Abstract
It is necessary to know the nutritional characteristics of forest trees when considering the effects of changes in nutrient environment due to air pollution on forests. The reasons why Pinus densiflora grows on a poor nutrition area like ridge were examined. It is considered that the soil is acidic and the main nitrogen source is ammonium nitrogen in locations where P. densiflora grows. P. densiflora seedlings grown by both hydroponics and soil cultivation grow better when the nitrogen source supplied is ammonium nitrogen. Nitrate nitrogen is barely detectable in the roots, needles, and xylem saps in P. densiflora. Free amino acids detected in these seedlings are mainly glutamine and arginine, and nitrogen is transferred from the root to the shoot in the form of glutamine most. Absorbed inorganic nitrogen appears to be rapidly assimilated into amino acids in relation with root respiration. In the needles of P. densiflora grown in the field, the concentrations of bases, such as calcium and magnesium, are lower than the concentrations in most other coniferous and broadleaf trees. In many plants such as Cryptomeria japonica, bases are thought to be necessary for electrical balance when nitrate nitrogen is absorbed. However, P. densiflora does not use such a mechanism. In case of P. densiflora, ammonium nitrogen is the main nitrogen source; on the other hand, nitrate nitrogen is reduced immediately after absorption. Furthermore amino acid synthesis progresses smoothly in the root. The quick reduction of nitrate nitrogen in P. densiflora tree might be the adaptation to acidic soil areas where bases are lacking.
