Strong non-growing season N uptake by deciduous trees in a temperate forest: A 15N isotopic experiment

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 N-15 isotopic experiment to trace N processes throughout the non-growing season in a temperate forest in northern China. The N-15-labelled inorganic N (NH4+ and NO3-) and (CN)-C-13-N-15-labelled organic N (glycine and tyrosine; equivalent to 150 mg N-15/m(2)) were applied to soils at mid-fall, and the N-15 recovery in various components of dominant evergreen and deciduous species was analysed. 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.