Abstract: Soil carbon (C) cycling in alkaline-saline soils has received great attention in recent years because of its potential role in climate change mitigation in arid regions, and it is sensitive to land use change. Understanding its impacts on soil carbon can help accurately assess carbon sink in saline-alkaline soil. Here, we quantified the effects of different land use types on the spatial distributions of soil inorganic C at a typical watershed in the arid region of northwest China. Soil samples were collected from the oasis in the Sangong River Basin, which covered different land use types, such as grassland, saline-alkali land, shrub land, planted land, as well as different types of croplands. The mean inorganic C content of the entire study area was 4.81 g·kg^-1. The mean value in cropland and planted forest was lower than 4.61 g·kg^-1, with more than 30% soil samples was concentrated in lower than 4 g·kg^-1. The value from other land use types was higher than 5 g·kg^-1, with more than 70% soil samples was concentrated in higher than 4 g·kg^-1. For all land use types, the mean value of inorganic C content was higher in natural landscape than that in irrigated landscape. The inorganic C content was obviously lower in alluvial-proluvial- delta than in alluvial plain (P<0.05), and the soil inorganic C content in different geomorphology zones were in the following order: middleupper alluvialproluvial delta <upper alluvial plain<lower alluvial plain<groundwater overflowing zone. The order of soil inorganic carbon density was: salinealkali land<shrub land<grassland<planted forest<cropland, with the minimum of soil inorganic C storage (1.17 kg·m^-2) in salinealkali land and the maximum of soil inorganic C storage (1.44 kg·m^-2) in cropland. Land use had a significant effect on the density of soil inorganic C at 0-20 cm layer. Soil inorganic C density in irrigated landscapes was higher than that in natural landscapes. The results of variance analysis, multiple linear regression, and stepwise regression showed that many factors had significant impacts on soil inorganic C content, with their relative contribution to the variation of SIC content following the order: geomorphology zone>land use type>electrical conductivity>crop type.

Key words: cucumber, autotoxicity, p-hydroxybenzoic acid, melatonin, alleviating effect.