emission was about 3.19×10<sup>7</sup> ton−Cyr−1.