Severe water scarcity and intense surface evaporation have suffered in the arid and semi-arid regions, leading to the widespread desertification and ecosystem vulnerability. Biological soil crusts (biocrusts) can be expected for surface assemblages during microbial mediation, where cyanobacteria, lichens, and mosses bind mineral particles into a cohesive layer. Functioning as a biologically active interface between the atmosphere and soil, biocrusts also dominate the heat transfer in the shallow subsurface. This study aims to systematically quantify the effects of the biocrusts on soil thermal properties and temperature. The object was taken from the bare soil and biocrusts of the aeolian sandy soil (moss crusts and cyanobacterial crusts) of the Loess Plateau. Controlled laboratory experiments and long-term field monitoring were combined to explore the soil thermal properties and temperature dynamic differences of two typical biocrusts and bare soil at 0-2 cm depth. A three-needle heat-pulse probe was employed to accurately identify soil thermal parameters: heat capacity, thermal conductivity, and thermal diffusivity. The results demonstrated that biocrusts significantly altered the physicochemical properties of surface soil, particularly on the field capacity and equivalent porosity (P < 0.001). Moss crusts increased field capacity by 120.0%, compared with bare soil, while total porosity reached 1.4 times. Furthermore, biocrusts further significantly influenced the soil thermal properties (P < 0.001). Laboratory measurements indicated that the heat capacity of moss crusts was 14.2% and 14.6% lower than that of bare soil and cyanobacterial crusts, respectively. Besides, the thermal conductivity values of cyanobacterial and moss crusts were reduced by 41.0% and 31.1%, respectively, compared with the bare soil, while their thermal diffusivity was also reduced by 39.3% and 19.5%, respectively. At field capacity, cyanobacterial crusts reached the highest heat capacity (1.88 MJ/(m³·K)), followed by bare soil (1.77 MJ/(m³·K)), and moss crusts were the lowest (1.63 MJ/(m³·K)). The thermal conductivity of bare soil exceeded that of biocrusts by 63.2% on average, while its thermal diffusivity was 55.0% higher. Field monitoring further confirmed that the heat capacity of cyanobacterial crusts (1.30 MJ/(m³·K)) and moss crusts (1.49 MJ/(m³·K)) decreased by 21.2% and 9.7%, respectively, compared with bare soil (1.65 MJ/(m³·K)). Moreover, the thermal conductivity and thermal diffusivity of cyanobacterial crusts were significantly lower than those of bare soil, with reductions of 31.3% and 26.9%, respectively. In contrast, moss crusts displayed higher thermal conductivity (0.78 W/(m·K)) and thermal diffusivity values (4.96 × 10^-7 m²/s), which were 1.2 times higher than those of bare soil. Furthermore, biocrusts consistently suppressed all thermal properties (P < 0.001), according to typical rainfall events (32.1 mm). Moreover, field temperature monitoring revealed that the average soil temperatures of cyanobacterial crusts and moss crusts reached 4.83 and 5.11 °C, respectively, exceeding the bare soil temperature (4.48 °C) by 0.35 and 0.63 °C. However, the warming effect was strongly suppressed during wet periods with higher rainfall. The temperature difference between the biocrusts and bare soil decreased by an average of 79.0%, compared with the dry periods. In summary, the biocrusts effectively modulated near-surface soil thermal properties. Basic physicochemical properties were altered, such as its bulk density and total porosity, organic matter content, and field capacity, thereby reshaping the solid-liquid-air phase composition of surface soil. Collectively, the solid-liquid-air configuration systematically modulated the thermal properties for the heat retention within surface soil. Consequently, biocrusts can be expected to regulate the surface energy balance and ecosystem restoration. This finding can also provide a scientific basis to balance surface energy for ecological restoration practices in arid and semi-arid areas.

Biocrust (biological soil crust) is one special type of the most important surface layer cover in the arid and semi-arid regions. There is also a strong impact on the water balance of surface soil. The biocrusts can rapidly fill the degraded patches to stabilize on the surface soil. Moreover, the biocrusts can also change the physicochemical properties of the surface soil, such as the bulk density, total porosity, field capacity, and organic matter content. This study aims to clarify the influence of the biocrust layer (0-2 cm) and its covered soil (0-15 cm) on evaporation in the Loess Plateau. A comparison was also made on the evaporation among the bare sand, cyanobacteria crusts (cyano-crusts), and moss crusts. Indoor simulated experiments were carried out to in-situ monitor the soil moisture. The results showed that there was a great variation in the effects of the different types of biocrusts on soil evaporation. The cyano-crust layer promoted evaporation, whereas, the moss crust layer inhibited it. Specifically, the biocrust layer has significantly enhanced the evaporation at the low intensity of evaporation (150 W). The average evaporation rates of the cyano-crust layer (0.28 mm/h) and moss crust layer (0.30 mm/h) were 12.0% and 20.0% higher than those of the bare sand (0.25 mm/h), respectively. Furthermore, the average evaporation rate of the cyano-crust layer was similar to that of the bare sand at high evaporation intensity (275 W). While the moss crust layer (0.54 mm/h) was 12.9% lower than that on the bare sand (0.62 mm/h). On the undisturbed soil columns, the average evaporation rate of cyano-crusts (0.18 mm/h) was 1.2 times higher than that of bare sand (0.15 mm/h) under low soil moisture. While the average evaporation rate of moss crusts (0.19 mm/h) increased by 26.7%, compared with the bare sand (0.15 mm/h). The cyano-crusts (0.25 mm/h) and moss crusts (0.30 mm/h) facilitated the evaporation with an average evaporation rate of 8.7% and 30.4% higher than those of bare sand (0.23 mm/h) under high soil moisture. While the moss crusts shared the significant stage behavior. The evaporation rate first decreased by 12.2% in stages 1 and 2, compared with the bare sand. However, the evaporation rate of the moss crusts in stage 3 was 37.8% higher than that of the bare sand. Correspondingly, the soil moisture is also monitored at different depths of layers. The biocrusts (especially the moss crusts) significantly increased the soil moisture from 0 to 10 cm. But the soil moisture at 20 cm was significantly lower than that of bare sand. The soil moisture of cyano-crusts was 11.3% lower than that of bare sand at 0-20 cm. Whereas the soil moisture of moss crusts was 43.9% higher than that of bare sand at 0-10 cm on average. Nevertheless, the soil moisture of moss crusts was 31.5% less than that of bare sand at a depth of 20 cm. In conclusion, the evaporation rate of the surface soil covered by biocrusts was higher than that of the bare sand. The loss of surface soil water was accelerated significantly, even though the biocrust layer shared an evaporation-inhibiting. The finding can provide a strong reference for the soil water balance on the surface soil evaporation in arid and semi-arid regions.