Soil moisture shifts under warming are associated with divergent topsoil nitrogen responses in permafrost-affected ecosystems.

Understanding how soil nitrogen (N) responds to climate warming is critical for predicting permafrost carbon-climate feedbacks, yet long-term N responses and their underlying mechanisms remain elusive. Here we show that divergent topsoil N responses to warming are associated with warming-induced shifts in soil moisture. Twelve-years of high-level warming increases topsoil N stocks in mesic alpine meadows but triggers substantial topsoil N losses in waterlogged swamp meadows. Enhanced topsoil N retention in the alpine meadow coincides with warming-driven soil drying, reduced gross protein depolymerization, and elevated microbial N immobilization. By contrast, declining topsoil N stocks in the swamp meadow coincide with elevated soil moisture, accelerated N transformation rates, greater plant N accumulation, and elevated potential N losses. A meta-analysis further indicates that such moisture-associated divergent N responses are widespread across permafrost-affected ecosystems. These findings highlight the necessity of hydrological shifts when projecting N cycling and permafrost-climate feedbacks under climate warming.