Effects of moss-dominated biocrusts on surface soil-water movement parameters in the Chinese Loess Plateau

Abstract: Biological soil crusts (biocrusts) are the common living skin of the land face in arid and semiarid regions. They greatly change the hydropedological features and soil-water movement of surface soil. But the influencing mechanism is still lacking, especially on water infiltration and retention. Taking the Loess Plateau of China as a study area, this study aims to explore the effects of moss-dominated biocrusts on the fundamental parameters in the surface soil-water movement, with emphasis on the characteristics of water infiltration and water retention. The moss biocrusts (naturally developed for > 30 years) and uncrusted soil formed on the aeolian sandy and loess soil were firstly collected in the Liudaogou watershed in the northern Loess Plateau. Afterwards, four treatments were carried out to measure the parameters of Boltzmann transformation, soil water diffusivity, infiltration, specific water capacity, and unsaturated hydraulic conductivity on-site or in the laboratory after sampling. A comparison was finally made on the soil-water movement parameters between the biocrusts and uncrusted soil. The results showed that the biocrust layer impeded the horizontal movement of surface soil water both in the aeolian sandy and loess soil. Specifically, the Boltzmann transformation parameter and soil water diffusivity of biocrusts decreased compared with the uncrusted soil. Moreover, the biocrusts highly decreased the water infiltrability of surface soil, subsequently the whole soil profile. The initial, stable, and average infiltration rates of biocrusts, the cumulative infiltration amount and unsaturated water conductivity of biocrusts decreased compared with the uncrusted soil. Additionally, the mean values of soil water parameters (including saturated water content, field capacity, wilting point, gravitational water content, available water content, and readily available water content) and specific water capacity at soil suction of 100 kPa of biocrusts were 40.9%-1 233.3% and 7.4%-1540.5% higher than that of the uncrusted soil, respectively, implying that the biocrusts greatly improved the soil water holding capacity and availability. Most importantly, the low infiltrability and high water holding capacity of moss biocrusts were closely related to the contents of sand and silt, as well as the organic matter in the biocrust layer. The biocrusts that developed on the aeolian sandy soil presented weaker effects on the soil infiltrability, but stronger effects on the water retention and availability, compared with the biocrusts on the loess soil. The reason was that there was a 1-2 cm thick layer with particular properties (e.g., soil texture, organic matter content, and bulk density) in the moss biocrusts. Therefore, the availability of soil water increased greatly, whereas, the diffusivity and infiltrability decreased significantly in this case. Consequently, the biocrusts can possibly make the upper soil (such as 0-50 cm) retain more water, while inhibiting the water infiltration into the deep soils after rainfall. Subsequently, the availability of soil water can be altered for the water use strategy of vegetation in fragile arid and semiarid ecosystems of the Loess Plateau.