N Utilization And Wood Structure Changes In Fast-and Slow-growing Populus Species Under Nitrogen Fertilization

Nitrogen fertilization can increase growth rate and productivity of trees such as Populusspecies. Wood properties can also been changed by short-term nitrogen treatment. We usedthree N treatments in which the level of ammonium nitrate was different in this study to showthe response of the two Populus species.Rooted cuttings of the fast-growing Populus species Populus popularis(Pg) and theslow-growing species Populus alba×P. glandulosa(Pp) were produced and planted in agreenhouse which conditions can be controlled strictly.4months later, three N treatmentswith different ammonium nitrate levels in LA nutrient solution were used. The NH4NO3levelwas adjusted to0mM (limiting N treatment),5mM (adequate N treatment) and10mM(luxuriant N treatment). After7weeks, the plants were harvested. Only the part of the plantwhich grew after the N treatment beginning were used for further experiments.The study reached conclusions as follows:(1) Nitrogen fertilization can increase growth rate and productivity of the two Poplar species.Increasing nitrogen supplied lead to the increase of nitrogen concentration in leaf androot．The root to shoot ratio of Pg reduced under N fertilization.(2) Photosynthetic rate of the two species noticeably raised with increasing N treatment levels.N fertilization increased leaf area of the two Poplar species. The content of chlorophyll andcarotenoid in Populus alba×P. glandulosa increased significantly (P<0.001), whereas therewas no significant difference under different N treatments in Populus popularis.(3)N fertilization changed the physiological parameters of the two species.Amino compoundscontent, soluble protein concentration and NR content increased under high N treatment.There were no significant changes in soluble sugars under the N treatments.(4)Vessel and fiber characteristics were affected by N fertilization. The samples from the twospecies supplemented with the10mM NH4NO3treatment contained significantly shorterfibers and vessels than the lower N treatments. Vessel and fiber width were larger under the Ntreatments. Thicker fiber cell walls were produced during high nitrogen treatment.High-nitrogen availability increased cellulose deposition in xylem fibers and the G-layer canbe observed.(5) Lignin content reduced after high N treatments in both two species, but there were no significant changes.As expected, increased biomass accumulation was observed and the formation of morechlorophyll in leaves significantly increased photosynthetic capacity under high-N conditions.Nitrogen fertilization made the fibers and vessels shorter and wider. High-nitrogen availabilityincreased cellulose deposition in xylem fibers and reduced the lignin content probablybecause of an increase of amino compounds or a decrease of carbohydrate under high Ntreatment.5mM NH4NO3may be a suitable concentration in Poplar cultivation.