[Effects of nitrogen forms on nutrient uptake and growth of trees]

Forest tree species take up both NH4(+)-N and NO3(-)-N forms of nitrogen (N) during their growth. However, the effects of N form on nutrition uptake and tree growth are different due to the different N forms. Generally, trees show preference to N source, 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 pH value 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 pH value. Contrary, uptake of NO3(-)-N will cause the increase of rhizospheric pH value. The change of rhizospheric pH value 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. Nevertheless, most plants grow better when both forms of N supplied simultaneously than either of N source supplied alone. For few similar studies were carried out with tree species, it needs to do more researches.