Nitrogen addition accelerates aboveground biomass sequestration in old-growth forests by stimulating ectomycorrhizal tree growth.

Examining whether nitrogen (N) enrichment promotes secondary tree growth in both young (YF) and old-growth forests (OF) is crucial. This will help determine how N addition influences plant carbon sequestration across successional phases in temperate forests. We conducted an eight-year N addition experiment (0, 25, 50, 75 kg N ha^-1 yr^-1) in YF and OF in northeast China to investigate the effects of enhanced in situ N deposition on tree growth. Our results indicated that N addition accelerated the accumulation of annual mean aboveground biomass (ΔAGB) of trees only in OF. Specifically, for the species co-occurring in both YF and OF plots, their ΔAGB in OF peaked under the medium N treatment (3.69 Mg ha^-1 yr^-1), which was 2.3 times higher than that of YF (1.58 Mg ha^-1 yr^-1). Regarding mycorrhizal types, only the ΔAGB of EcM-associated trees peaked under the high N treatment (2.81 Mg ha^-1 yr^-1), increasing by 126.6% compared to the control (1.24 Mg ha^-1 yr^-1). This increase in biomass primarily came from large trees with a DBH ≥15 cm, most of which are EcM -associated species, such as Pinus koraiensis. In conclusion, continuous N addition increases nutrient supply and alleviates N limitation in old growth forest, leading to faster biomass accumulation. The growth of large-diameter trees with EcM-associated may contribute significantly to aboveground biomass accmulation under N addition. Nutrient limitation is dependent on stand age, mycorrhizal type and size, so these factors must be considered when assessing forest nutrient limitations.