Raw_data_N_adition_increased_SOC_PNAS.xlsx

Boreal forest soil organic carbon (SOC) plays a critical role in mitigating climate change, as these forests contribute around 45% of the total forest carbon sink. However, the effects of nitrogen (N) deposition – a major anthropogenic driver of ecosystem changes – on SOC accumulation remain poorly understood. While some studies report positive impacts of N addition on SOC sequestration due to enhance forest productivity and decrease in soil C release, others suggest neutral or negative effects due to negative effects of excess N and other nutrient limitations, reflecting ecosystem-specific responses and highlighting a need for holistic mechanistic insights. Thus, we conducted a long-term N addition experiment in a boreal forest to investigate whether and how N addition affects SOC, as well as how this effect is regulated by plant-soil-microbe synergies. We found that SOC significantly increased with N addition, driven by the combined responses of plants, soil, and microbes. Specifically, N addition enhanced plant nutrient uptake and photosynthetic capacity, leading to increased plant carbon input. N addition also induced soil acidification due to shifts in plant N preferences, which suppressed decomposition and altered microbial communities by enhancing microbial carbon use efficiency toward higher carbon stabilization. Our study provides a holistic mechanistic understanding of how N addition enhances SOC sequestration through synergistic plant-soil-microbe associations, offering important insights into nutrient management strategies for mitigating climate change.