Abstract: Watersoluble inorganic ions such as NH₄⁺ are the major components of fine particles (PM_2.5). The absorption, distribution and utilization of NH₄⁺ on the leaf surface of Populus tomentosa and P. deltoids × P. nigra seedlings were studied by using the ¹⁵N trace technique. Our results showed that ¹⁵NH₄+ was absorbed rapidly by P. tomentosa and P. deltoids × P. nigra leaves and the rates of absorption all peaked 6 hours after solution application. Significant differences were observed in the absorption rate between two poplar species, with the maximum absorption rate of P. tomentosa being 3 times that of P. deltoids × P. nigra. The content of ¹⁵N in leaves peaked at the same time (24 hours after treatment) in the two species, but the maximum amount of ¹⁵N of P. tomentosa was about 4 times that of P. deltoids × P. nigra. Foliar application of ¹⁵Nlabeled ammonium sulfate (^15NH₄SO₄) solution led to a significant increase of total nitrogen level in the leaves of both poplars. The leaf total N contents of P. tomentosa and P. deltoids × P. nigra were 1.26 and 1.36 times that of control 7 days after treatment, respectively. The Ndff values of different organs showed that the 15N absorbed by functional leaves was transported to both the upper and lower parts of the plants, which was mainly stored in stems of P. tomentosa, but transported to roots of P. deltoids × P. nigra. Foliar application of ¹⁵NH₄SO₄ enhanced the net photosynthetic rate of both poplars to varied degrees 7 days after treatment, but no significant effect on stomatal conductance was observed.

Key words: MaxEnt, giant panda, species distribution model, Biomod2