Nitrogen uptake and allocation in Populus simonii in different seasons supplied with isotopically labeled ammonium or nitrate

Key message The total uptake of 15 NO 3 -N was twofold higher than that of 15 NH 4 -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO 3 -N had good growth with high photosynthetic rate and total biomass. Abstract Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of 15 NH 4 -N, 15 NO 3 -N, 15 NH 4 NO 3 , and NH 4 15 NO 3 in May, July, and September. The total 15 N uptake was twofold higher when supplied with sole 15 NO 3 -N compared to sole 15 NH 4 -N in all the investigated seasons. In the presence of ammonium nitrate ( 15 NH 4 NO 3 and NH 4 15 NO 3 ), the total 15 N uptake was two times higher when supplied with NH 4 15 NO 3 compared to 15 NH 4 NO 3 . Per unit biomass, the 15 N-uptake ability of fine roots was higher in May and July compared to that in September. 15 N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH 4 -N, NO 3 -N, and NH 4 NO 3 for 8 weeks. The seedlings fertilized with NO 3 -N had good growth with high photosynthetic rate and total biomass indicating that NO 3 -N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.