Warming winter disrupts mycorrhizal phenology and plant-fungal nutrient cycling.

By experimentally advancing snowmelt in a sub-alpine meadow, we demonstrate that warming winters cause phenological asynchrony between host plant and arbuscular mycorrhizal fungi. Early snowmelt caused early onset of greening and diminished extraradical hyphal production by mycorrhiza. Plant-fungal asynchrony was accompanied by smaller pools of available P and N, suggesting early, direct nutrient adsorption by plants lowered host investment in mycorrhizal fungal partners. The mycorrhizal symbiosis was weakened during a critical period of nutrient exchange. These findings point to the vulnerability of belowground mutualisms to warming winters, and highlight the need to incorporate fungal phenology into predictions of ecosystem responses to climate change. Future work scaling early snowmelt effects on belowground microbes to watershed N & P dynamics, or loss of N to the atmosphere, may provide a clearer picture on the relationships between warming winter, nutrient cycling, and plant-fungal asynchrony. The AM fungal community was highly variable across the growing season, indicating that mycorrhizal responses to warming are context-dependent on seasonal variation of soil resources and trait-based community assembly. Together, our results show that warming winters disrupt the seasonal timing of plant fungal interactions, potentially triggering nutrient-limitation feedbacks with potential consequences that extend from soil microbial processes to watershed-scale N and P cycling. By revealing both the sensitivity of belowground mutualisms to warming and the strong seasonal shifts in AM fungal community structure, this study underscores the importance of integrating fungal phenology, microbial functional traits, and nutrient-cycling mechanisms into predictions of ecosystem stability under continued climate change.