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 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...