A Comprehensive Review of Natural Nitrogen Deposition and Artificial Addition in Boreal Forests: Effects on carbon sequestration, the soil carbon cycle, under changing climatic conditions

Boreal forests constitute one of the largest terrestrial carbon (C) reservoirs globally, storing substantial amounts of C in vegetation, soils, and permafrost. Nitrogen (N) availability exerts a major constraint on productivity and biogeochemical cycling in these ecosystems, rendering them particularly sensitive to changes in atmospheric N inputs under global change. Both natural N deposition and artificial N addition have intensified over recent decades, resulting in complex and scale-dependent ecological consequences. Here, we synthesize current knowledge on how natural N deposition and experimental or management-driven N addition influence boreal forests ecosystems, with a particular focus on C sequestration, soil C cycling, and associated ecological processes. We conducted a systematic literature survey of studies published between 1997 and 2025, compiling quantitative evidence on ecosystem responses spanning tree physiology and growth, understory vegetation dynamics, soil microbial communities, soil organic matter mineralization, organic and mineral soil horizons, and dissolved organic matter production and export. Collectively, these studies demonstrate that modest N inputs can enhance forest productivity and C accumulation in N-limited boreal systems, whereas sustained or high N loading often induces nonlinear responses, including altered species composition, shifts toward nitrophilic understory communities, soil acidification, and increased risks of C and N losses. These effects are further modulated by interactions with climate warming, hydrological change, and permafrost dynamics, which jointly regulate N availability and microbial activity. We highlight key uncertainties related to N input spatial representativeness, N form and dose, and long-term ecosystem feedback. Additionally, we identify critical research priorities for improving predictions of how boreal forests may respond to future N deposition, which are crucial for formulating boreal forests management strategies aimed at enhancing C sequestration capacity under increasing N deposition, thereby contributing to net emission goals. Ultimately, future research should focus on long-term monitoring and multi-scales assessments of N deposition impacts, identifying ecological thresholds and mechanistic ecosystem response pathways, and should integrate N management with policy and conservation strategies in boreal forests.