Seasonal changes in nitrate assimilation of boreal woody species: importance of the leaf-expansion period

Key message Nitrate served as an important nitrogen source for dominant deciduous tree species, especially during their leaf expansion period, even in boreal forests, where nitrate availability was assumed to be low. Abstract Temporal changes in leaf nitrate assimilation with leaf growth were intensively investigated in boreal tree species to demonstrate the contribution of nitrate as a N source and to determine temporal changes in the contribution of nitrate during leaf ontogeny. Leaf area, mass, nitrate reductase activity (NRA), N concentration, and δ 15 N were repeatedly measured in developing leaves of naturally grown Alnus crispa , Betula neoalaskana , and Populus tremuloides during their leaf expansion period. Alnus crispa and B. neoalaskana showed distinct peaks in NRA during leaf expansion, whereas P. tremuloides did not. The highest peak in NRA occurred for A. crispa , whereas it had low NRA during the summer. Peak NRA in B. neoalaskana was lower than that of A. crispa ( p < 0.01, ANOVA), although it showed higher NRA during summer ( p < 0.01, ANOVA). All species showed clear decrease in N concentration through the leaf expansion period, but total N content per leaf increased. Only the N-fixing species A. crispa showed a rapid change in δ 15 N during the leaf expansion, and the decline indicated the changes in N source during the leaf development. The results indicate leaves of target species assimilated nitrate during the leaf expansion period, consuming immense energy, although leaves were considered a carbon sink during the early leaf expansion period. We suggest the early onset of leaf growth due to climate warming could influence plant nutrition via asynchrony between supply and demand for energy during spring.