Temporal and spatial variations in soil organic carbon sequestration following revegetation in the hilly Loess Plateau, China

Land use change is one of the major factors that affect soil organic carbon (SOC) variation and global carbon balance. Since the late 1970s. a significant area of the hilly Loess Plateau has undergone major land use changes during several revegetation programs, especially in the Grain for Green Project. However, so far there has not been a comprehensive study to determine temporal and spatial variations in SOC due to revegetation in this region, which hampers accurate predictions of the SOC sequestration potential and the land use change impacts. In this study, slope cropland and five typical revegetation types in the hilly Loess Plateau were selected, and then classified into 36 groups according to different revegetation years and landforms to investigate temporal and spatial variations of SOC and the impacts of relevant factors following revegetation. The results showed that the SOC concentration in the top soil horizon (0-5 cm) increased most significantly with revegetation and that increases in SOC slowed with increasing soil depth, but coefficient of variations in different soil horizons indicated that revegetation could cause SOC concentration differences at up to a 30 cm depth. Temporal variation in SOC occurred in two main phases in replanted cropland: in the first 10 or 15 yr, the profile mass of SOC (SOC density, SOCD) increased slightly; after this phase. SOCD increased significantly, with sequestration rates of 0.69, 0.55 and 0.24 t.ha(-1).yr(-1) (revegetation 10 to 35 yr) for planted woodland, planted shrubland and abandoned cropland, respectively. The SOC sequestration rate in wild grassland was 0.23 t.ha(-1).yr(-1) (revegetation 10 to 35 yr), which was similar to that in abandoned cropland. In contrast, the SOCD in wild shrubland increased rapidly in the first 10 yr, with a rate of 0.93 t.ha(-1).yr(-1), and then by 0.56 t.ha(-1).yr(-1) over the next 25 years. SOCD spatial differences in different landforms had reached several times the annual SOCD increment and followed a new trend after revegetation: SOC sequestration in shady slope areas was significantly higher than in sunny slope areas, but no significant difference was found between gentle slopes and steep slopes after revegetation. A general linear model was used to identify the factors that were most relevant to SOC variation. Revegetation years accounted for nearly half of the total contribution to SOC variation (42%), and land use type was responsible for 33% of SOC variation. SOCD distribution in a watershed confirmed that variables of land use and years after revegetation dominated SOC variation. Landforms had a small influence on SOC variation at the regional scale, but the influence of the slope aspect was still large. The SOC sequestration rate was about 0.21 to 0.64 t.ha(-1).yr(-1) with revegetation in the hilly Loess Plateau, which was highest in wild shrubland, followed by planted woodland, planted shrubland, abandoned cropland and wild grassland. Considering the large area of revegetation and relatively high SOC sequestration rate, SOC sequestration in this region should contribute significantly to decreasing the carbon concentration in atmosphere. (C) 2012 Elsevier B.V. All rights reserved.