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 anin-vivo bioassay for nitrate reductase activity (NRA) were used to monitor the seasonal variations in nitrogen assimilation among four coexisting dominant tree species, includingPinus koraiensis, Tilia amurensis, Fraxinus mandshurica andAcer mono, in a virgin mixed broad-leaved/Korean pine (Pinus 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 also in soils, which indicated climax woody species had a relative preference for ammonium nutrition. There was a positive relationship between tree nitrogen nutrition use and soil nitrogen nutrient supply. Utilization of ammonium 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 of virgin mixed broad-leaved//Korean pine forest.