2152

It has been recognized recently that trees can assimilate NO₂ directly through leaf stomata. Both laboratory and field studies have measured the foliar NO₂ deposition velocity, which could be determined by some environmental factors, e.g. light irradiation intensity, ambient NO₂ concentration, and leaf characteristics. However, the NO₂ uptake capacity and allocation of foliar uptake NO₂ under these environmental factors remain unclear. To clearly understand the foliar NO₂ uptake process and refine the forest NO₂ uptake models, we conducted a dynamic ¹⁵NO₂ fumigation experiment.

We selected Fraxinus mandshurica (F. mandshurica), Pinus koraiensis (P. konraiensis), Quercus mongolica (Q. mongolica), and Larix gmenilii (L. gmenilii) saplings, four dominant tree species in temperate forests of northeastern China, as our experimental materials. Meanwhile, we chose a pair of broad-leaved and coniferous tree species (F. mandshurica and P. konraiensis) to perform fumigation experiment under dark/light irradiation and another pair (Q. mongolica and L. gmenilii) to perform fumigation experiment with soil N addition. All saplings were dynamically fumigated with 50 ppb ¹⁵NO₂ for 8 h and destructively sampled immediately after fumigation. We rinsed the samples surface with purified water, dried and grinded all samples, then measured the ¹⁵N abundance in leaves, twigs, stems and roots with EA-IRMS.

The results showed that tree saplings can absorb NO₂ under both dark and light irradiation treatments. The total ¹⁵N recovery ranged between 30 to 80% under the light condition in all species. Under the dark condition, the total ¹⁵N recovery were (29.8±9.16) % and (1.1±0.47) % for F. mandshurica and P. konraiensis, which were significantly lower than under the light condition, (59.6±5.2) % and (8.8±2.5) %, respectively. With the soil N addition, the total ¹⁵N recovery in Q. mongolica ((56.2±8.8) %) were significantly larger than non-N addition ((27.6± 4.8) %), while L. gmenilii showed the opposite result that the total ¹⁵N recovery ((31.7±7.8) %) significantly decreased, compared to that without N addition ((73.6±4.3) %). These results are likely attributed to different amount of N demand for different tree species, more N needed for Q. mongolica than L. gmenilii. Moreover, coniferous species could assimilate more N through foliar uptake than broad-leaved species, probably due to bigger leaf surface areas of coniferous trees. After 8 h fumigation, the largest proportion of ¹⁵NO₂ was recovered in leaves in all species and treatments, accounting for 60-97%, which indicates that NO₂ stays in leaves in a short-term period after foliar assimilation. However, further studies are needed to explore the transformation of foliar incorporated NO₂ to other organs in a long-term scale.

This study quantified the foliar NO₂ uptake capacity of different tree species and figured out the effects of light irradiation and soil nitrogen availability on foliar NO₂ uptake. Our results would provide references for the model estimation of canopy NO₂ uptake magnitude at a regional scale.