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

<p dir="ltr">Global forest ecosystems broadly benefit from international policies involving the addition of artificial N-compound fertilizers, yet the mechanisms underlying how this affects carbon cycling and nutrient dynamics in boreal forests remain poorly understood. To redress this knowledge gap, in this review, we synthesize findings on the effect of nitrogen (N) addition on the above- and below-ground components of boreal forests. N deposition alters forest microbial community structures, thereby influencing nutrient cycling processes. The effects of N addition are modulated by temperature, precipitation, soil properties, and forest community characteristics, and complex interactions can arise due to climate change and other environmental drivers. By integrating data from multiple field experiments, we establish that N input at low concentrations can enhance tree growth and carbon uptake, whereas excessive N deposition may suppress plant growth and exacerbate soil acidification. This information is crucial for formulating forest management strategies aimed at enhancing carbon sequestration capacity under increasing N deposition, thereby contributing to carbon peaking and neutrality goals. In closing, we highlight remaining knowledge gaps and discuss future research directions in this field.</p>