Objective  Biological crusts are widely distributed in arid and semi-arid areas and significantly affect the saturated hydraulic conductivity (Ks) of soil. However, the effect of freeze-thaw on Ks of soil covered by biological crusts is unknown. In order to investigate the Ks changes of soil covered by different types of biological crusts under freeze-thaw conditions, taking the mixed (50% algae crust + 50% mossy crust) and moss crust (coverage above 90%) soil in the Liu Dao gou watershed of Shen Mu County as the research subjects.

  Method  Simulated freeze-thaw experiment was carried out to determine the Ks of soil covered by biological crust under different freeze-thaw alternating times (3, 7, 13 and 20) and initial water contents (4%, 6%, 8%, 12% and 15%).

  Results  The results showed that: (1) The soil Ks with moss crust and mixed crust decreased under freeze-thaw conditions compared with bare soil. (2) The Ks with moss crust and mixed crust increased first and then decreased with the increase of initial moisture content under the same freeze-thaw times. Under the same initial moisture content, the Ks of the two crusts increased gradually with the increase of freezing and thawing times. (3) After freeze-thaw, the Ks of moss crust covered soil was significantly greater than that of mixed crust-covered soil. Under the same freeze-thaw condition, the difference of Ks between the two kinds of crust after freeze-thaw for 3 times and 7 times was the largest at the initial moisture content of 8%, and the Ks of moss crust-covered soil was 2.1 and 2.3 times that of mixed crust, respectively. (4) Freezing and thawing affected Ks, bulk density, crust thickness, soil organic matter and > 0.25 mm aggregates in the lower layer of crust. The freezing and thawing times had a very significant effect on the thickness and organic matter content of crust (P < 0.01), the bulk density of crust (P < 0.05), and the initial water content had a very significant effect on the content of > 0.25 mm aggregates (P < 0.01). (5) The Ks of moss crust and mixed crust covered soil under freeze-thaw environment was significantly positively correlated with the times of freeze-thaw (P < 0.01), and negatively correlated with the bulk density of crust (P < 0.01). Partial least squares regression analysis was performed between the two crusts Ks and other factors. The results showed that the main influencing factors for Ks were crust bulk density > freezing and thawing times > crust thickness, while the mixed crust was freezing and thawing times > crust bulk density.

  Conclusion  The alternation of freezing and thawing has a significant effect on the Ks of biological crust covered soil, and the freezing and thawing effect mainly affects the soil Ks through the crust thickness, crust bulk density and the content of large aggregate particles and organic matter in the lower soil.