Abstract
Soil fungi are key regulators of forest carbon cycling and their responses to global change have effects that ripple throughout ecosystems. Global changes are expected to push many fungi beyond their environmental niches, but there are relatively few studies involving multiple, simultaneous global changes. Here, we studied soil fungal diversity, community composition, co-occurrence patterns, and decomposition gene responses to ten years of soil warming, nitrogen additions, plus their interaction. We specifically examined whether there were fungal community characteristics that could explain changes in soil carbon storage and organic matter chemistry in chronically warmed and fertilized soil. We found that fungal communities in warmed soils are less diverse, shift in composition, favor hyper-dominance by just a handful of mycorrhizal fungal species, and harbor lower manganese peroxidase but higher hydrolytic enzyme encoding gene potentials. Nitrogen additions did not significantly affect fungal composition but also reduced fungal diversity and favored over-dominance by a unique set of mycorrhizal species. Warming alone and warming × nitrogen additions also reduced negative but increased positive fungal co-occurrence probabilities, promoting species co-existence. Negative fungal co-occurrence was positively correlated to soil carbon content while the proportion of fungal hydrolytic enzyme encoding genes was negatively correlated. This may reflect fungal life history tradeoffs between competition (e.g., reduced negative co-occurrence) and resource acquisition (e.g., higher abundance of hydrolytic enzyme encoding genes) with implications for carbon storage.