Intermediate grazing intensities by sheep increase soil bacterial diversities in an Inner Mongolian steppe

Abstract

Ungulate grazing is known to play a crucial role in regulating nutrient cycling and controlling plant community structure and productivity in grassland ecosystems. However, little is known about the effects of grazing intensities on soil bacterial community structure and diversity, particularly at the long-term scale. In this study, we measured plant biomass and diversity, soil characteristics and bacterial community structure, and diversity in a 16-year field experiment that had four grazing intensity treatments (non-grazed, CK; low-intensity grazing (LG), moderate-intensity grazing (MG), and high-intensity grazing (HG)) in an Inner Mongolian typical grassland. The CK, LG, MG, and HG sites were grazed by 0.00, 1.33, 4.00, and 6.67 sheep ha-1, respectively. Bacterial community structure and diversity under grazing intensity treatments were assessed with PCR amplification of DNAs extracted from soils and denaturing gradient gel electrophoresis (DGGE) separation. The results showed that the CK soil had higher moisture, organic C, NH4 +-N, and NO3 --N concentrations than grazed soils, and the HG treatment had the lowest plant biomass and diversity across all the treatments. Principal component analysis of DGGE patterns showed that the LG and MG treatments were different from the CK and HG treatments. In addition, soil bacterial diversities in the LG and MG treatments were significantly higher than those in the other treatments. The relationships between environmental variables and soil bacterial community structure were assessed using redundancy analysis, and we found that soil moisture content, Artemisia frigida biomass, and pH were the best indicator of the changes in soil bacterial community structure among all the treatments. Overall, our results indicated that intermediate grazing intensities (LG and MG) increased soil bacterial diversities, and along with previous studies in this area, we suggested the MG treatment was the most suitable management practice in the Inner Mongolian steppe, not only supporting greater livestock amounts but also harboring greater bacterial diversity.