Dynamics of nitrogen nutrition of coexisting dominant trees in mixed broad-leaved/Korean pine forest

Chemical analysis of ammonium, nitrate and total nitrogen in tree leaves and roots and an in-Vivo bioassay for nitrate reductase activity (NRA) were used to monitor the seasonal variations in nitrogen assimilation among four coexisting dominant tree species, including Pin us koraiensis, THin amurensis, heXinus mandshurica and Acer mono, in a virgin mixed broad-leaved/Korean pine (Pin us koraiensis) forest. The soil study included individual horizons of L+F (0-5 cm), Ah (5-11 cm) and AW (11-25 cm). All four species had nitrate and ammonium in their roots and leaves, and also NRA in leaves. This indicated that these coexisting species were adapted to ammonium + nitrate nutrition. A negative correlation existed between nitrate use and ammonium use. Ammonium concentration was higher than that of nitrate in tree leaves and roots, and aIso in soils, which indicated climax woody species had a relative preference for ammonium nutrition. There was a positive retationship between tree nitrogen nutrition use and soil nitrogen nutrient supply. Utilization of amrnonium and nitrate as well as the seasonal patterns differed significantly between the species. Peaks of ammonium, nitrate, NRA and total nitrogen in one species were therefore not necessarily synchronous with peaks in other species, and which indicated a species-specific seasonal use of nitrogen. The species-specific temporal differentiation in nitrogen use might reduce the competition between co-existing species and may be an important mechanism promoting stability bf virgin mixed broad-leaved