Abstract:Spatial variability of soil nutrients provides useful information for improving agricultural practices and ecological management. Peak-cluster depression forms a typical Karst landscape with agriculture mainly based in depressions, such as in the Karst region in south-west China. In this study, the depression area in a typical landscape unit selected from the Karst region of Guangxi province was investigated for the spatial variability of soil organic carbon (SOC) and available phosphorus (AP) in the surface soil (0-15 cm). Grid sampling method (20 m×20 m) and geological statistics were applied to analyze the spatial distribution of SOC and AP contents. Sample variograms of SOC and AP were fitted well by spherical and exponential models, respectively. Nugget effects due to random errors or variability within the distance of the sampling interval (20 m) contributed to 37.9% of the overall variation for SOC, and 49.8% for AP. This suggests that the spatial variation in SOC was mainly controlled by structural factors, which mainly depended on topographic conditions and land use types in the study area, but AP rather more controlled by random factors mainly due to the variability of fertilizer application which differed widely in the depression. Correlation length of SOC was about 135.5 m, which had similar scope to the average range of difference in land use types. Based on directional semi-variogram analysis, zonal anisotropy, and lower sill and longer range in the long axis direction were found in the spatial distribution of SOC in the depression. AP also had anisotropy, but the pattern in its spatial distribution was quite different from that of SOC, as AP had much longer correlation length (413.4 m) and showed clearly a drift trend which enhanced variability in the long axis direction, presumably due to the random application of P-fertilizers. If the effects of drift trend were subtracted, the correlation length of AP decreased to 167.4 m. This study suggests that the pattern in the spatial distribution of SOC in the depression area in Karst regions was mainly controlled by structure factors (topographic conditions and land use types), but for AP rather more by random factors such as fertilizer application.