Spatiotemporal heterogeneity of soil nutrients was put forward to indicate the distribution and changes of soil nutrients. Comparing with non-karst regions, former studies had shown that soil nutrients were inversed in karst peak-cluster depression, namely nutrient-inversion. The reasons for nutrients inversion in karst peak-cluster depression were obscure. Therefore, this study aimed to explore the impacts of vegetation succession and topographical attributes on surface soil nutrients and validate whether it is universal for nutrients inversion.
An experiment was designed to compare the distribution of soil nutrients under three mainly vegetations to theoriginal forest. In order to explore the temporal and spatial variability of soil nutrients and its controlling factors, the soil samples of topsoil (0-15cm) were collected in two typical Karst peak-cluster with four vegetation succession stages (grassland, shrubland, plantation forest, original forest) in northwest Guangxi, southwest China. A total of 202 samples were collected from twelve sampling lines (each vegetation type had three sample lines for comparison), including 48 grassland samples, 46 shrubland samples, 44 plantation forest samples and 64 original forest samples. Analysis of variance (ANOVA) was utilized to analyze the differences of topsoil nutrient contents for different vegetation types and topographical attributes, respectively. The results showed that there were significant differences of topsoil nutrients (soil organic carbon, total nitrogen, carbon/nitrogen ratio, total phosphorus, total potassium, available nitrogen, available phosphorus, and available potassium) and soil pH values among different vegetation succession stages. Except for the total potassium, other nutrient contents increased with vegetation succession. The slope position had different effects on soil nutrient contents. Many influencing factors affected the variability of nutrients at the same time, such as human disturbance and its intensity, soil physical properties, vegetation community, ground litter and so on. Carbon and nitrogen of original forest, as well as phosphorus and potassium of other vegetation succession stages, was accorded with the "nutrient depression effect", which meant that nutrient reduced with the increase of elevation. In addition, carbon and nitrogen of grassland, shrubland and artificial forest increased with elevation. It was due to the disturbance of human activities.
In addition, Our study indicates that vegetation succession plays a positive role in soil nutrient accumulation and thegovernment should keep on carrying out to protect vegetation from human disturbance. Phosphorus (available phosphorus 3.55-11.41 mg/kg) and potassium (available potassium 64.05-105.75 mg/kg) are always limiting nutrient factors in karst peak-cluster depression regions. Additionally, nutrient-inversion is not a common phenomenon and if there was no interference of human, the distribution of soil nutrients in Karst peak-cluster depression will accord with other regions, following the "nutrient depression effect".