Grazing Density Effects on Cover, Species Composition, and Nitrogen Fixation of Biological Soil Crust in an Inner Mongolia Steppe

Abstract

Biological soil crusts (BSCs) are important in many arid and semiarid ecosystems for their abilities to fix atmospheric nitrogen (N) and stabilize surface soil. Grazing disturbance has a profound influence on abundance, species composition, and ecological functioning of BSCs. To elucidate the effects of grazing on BSCs in Inner Mongolia grasslands, an investigation was conducted in a typical steppe that had previously been subjected to long-term grazing with six grazing densities (control: 0 sheep ? ha21, very light: 4 sheep?ha21, light: 8 sheep?ha21, medium: 12 sheep?ha21, heavy: 16 sheep?ha21, and very heavy: 20 sheep?ha21). Cover, species composition, potential N-fixing activity, and potential N input of BSC indicate that long-term grazing significantly reduced the importance of BSC in N input and soil stabilization. Such reductions were highly related to grazing density. Very light grazing had no significant effect on the role of BSC in soil stabilization, but resulted in a 13.3% reduction in BSC N input potential. Heavy and very heavy grazing led to a decrease of potential N input by one order of magnitude, and a decline of BSC function via a shift from high coverage of an attached group-dominated BSC community to a low coverage of a vagrant group-dominated community. Constraining grazing level to a very light density—and to a lesser extent, a light grazing density—is likely a preferred practice for conserving BSC and the ecological services it provides in N fixation and soil stabilization.