Abstract: Biological crusts (Biocrusts) are important surface active coverings in arid and semi-arid regions, which affect the content of soil organic carbon (SOC), SOC labile fractions and stability of SOC through photosynthetic carbon fixation. At present, studies on the variation characteristics of SOC, SOC labile fractions and the stability of SOC in biocrusts are rather limited. In this study, two types of typical biocrusts (moss crusts and algae crusts) were selected along a precipitation gradient from northwest to southeast in the Mu Us Sandland (straight line distance 188 km) by measuring soil organic carbon (SOC), soil microbial biomass carbon (MBC), water soluble carbon (DOC), particulate carbon (POC), easily oxidizable carbon (ROC). We aimed to explore the effects of biocrusts on the stability of SOC and carbon decomposition across the precipitation gradient. Results showed that:1) Two types of biocrusts significantly increased the contents of SOC, MBC, DOC, POC, ROC and stability of SOC. Moss crusts increased SOC contents by 1.6 to 2.6 times as that of algae crusts. 2) The lowest SOC contents of the two types of biocrusts were 6.43 g·kg^-1 and 14.50 g·kg^-1 respectively, which showed an increasing trend with increasing precipitation along the gradient. 3) With the increases of precipitation, the decomposition time of moss litters gradually decreased. The decomposition coefficient of moss litters during the study period (From July to Feb-ruary of the next year) ranged from 0.010 to 0.014, which was significantly lower than that of vascular plants. The carbon release of moss litters from northwest to southeast was 8.09, 10.89, 12.88 g·kg^-1, respectively. 4) Results of canonical correspondence analysis showed that water vapor partial pressure, actual evapotranspiration, annual average temperature, subsurface short-wave radiation, potential evapotranspiration and vapor pressure difference were the key climate factors affecting the content of SOC and its active components. Silt content was the main soil factor affecting the content of SOC and its active components.

Key words: Mu Us sandland, biological crusts, soil organic carbon, precipitation gradient, contribution rate