Plant litter inputs increase microbial metabolic capacity more than warming in High-Arctic soils

Global warming has accelerated vegetation expansion across high northern latitudes, reshaping tundra landscapes and altering the balance between carbon uptake and release. As shrubs and other plant functional groups increase in biomass and distribution, the quantity and quality of plant-derived carbon entering soils through root exudates, litter deposition, and rhizosphere processes are expected to change substantially. However, the consequences of these enhanced carbon inputs for soil microbial activity, nutrient cycling, and other key functional processes remain poorly studied, particularly in remote Arctic regions where logistical constraints limit long-term ecological research. This knowledge gap is especially critical given that many of these areas are simultaneously experiencing rapid permafrost thaw, a process that can further modify soil hydrology, organic matter accessibility, and microbial metabolism. Understanding how vegetation expansion interacts with permafrost degradation to shape soil function is therefore essential for predicting future carbon–climate feedback in Arctic ecosystems. To address this knowledge gap, we conducted a four-year field experiment in the High-Arctic (northern Greenland) to test how plant litter inputs affect the microbial metabolic potential and functional processes in the active layer and thawing permafrost soils. For this, we simulated three distinct scenarios: 1) litter amendment effects on active layer soils, 2) permafrost thawing without litter amendment, and 3) permafrost thawing with litter amendment. Our results showed that plant litter inputs had a stronger effect than permafrost thawing (as a proxy for warming) alone, significantly increasing the abundance of genes involved in the breakdown of complex carbon substrates (cellulose, hemicellulose, pectin, and chitin), thereby altering carbon cycling pathways. Litter also increased the abundance of genes involved in nitrogen cycling, ultimately enhancing microbial growth and respiration rates and promoting overall microbial metabolic activity. Our four-year field experiment demonstrates that vegetation expansion—through litter inputs—drives more profound changes in microbial metabolic capacity than warming, with potentially direct implications for carbon dioxide and methane production. Monitoring vegetation-driven transformations of Arctic soil microbiomes is essential for predicting high-latitude greenhouse gas feedback in a warming climate.