Advancing predictive understanding of tree organic and inorganic nitrogen uptake across forest biomes

Plants uptake nitrogen (N) from soils in inorganic forms, such as ammonium (NH4+) and nitrate (NO3−), but also in the form of organic compounds like amino acids. Despite extensive research on terrestrial N cycling, the patterns and underpinning mechanisms of inorganic and organic N uptake by tree species across forest biomes remained very uncertain. To address this knowledge gap, we conducted field-based hydroponic labelling experiments on 34 tree species spanning from temperate to subtropical and tropical climate zones. We assessed uptake rates of nine common amino acids (15N and 13C dual-labelled) alongside with NH4+ and NO3− (15N-labelled) at micromolar concentrations. Root morphological traits, soil chemical properties, soil N pool sizes and microbial N functional genes were determined to assess their role in explaining differential N uptake among tree species and forest biomes. Our results demonstrated stable N uptake rates and preferences across all forest biomes but showed large differences among N forms. Such N uptake was predominantly affected by N intrinsic properties, followed by effects of soil properties and microbial N functional genes on soil N availability, while controls by tree root traits were weakest. Mean uptake rates of single amino acids contributed to 39 % of the total root N uptake, with NH4+ showing the highest (56 %), and NO3− showing the lowest uptake rates (5.0 %). Uptake rates of positively charged and high N% amino acids such as arginine, histidine, and lysine were fastest, i.e., 0.98, 0.81, and 0.78 μg N g−1 d. w. root h−1, respectively. Nitrogen uptake rates were faster when trees have longer and thinner fine roots, in soils with higher pH and phosphorus (P) availability and faster microbial N turnover. Our findings highlight the important role of organic N and NH4+ for tree nutrition and reveal how tree N uptake is influenced (in increasing importance) by tree root morphological traits, soil microbial N functional composition, soil resource availability, and N form intrinsic properties. These findings provide profound quantitative and predictive insights into our understanding of forest N sink processes, offering a scientific foundation for optimizing global forestry N management strategies in the context of environmental change. © 2025 Elsevier Ltd.