Soil microbial community structure is unaltered by grazing intensity and plant species richness in a temperate grassland steppe in northern China

Abstract

Grazing intensity influences the productivity and sustainability of grassland systems through modifying aboveground biomass, soil microbes, and nutrient dynamics. Although heavy grazing is a key factor associated with grassland degradation in China and alleviating grazing intensity can assist grassland vegetation restoration, the influence of variable grazing intensity on belowground processes is less understood. We examined the effects of four grazing intensities (0, 1.00, 1.43 and 2.33 sheep-units ha−1 year−1) applied during five growing seasons on soil microbial biomass and community structure of a grassland ecosystem in Hebei Province, China. Within each grazing intensity, vegetation and soil were sampled in three patch types: low and high plant species richness and one composed of the dominant species (i.e., Leymus chinensis [Trin.], Tzvel. and Carex duriuscula C. A. Mey.). We sought to clarify whether soil microbial communities are influenced by an interaction between grazing intensity and patch type. Grazing significantly (P < 0.05) decreased soil microbial biomass and increased the ratio of bacterial-to-fungal biomass relative to the non-grazed control, but differences were not significant among the grazing intensities. Patch type did not significantly impact microbial biomass (P = 0.12), but the interaction between patch type and grazing intensity was significant (P < 0.05). The highest microbial biomass value (19.20 ± 0.51 μg · g−1) was found in non-grazed, low diversity patches. In addition, principal components analysis revealed that grazing intensity explained 25.42 and 54.53% of the variation among bacterial and fungi communities, respectively. Grazing, regardless of intensity, also resulted in bacterial communities distinct from the non-grazed control. Redundancy analysis also revealed that soil pH and organic carbon had the strongest influence on bacterial community structure, soil electrical conductivity and NO3−-N primarily influenced fungi community structure. Collectively our results indicate that microbial communities in semiarid grasslands can remain structurally stable even when subjected to persistent environmental change imposed by grazing pressure.