Fencing Decreases Microbial Diversity But Increases Abundance In Grassland Soils On The Tibetan Plateau

Fencing usually increases soil organic carbon and nutrients and eventually enhances soil microbial diversity; however with different or even conflicting results, the mechanisms underlying the diversity reduction remain unclear. Aiming to reveal the mechanism of diversity reduction, we explored the soil microbial diversity and community structure in fenced areas (5-and 10-years enclosure) and grazing during a growing season in arid and semiarid steppe, using Illumina sequencing of 16S and 18S rRNA genes. The results revealed that fencing and season both substantially increased soil total organic carbon (TOC), while the enhancing TOC substantially reduced soil pH. Fencing significantly increased soil bacterial and eukaryotic abundance due to the enhancing TOC, which was indicated by positive correlation (p < .05). Contrastingly, fencing significantly decreased soil bacterial and eukaryotic diversity. Our results further revealed that fencing- and season-driven change in soil water content and soil pH played key roles in reducing the bacterial and eukaryotic diversity, respectively. Distance-based linear model demonstrated that fencing dominantly drove the soil bacterial and eukaryotic community variations by explaining 15.64 and 24.88%, respectively, and redundancy analysis showed that the fencing effect was dependent on growing months. Bacteria were dominated by Cyanobacteria and Proteobacteria, and eukaryotes were dominated by Streptophyta, Ascomycota and Basidiomycota. Cyanobacteria relative abundance remained stable from May to September in fencing but substantially increased in grazing. Our findings offer a new insight into the mechanism of the soil microbial diversity reduction, and reveal the season-dependency of fencing effect in arid and semiarid grasslands.