# Warming‐Induced Effects on Microbial Communities and Nitrogen Cycling Capacity in Tundra Litter Are Modulated by Herb Abundance and Litter Quality

## TL;DR

Warming in the Arctic tundra changes plant litter and indirectly affects soil microbes and nitrogen cycling, which could boost plant growth and ecosystem respiration.

## Contribution

This study reveals how warming indirectly alters microbial nitrogen cycling in tundra litter through shifts in herb abundance and litter quality.

## Key findings

- Warming indirectly modifies microbial nitrogen cycling through changes in herb dominance and litter mass.
- Warming strengthens connections between litter nitrogen processes and soil δ15N signatures.
- Vegetation shifts from warming may intensify nitrogen cycling with feedbacks on plant growth and respiration.

## Abstract

Climate warming is changing tundra vegetation in the Arctic, with implications for plant litter properties. Warming may thus modify bacterial and fungal communities and their nitrogen (N) cycling capacity in the litter layer, which in turn can affect plant N availability. To address potential warming effects, we characterized the responses of bacterial and fungal communities and their genetically encoded capacity for inorganic N‐transformations in the litter layer, as well as 15N natural abundance in the underlying soil layer as an integrated measure of N processes in the soil, in 16 long‐term alpine and Arctic tundra warming experiments distributed across 12 circumpolar locations. Although abundance, diversity, and composition of microbial communities were structured by the local conditions rather than experimental warming, warming indirectly modified microbial communities and their capacity for N transformations through changes in litter quality. Specifically, experimental warming resulted in stronger connections between the capacity for nitrification, denitrification and N‐fixation in the litter and the δ15N signature in the soil. These warming‐induced connections were mainly mediated by increased dominance of herbs but also increased litter mass. These findings suggest accelerated inorganic N cycling in the litter layer with warming, particularly coupled to local abundance of herbs, which can create positive feedback on plant growth as well as ecosystem respiration. Thus, microbial communities in the litter may contribute to an intensification of ongoing vegetation shifts across the tundra biome.

Climate warming affects tundra vegetation, with implications for plant litter and associated bacterial and fungal communities. This study examined litter bacterial and fungal communities and their nitrogen (N) cycling potential across 16 long‐term warming experiments in arctic and alpine tundra. While local conditions primarily shaped microbial communities, warming indirectly affected microbial N‐cycling potentials through changes in herb dominance and litter mass, resulting in enhanced links between litter N cycling potentials and δ15N in the underlying soil. Our results suggest that warming‐driven vegetation changes may intensify N‐cycling with potential positive feedbacks on plant growth and ecosystem respiration across the tundra biome.