Spatial variability in soil organic carbon and its influencing factors in a hilly watershed of the Loess Plateau, China

Abstract

In the last 30years, a number of large-scale multi-ecological projects have been implemented on the Chinese Loess Plateau, where there has been a significant trend toward ecological restoration, which will certainly affect the spatial distribution of soil cabon sequestration. We investigated 180 topsoil sites (to 20cm depth) to enhance the understanding of the spatial patterns of soil organic carbon (SOC) and its dominant influencing factors in the Luoyugou watershed (72km2), which is a typical watershed in the gully-hill area of the Loess Plateau, China. The results indicate that the SOC in the Luoyugou watershed follows a distinct spatial pattern, whereby the SOC increases as altitude increases. The SOC of the mountain ridge area (high elevation area) is measurably higher than that of the sides of the valley. Four spatially interpolated methods (IDW, Ordinary Kriging, land use types and multiple linear regression) were used to express the SOC spatial patterns, but all of them have low estimate accuracy because the SOC in the hilly loess area is highly heterogeneous given the complex topography and land use/cover. The estimate accuracy should improve with the Normalized Difference Vegetation Index (NDVI), given auxiliary information. SOC in the Luoyugou watershed is influenced by altitude, land use, and NDVI. Altitude has a significant relationship with SOC in the Luoyugou watershed. Annual precipitation significantly increases with increased altitude, and land use/vegetation cover changes between terraced cropland, orchard to grassland, and woodland. The SOC of Luoyugou has no significantly correlated relationship to topographic indices (slope, upslope area, and TWI), which have been profoundly disturbed by human activity, particularly by the conversion from sloped land to terraced cropland. The study results show that afforestation can significantly increase SOC. The SOC in immature forests (10years old) is 17.91% higher than that in terraced cropland, but 32.25% lower than that in 30-year-old forests. The SOC concentration of 30-year-old forest is significant higher than that in other land cover. On a 10-year time scale, orchard is not a good ecological restoration type for increasing SOC; the contribution to enhancing SOC is not obvious and even shows a weak decreasing trend. In general, the SOC sink role of the Loess Plateau is being enhanced by ecological restoration.