Abstract: Though livestock grazing is the most common grassland management, its effects on species interactions and spatial patterns of soil microbial communities are still under-investigated. By using the nested sampling strategy, we investigated soil prokaryotic and fungal communities in alpine steppes being either grazed or fenced on the Qinghai-Tibet Plateau (QTP), focusing on their species interactions indicated by the co-occurrence network and spatial patterns based on changes in network node numbers along the increasing sampling area. Our results showed that grazing consistently enhanced the network complexity of the soil prokaryotic community in different sites, possibly because grazing enhanced environmental heterogeneity and resource pulses while restricting microbial dispersal through physical disturbances. In contrast, grazing inconsistently influenced the fungal network complexity, likely due to the deterministic, dominantly assembled fungal community and heterogeneous grazing effects on environmental properties in different sites. Different from the spatial pattern for the community richness, the richness for interacted or co-occurred species tended to increase along the sampling area before 64 m² for fungi and 256 m² for prokaryotes, respectively, but then decreased after these tipping points, no matter with grazing or fencing. These results demonstrate that the grazing effects on microbial network complexity depend on community assembly mechanisms, and changes in the network node number along the sampling area presented a reverse U shape spatially. These findings provide novel clues for a comprehensive understanding of grazing consequences in alpine grasslands.

Key words: grassland grazing, spatial pattern, co-occurrence network, microbial community assembly, alpine steppe