Changing snow conditions and shrub expansion alter above- and belowground seasonal dynamics in alpine grasslands

Climate change is disproportionately impacting mountain ecosystems, leading to large reductions in winter snow cover, earlier spring snowmelt and widespread shrub expansion into alpine grasslands. Yet, the combined effects of shrub expansion and changing snow conditions on abiotic and biotic soil properties remains poorly understood. We used complementary field experiments to show that reduced snow cover and earlier snowmelt have effects on soil microbial communities and functioning that persist into summer. However, ericaceous shrub expansion modulates a number of these impacts and has stronger belowground effects than changing snow conditions. Ericaceous shrub expansion did not alter snow depth or snowmelt timing, but did increase the abundance of ericoid mycorrhizal fungi and oligotrophic bacteria, which was linked to decreased soil respiration and nitrogen availability. Moreover, by combining molecular sequencing, enzyme assays, greenhouse gas flux measurements, soil biogeochemical analyses, and ¹⁵N labelling, we show that reduced winter snow cover and shrub expansion alter the seasonal dynamics of plant growth (i.e., net ecosystem exchange and plant N-uptake), with important consequences for the seasonal dynamics of soil microbial communities, their functioning, and alpine biogeochemical cycles. In conclusion, our findings suggest that changing winter snow conditions have cross-seasonal impacts on biotic and abiotic soil properties, but shifts in vegetation can modulate belowground effects of future alpine climate change.