Abstract: As one of the important physical properties of soil, swelling characteristic plays an important role in affecting the water conductivity, water holding capacity, corrosion resistance of soil and the formation and development of soil structure. In order to investigate the swelling characteristics and influencing factors of biocrusts, we took moss-dominated biocrusts and bare soil from aeolian and loessal soil on the Loess Plateau, and investigated their differences in swelling rates under different factors (initial water content, dry-wet cycle, freeze-thaw cycle, temperature) using soil dilatometer. The relationships between the swelling rates of the samples and the environmental factors or biocrusts properties were also analyzed. The results showed that the swelling rate of aeolian soil with biocrusts was 1.93%, which was 8.65 times higher than that without crust; the swelling rate of the loessal soil with biocrusts was 2.05%, which was 76.68% lower than the loessal soil without crust. At the same time, the biomass and thickness of biocrusts on aeolian soil were linearly positively correlated with the swelling rate (P < 0.05), and they were quadratic functions (P=0.02) and positive correlations on the loessal soil (P=0.02), respectively. The initial water content also affected the maximum swelling rate and stable swelling time, and the degree of influence was that the aeolian soil was much larger than the loessal soil (including the biocrusts and bare crust); The degree of dry-wet cycle numbers affected the swelling rate of bare soil more than that of biocrusts. The dry-wet cycle changed the swelling rate of bare soil on aeolian soil and loessal soil from 50.00% to 620.00% and from -2.28% to 10.81%, respectively; and the swelling rate of the biocrusts on both soils changed from -5.70% to 10.88% and from -10.24% to -21.46%, respectively. Under the freeze-thaw cycle, the swelling rates of four soils decreased to varying degrees, with a decrease of 0 to 18.54%. The swelling rate of the loessal soil without crust was greatly affected by the temperature, and the swelling rate of loessal soil without crust at 50℃ was 1.17 and 1.21 times at 25℃ and 35℃, respectively. The studies have shown that the biocrusts significantly changed the swelling of the surface soil of aeolian soil and loessal, but the extent and direction of its impact depended on the soil type. At the same time, the swelling of biocrusts is affected by several key factors such as water content, temperature, wet and dry, and freeze-thaw cycles.

Key words: soil swelling rate, soil swelling and shrinkage, initial water content, dry-wet cycle, freeze-thaw cycle, temperature