Divergent effects of snow exclusion on microbial variables across aggregate size classes

Projected changes in winter climate can have large implications for the functioning of terrestrial ecosystems. In particular, increased soil frost associated with reduced insulating snow cover can affect the structure and activity of soil microbial communities in cold ecosystems, but little known about the variability of these effects among the fractions of soil aggregates. We used a snow-exclusion experiment to examine the influence of increased soil frost on microbial biomass and activity in aggregate fractions in a Tibetan alpine spruce forest. We measured the concentrations of phospholipid fatty acids (PLFAs) and the activities of extracellular enzymes involved in carbon and nutrient cycling in soil aggregate fractions (<0.25 mm, 0.25–2 mm and >2 mm) during early thawing years of 2016 and 2017. We found that snow exclusion reduced the concentrations of PLFAs (total, bacterial and fungal) and the activities of enzymes (β-glucosidase, β-N-acetyl-glucosaminidase and acid phosphatase) in three aggregate fractions due to severe abiotic environments, but did not affect the microbial community or enzymatic stoichiometry. Although they varied across the aggregate fractions, soil microbial variables responded to snow exclusion significantly only in the small macroaggregates (0.25–2 mm), which indicated that aggregate size may have a stronger effect than did snow exclusion on microbial variables. Notably, A significant decrease of PLFAs and enzymatic activities in the small macroaggregates under snow exclusion revealed that soil microbes in this fraction were more sensitive to changes in snow cover than in the other aggregate fractions. These findings highlight the ecological importance of microbial processes in aggregates in Tibetan forests experiencing large decreases in snowfall.