Effects of typical biocrusts on soil thermal properties and temperature in a small watershed of the Chinese Loess Plateau

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.