Abstract: Forest tree species take up both NH₄⁺-N and NO₃^--N forms of nitrogen (N) during their growth. However, the effects of Nform on nutrition uptake and tree growth are different due to the different Nforms. Generally, trees show preference to Nsource, and most conifers prefer to NH₄⁺-N while the preference to N form of broadleaved trees is unclear. The preference may be correlated with the pHvalue of the site on which the trees grown. The trees grown on acid soil generally appear preference to NH₄⁺-N while trees grown on litmusless or alkaline soil prefer to NO₃^--N. Fungus also can alter the tree uptake characteristic to NH₄⁺-N and NO₃^--N. In some species, the NO₃^--N uptake capacity increased remarkably while uptake capacity of NH₄⁺-N keep consistent when fungus were inoculated. NH₄⁺-N and NO₃^--N have different effects on other nutrient ions uptake because of the difference charge of NH₄⁺-N and NO₃^--N. In general, NH₄⁺-N accelerates the anion absorption while inhibits cation absorption. The effects of nitrogen forms on nutrition uptake are related with not only the charge of NH₄⁺-N and NO₃^--N, but also the restrain effects among other ions. Sometimes, NH₄⁺-N inhibits NO₃^--N uptake, but the mechanism is not very clear now. The form of which nitrogen is taken up by trees will induce the change of rhizospheric pH value. Uptake of NH₄⁺-N results in the decrease of rhizospheric pHvalue. Contrary, uptake of NO₃^--N will cause the increase of rhizospheric pHvalue. The change of rhizospheric pHvalue will affect nutrient availability and thus affect nutrition uptake and utilization by trees. Trees grow differently after NH₄⁺-N and NO₃^--N uptake. When supplied with NH₄⁺-N, some confers that prefer to NH₄⁺-N grow better while a few ammonium-preference conifers hardly show the better growth. N evertheless, most plants grow better when both forms of N supplied simultaneously than either of Nsource supplied alone. For few similar studies were carried out with tree species, it needs to do more researches.

Key words: Tree nutrition, Nitrate, Ammonium, Rhizosphere pH