Tree Saplings Readily Take Up NO₂ During Foliar ¹⁵NO₂ Fumigation

Forest canopy nitrogen dioxide (NO₂) uptake is recognised as a potentially important N input. However, how N addition influences foliar NO₂ uptake and allocation of assimilated NO₂ in different tree organs has been poorly quantified. We conducted a ¹⁵NO₂ fumigation experiment using 3-year-old saplings of Fraxinus mandshurica, Pinus koraiensis, Quercus mongolica (Q. mongolica) and Larix gmelinii (L. gmelinii) and measured assimilation into different tree organs. Total ¹⁵N recovery ranged from 9% to 74% in the light and varied with species. With soil N addition, ¹⁵N recovery increased in Q. mongolica but decreased in L. gmelinii, which we attributed to opposite responses of stomatal density and leaf area between these two species to soil N addition. These indicate that foliar NO₂ uptake amounts are likely associated with tree N demand. Leaves were the dominant sink for N derived from ¹⁵NO₂ and accounted for 60% to 97% of total recovery, suggesting that most foliar-assimilated NO₂ is initially stored in leaves. Our study indicates that tree canopies could assimilate 0.51 ± 0.07 kg N ha^-1 y^-1 atmospheric NO₂ in temperate forests, which provides references for model large-scale modelled estimates of canopy NO₂ uptake. These data improve the understanding of N cycling between atmosphere and forest ecosystems.