Abstract: The intensification of global climate change may increase temperature and precipitation in drylands. Biocrust is one of the important surface covers in drylands. The responses of soil organic carbon (SOC) fractions and their stability in biocrusts-covered soil to climate warming and wetting remain unclear. We investigated the response of SOC fractions and stability under moss-dominated biocrusts in the Loess Plateau to two years warming (approximately 3 ℃ increase) crossed with 10%, 30% and 50% increases in precipitation. The results showed that: 1)The contents of SOC and its fractions in the surface layer covered by moss biocrusts were significantly increased by both warming and warming combined with wetting, whereas SOC stability was decreased. Both warming and warming combined with wetting increased the contents of SOC, labile organic carbon (LOC, including microbial biomass carbon, dissolved organic carbon and easily oxidized organic carbon) and recalcitrant organic carbon (ROC) by 28%, 51% and 24% on average, respectively. They also increased the labile index of organic carbon (LIC) by an average of 28%. In contrast to only warming, the treatments that combined warming and wetting resulted in average increases in SOC, LOC and ROC of 25%, 30% and 22%, together with a 15% rise in LIC. 2) There were significant and positive correlations between the contents of SOC, LOC and ROC and biocrust cover, moss biomass, moss density, and ammonium and nitrate contents under simulated warming and wetting. Furthermore, annual precipita-tion accounted for an average of 88% of the variation in SOC components. In conclusion, the warming and wetting climate changed the carbon conversion strategies of biocrusts, increased the content and activity of SOC components and decreased SOC stability. Therefore, in future intensified warming and wetting scenarios, SOC content in moss-dominated biocrust-covered soils in the Loess Plateau region may increase, with more pronounced dynamic changes in soil carbon pools.

Key words: climate change, biological soil crusts, carbon cycle, carbon transformation, Loess Plateau