Niche partitioning in nitrogen uptake among subtropical tree species enhances biomass production

Nitrogen (N) is a main nutrient limiting plant growth in most terrestrial ecosystems, but so far it remains unknown which role plant N uptake plays for the positive relationship between species richness and productivity. An in situ¹⁵N labeling experiment was carried out by planting four subtropical tree species (i.e., Koelreuteria bipinnata, Lithocarpus glaber, Cyclobalanopsis myrsinaefolia and Castanopsis eyrei) in pots, at richness levels 1, 2 and 4 species per pot. Plant N uptake preference for inorganic N form of NO₃^- to NH₄⁺ and organic N form of glycine, as well as biomass and plant functional traits was evaluated under different tree species richness level. Overall, pot biomass productivity increased with tree species richness. Biomass of the most productive species, K. bipinnata increased, but not at the expense of a decreased growth of the other species. In mixtures, the species shifted their preference for the inorganic N form, from NO₃^- to NH₄⁺ or vice versa. The uptake preference for glycine remained stable along the species richness gradient. Plant N uptake was well correlated with numerous functional traits, both aboveground, such as height and shoot diameter, and belowground, such as root diameter and root length. We conclude that increased ecosystem biomass production with tree species richness could be largely explained by niche partitioning in N uptake among tree species. Our findings highlight that niche partitioning for N uptake should be a possible important mechanism maintaining species diversity and ecosystem production in subtropical forests.