[Distribution characteristics of exogenous carbon in different carbon fractions in biocrusts-covered soil]

The distribution characteristics of exogenous carbon (C) in the C fractions of biocrusts-covered soil are critical for understanding the geochemical cycling of C with biocrusts in drylands. A ¹³C pulse labeling experiment was conducted for moss-dominated biocrusts-covered soil and bare soil on the Loess Plateau of China with semiarid climate, with the content of ¹³C in different C fractions being continuously measured to determine the biocrust effects on the distribution of exogenous C in each C fraction. Our results showed that, 1) the ¹³C abundance of each C fraction in the biocrusts-covered soil was steadily changed with time, due to the relatively low rate of nutrient cycling in the biocrusts-covered soil and also to the relatively low biomass of moss in the biocrusts-covered soil as compared with vascular plants. 2) The ¹³C content of each C fraction in the biocrusts-covered soil was significantly higher than that in the bare soil. Specifically, the ¹³C content of total organic C (TOC), microbial biomass C (MBC), and dissolved organic C (DOC) in the biocrusts-covered soil was 0.258, 0.078, and 0.004 mg·kg^-1, respectively, which was 3.1, 18.5, and 2.6 times higher than that in the bare soil. Moreover, the ¹³C content in the moss of the biocrusts-covered soil was 1.45 mg·kg^-1. 3) The presence of biocrusts changed the distribution characteristics of each C fraction, with the newly assimilated C being mainly distributed in active organic C and biological components of the biocrusts-covered soil. In the biocrusts-covered soil, the ¹³C distribution in MBC (30.6%) was higher than that in DOC (1.7%), and the ¹³C distribution in the C of moss was 20.3%. 4) The transferred amount and storage capacity of MB¹³C in the biocrusts-covered soil were 15.7 and 19.5 times of that in the bare soil, respectively. The turnover rate of MB¹³C in the biocrusts-covered soil and bare soil was 2.94 and 3.30 times per month, respectively, implying that the turnover time of MB¹³C in the biocrusts-covered soil was 1.1 times longer than that in the bare soil. In conclusion, biocrusts could greatly change the distribution characteristics of each C fraction and increase C turnover rate, highlighting its important roles in C cycling in dryland ecosystems.