Effects of livestock grazing intensity on soil biota in a semiarid steppe of Inner Mongolia

Intensive livestock is known to significantly affect soil physical and chemical parameters in steppe ecosystems. However, the effects on soil biological parameters still remain unknown. We hypothesized that intensive grazing would significantly decrease the size and diversity of soil biota due to deterioration of the soil environment and reduction in vegetation cover, while the adapted grazing intensity would improve the biological parameters. Soil samples were collected from five sites with different grazing intensities and history in a semiarid steppe of Inner Mongolia in August 2005. Two sites were longterm ungrazed since 1979 (UG79) and 1999 (UG99), one had been moderately grazed in winter (WG), one continuously grazed moderately (CG) and one longterm site was heavily grazed (HG). Soil microbial biomass carbon (C), basal respiration (BR), catabolic diversity of soil microbial communities, protozoa and nematodes abundance were measured. Soil physicochemical variables were also measured to establish the relationships between soil biological parameters and key soil physical and chemical properties. Soil microbial biomass C, BR, biomass specific respiration (qCO(2)) and soil protozoa abundance were significantly lower at the HG site compared to the UG79 site, but no clear differences were found in the other sites. However, soil nematodes abundance increased with increasing grazing intensity, and the abundance of soil amoeba were greater in CG than in the other sites. Principal component analysis (PCA) of Biolog data revealed large differences in catabolic capacity of soil microbial communities between UG79, HG and UG99, WG, CG. However, Shannon's diversity index did not indicate marked effects of grazing intensity on substrate catabolic community structure. In conclusion, heavy grazing negatively affected soil microbial biomass, activity and protozoan abundance, but positively influenced soil nematodes abundance and did not affect soil microbial catabolic diversity. Based on these results, CG may provide an appropriate grazing intensity to be used in the long term in the semiarid steppe of Inner Mongolia.