The lower water release capacity of biocrusts under higher soil water suction is beneficial for drylands

Water retention capacity (WRC) of soil is the ability of soil to hold and retain water, which reflects soil water availability and soil drought resistance. Biological soil crusts (biocrusts), the ubiquitous living cover in drylands, regulate water holding capacity (WHC) by changing surface soil physico-chemical properties. However, to date, the water release capacity of biocrusts remains unclear, which hinders a deep understanding of the WRC of biocrusts. Therefore, we examined the WRC by analyzing the WHC and water release capacity of five types of biocrusts at different successional stages: light cyanobacteria (LC), dark cyanobacteria (DC), cyanobacteria mixed with a few mosses (CM), moss mixed with small patches of cyanobacteria (MC), and moss (MS) biocrust. Bare soil (CK) was used as the control. The results showed that there was no significant difference in WHC between CK and the five types of biocrusts. However, biocrusts displayed different water release capacities at different soil water suctions. Biocrusts maintained a higher water release capacity when the soil water suction ranged from 200 cm H2O to 800 cm H2O (the soil volumetric water content (theta(v)) ranged from 32% to 22%); furthermore, there were no significant differences in the water release capacity of different types of biocrusts. The biocrusts exerted a lower water release capacity compared to bare soil when the soil water suction ranged from 800 cm H2O to 3000 cm H2O (theta(v) ranged from 22% to 11%), which was due to the increases in the > 0.25 mm aggregate content and the soil organic matter (SOM) contents. Biocrusts exhibited a better WRC due to their lower water release capacity under relatively dry conditions (11% < theta(v) < 22%), which enables biocrusts to maintain longer wet duration and benefits their development. Hence, good development of biocrusts is considered beneficial for the restoration, stability and sustainability of drylands with limited water resources are limited.