Litter cover promotes biocrust decomposition and surface soil functions in sandy ecosystem

Ecological restoration of sandy inland ecosystems is important for achieving global sustainability. In the world's semi-arid regions, soil crusts play crucial roles in maintaining ecosystem functioning. However, the true extent of soil quality improvement during the development of crusts is an issue not solved. In this study, four development stages of natural soil crusts, i.e., physical crusts (PC), biocrusts (BC), litter covered biocrusts (LBC) and litter crusts (LC) were selected in a semi-arid sandy ecosystem, along with a bare sandy land (BSL) as control area, to evaluate soil physicochemical properties at different soil depths. The coverage of litter (mainly leaves of Populus simonii) increased soil moisture and reduced soil bulk density. Compared with BC, the content of total soil organic matter (SOM) decreased in LBC by 13.83% and increased in LC by 36.57%. In contrast with BC, LC promoted a significant increase in soil nutrients, such as total nitrogen (30.30%), total phosphorus (46.89%) and available potassium (34.40%) in the topsoil layer (0–2 cm). Besides, LC contained higher clay and silt contents (10.47% and 29.81%) and lower sand content (−1.02%) than BC. In the 0–10 cm soil layer, the D (fractal dimension of the soil particle size distribution) of LC was the largest, with a value 5.71%, 6.1%, 2.44% and 0.93% higher than D in BSL, PC, BC and LBC, respectively. These findings reveal that litter covering facilitate the disintegration of BC, which further forms LC, and these processes clearly promote the enhancement of soil quality under sandy semi-arid conditions. Our findings are particularly important for predicting the transformation processes of sandy soil crusts and are of interest in ecological restoration programs.