Nitrogen (N) is a critical element for vegetation growth and subsequent carbon (C) and nutrient cycling in terrestrial ecosystems. Plant N uptake, the only pathway for plants to directly obtain N from soils, is a bottleneck process for ecosystem C and N cycling. Ecological theories predict that deciduous trees remain dormant and do not take up N during winters as no growth occurs during this season.

In this study, we adopted a ¹⁵N isotopic experiment to trace N processes throughout the non-growing season in a temperate forest in northern China. The ¹⁵N-labeled inorganic N (NH₄⁺ and NO₃⁻) and ¹³C¹⁵N-labeled organic N (glycine and tyrosine) (equivalent to 150 mg ¹⁵N m^-2) were applied to soils at mid-fall, and the ¹⁵N recovery in various components of dominant evergreen and deciduous species was analyzed.

We found that soil N transformation remained active in the winter and microbial N immobilization reached its peak in late winter. Surprisingly, deciduous species maintained a high N uptake that was comparable with the evergreen species throughout the non-growing season. Perennial herbs did not take up N until the next spring. All plant species acquired inorganic N and simple amino acids, while only the tree species utilized complex amino acids. Throughout the non-growing season, evergreen and deciduous trees showed higher uptake rates for NH₄⁺ and glycine than NO₃⁻ and tyrosine, while deciduous shrubs and herbs showed a stronger preference for NO₃⁻ over other N forms.

Synthesis: The finding that deciduous trees have strong N uptake in the non-growing season challenges the conventional viewpoint that deciduous trees remain dormant during non-growing seasons. This mechanism might supplement the algorithm in the model representation of N-limited temperate forest ecosystems.