A B S T R A C TThe natural environments in the semiarid regions of the Chinese Loess Plateau (CLP) are fragile due to the serious soil erosion and the weak ecological services of the plants. To ascertain and then evaluate a sustainable land-use pattern in these regions, we selected six typical land-use patterns (i.e., a farmland, a natural grassland, a homogeneous shrubland (S), a mix of shrubland and cultivated grassland (S-Alf), a mix of shrubland and orchard (S-O) and a mix of shrubland and grassland (S-G)) on the plateau and then measured the soil water, related soil properties and plant root indices to a depth of 1800 cm. We also measured the aboveground net primary productivities (ANPPs). The mean soil water content (SWC) within the 0-1800 cm profile was significantly highest (15.2%) in farmland, followed by grassland (11.4%) and S-Alf (8.0%). The available water (AW), the ratio between AW and AW capacity, and the thickness of the dried soil layers also demonstrated that farmland had the best conditions of soil water, followed by grassland and shrubland. The aboveground biomasses of grassland in both non-growing (140 g m À2 ) and growing (370 g m À2 ) seasons were significantly higher than those of shrublands. The ANPPs of the grassland (2.0 g m À2 d À1 ) demonstrated a similar trend. The patterns of land use (including the mixtures of different plant species) greatly affected the patterns of vertical distribution and quantities of soil water within the 1800-cm profile. The data for the soil-water regime and the ANPP further indicated that grassland would be an optimal use of the land for these semiarid regions. This information should be useful to the ecological scientists and policy makers for developing strategies for the sustainable management of vegetation on the CLP and possibly other water-limited regions around the world.

An accurate evaluation of soil organic C (SoC) stocks is important to C management and to understanding fully the role of soils in the C cycle. However, SoC stocks in deep soils and the factors that affect them have been largely ignored. We measured SoC stocks and other soil properties to a depth of 500 cm (n = 73) under three land uses in the Lao ye Man Qu watershed on the Chinese Loess Plateau. Similar patterns in the vertical distributions of SoC stocks were found under cropland, grassland, and shrubland. However, for the 0-to 200-cm soil layer, SoC stocks were significantly lower under shrubland than under either cropland or grassland. The SoC stocks under cropland, grassland, and shrubland in the 0-to 100-cm layer were 2.64 ± 0.67, 2.50 ± 0.69, and 1.99 ± 0.22 kg m −2 , respectively; in the 0-to 500-cm layer they were 8.34 ± 2.26, 8.37 ± 2.01, and 7.26 ± 1.00 kg m −2 , respectively. Significantly higher SoC stocks occurred in loamy than sandy soils (P < 0.01), and they were lower at the mid than at the upper and lower slope positions. However, SoC stocks were similar in sunlit and shaded soils. Pearson correlation analysis indicated that land use, soil texture, and soil water content significantly affected SoC stocks in each 100-cm soil layer to a depth of 500 cm. Effects of topographic features and pH varied with increasing depth. A considerable amount of SoC was stored in deep soils, indicating the SoC content dependence on time and depth. This information is essential for evaluating C fluxes, estimating C stocks, and for managing the C cycles in regions around the world with deep soils.

Spatial heterogeneity of soil water retention curves (SWRC) plays an important role in modelling the movement of water and solutes in soils. To characterize the spatial variability of SWRC parameters fitted by the van Genuchten (VG) equation, and then identify factors that closely correlated with the VG parameters, we collected undisturbed and disturbed soil samples from the upper 0-5 cm soil layer at 382 sampling sites across the entire Loess Plateau of China ($620 000 km 2 ). We determined the SWRC and six other soil properties as well as nine environmental factors for each site. Results showed that the saturated water content (u s ) and a curve-shape parameter that is related to soil pore size distribution (n) were weakly spatially variable (coefficient of variation (CV) ¼ 15%), and that the residual soil water content (u r ) and a scaling parameter that is related to the inverse of the air entry pressure (a) were moderately (CV ¼ 96%) and strongly variable (CV ¼ 136%), respectively. Semivariograms of ffiffiffiffi u r p and log(a) were best-fitted by an isotropic exponential model, while those of u s and log(n) were fitted by an isotropic spherical model. Fractal analysis indicated that log(a) demonstrated the most short-range variation, while log(n) reflected the importance of long-range variation. Bulk density and the contents of clay, silt, sand and soil organic carbon contributed greatly to the variations in the VG parameters (except a). However, the vegetation, topographic and climatic elements as well as land use also usually explained some of the variation. The VG parameters, at the regional scale, demonstrated different characteristics of spatial variation, which comprehensively reflected the combined effects of processes involving the soil, vegetation, topography and climate as well as effects of land use, which is often determined by human activity.