Uptake of various nitrogen forms by co-existing plant species in temperate and cold-temperate forests in northeast China

The uptake of different soil nitrogen (N) forms by co-existing plant species is ecologically crucial, but in situ studies conducted to investigate this phenomenon across forest ecosystems are still limited. Here, stable isotope tracer techniques (C-13 and N-15)were used in the field, examining the uptake of inorganic and organic N by coexisting plant species, plasticity in N preference, and difference in the N uptake between layers of forest in temperate and cold-temperate forests in northeast China. Amino acid-N composed an important part (29% avg.) in the pool of available soil N (inorganic N and amino acid-N). The vegetation could take up glycine-derived N (intact uptake from 48% to 99%). With variation in habitat, Betula platyphylla and Sphagnum palustre modified their competitive abilities for uptake of intact glycine and apparent N preferences, illustrating the plasticity in the N preferences of plants. Due to the plasticity, the co-existing species appeared to show various N preference strategies in these forests. Canopy trees took up more glycine-derived N than understory plants, and vice versa for the inorganic N, which may be related to differences in the intensity of photosynthesis. This study indicates that soil amino acids are a potentially important N source, and various choices for N nutrition may alleviate interspecies competition and contribute to species co-existence in forest ecosystems in northeast China. Plasticity in N preference could underlie N source partitioning among the co-existing species. With ongoing N deposition in these and many other forest ecosystems, this research helps to clarify how soil-plant N cycling varies between canopy layers and the feedbacks that are taking place.