Evidence shows that many tree species can take up organic nitrogen (N) in the form of free amino acids from soils, but few studies have been conducted to compare organic and inorganic N uptake patterns in temperate and tropical tree species in relation to mycorrhizal status and successional state. We labeled intact tree roots by brief ¹⁵N exposures using field hydroponic experiments in a temperate forest and a tropical forest in China. A total of 21 dominant tree species were investigated, 8 in the temperate forest and 13 in the tropical forest. All investigated tree species showed highest uptake rates for NH₄⁺ (ammonium), followed by glycine and NO₃⁻ (nitrate). Uptake of NH₄⁺ by temperate trees averaged 12.8 μg N g⁻¹ dry weight (d.w.) root h⁻¹, while those by tropical trees averaged 6.8 μg N g⁻¹ d.w. root h⁻¹. Glycine uptake rates averaged 3.1 μg N g⁻¹ d.w. root h⁻¹ for temperate trees and 2.4 μg N g⁻¹ d.w. root h⁻¹ for tropical trees. NO₃⁻ uptake was the lowest (averaging 0.8 μg N g⁻¹ d.w. root h⁻¹ for temperate trees and 1.2 μg N g⁻¹ d.w. root h⁻¹ for tropical trees). Uptake of NH₄⁺ accounted for 76% of the total uptake of all three N forms in the temperate forest and 64% in the tropical forest. Temperate tree species had similar glycine uptake rates as tropical trees, with the contribution being slightly lower (20% in the temperate forest and 23% in the tropical forest). All tree species investigated in the temperate forest were ectomycorrhizal and all species but one in the tropical forest were arbuscular mycorrhizal (AM). Ectomycorrhizal trees showed significantly higher NH₄⁺ and lower NO₃⁻ uptake rates than AM trees. Mycorrhizal colonization rates significantly affected uptake rates and contributions of NO₃⁻ or NH₄⁺, but depended on forest types. We conclude that tree species in both temperate and tropical forests preferred to take up NH₄⁺, with organic N as the second most important N source. These findings suggest that temperate and tropical forests demonstrate similar N uptake patterns although they differ in physiology of trees and soil biogeochemical processes.

Plants produce thousands of compounds, collectively called the metabolome, which mediate interactions with other organisms. The metabolome of an individual plant may change according to the number and nature of these interactions. We tested the hypothesis that tree diversity level affects the metabolome of four subtropical tree species in a biodiversity–ecosystem functioning experiment, BEF‐China. We postulated that the chemical diversity of leaves, roots and root exudates increases with tree diversity. We expected that the strength of this diversity effect differs among leaf, root and root exudates samples. Considering their role in plant competition, we expected to find the strongest effects in root exudates. Roots, root exudates and leaves of four tree species (Cinnamomum camphora, Cyclobalanopsis glauca, Daphniphyllum oldhamii and Schima superba) were sampled from selected plots in BEF‐China. The exudate metabolomes were normalized over their non‐purgeable organic carbon level. Multivariate analyses were applied to identify the effect of both neighbouring (local) trees and plot diversity on tree metabolomes. The species‐ and sample‐specific metabolites were assigned to major compound classes using the ClassyFire tool, whereas potential metabolites related to diversity effects were annotated manually. Individual tree species showed distinct leaf, root and root exudate metabolomes. The main compound class in leaves was the flavonoids, whereas carboxylic acids, prenol lipids and specific alkaloids were most prominent in root exudates and roots. Overall, plot diversity had a stronger effect on metabolome profiles than the local diversity. Leaf metabolomes responded more often to tree diversity level than exudates, whereas root metabolomes varied the least. We found no uniform or general pattern of alterations in metabolite richness or diversity in response to variation in tree diversity. The response differed among species and tissues. Synthesis. Classification of metabolites supported initial ecological interpretation of differences among species and organs. Particularly, the metabolomes of leaves and root exudates respond to differences in tree diversity. These responses were neither linear nor uniform and individual metabolites showed different dynamics. More controlled interaction experiments are needed to dissect the causes and consequences of the observed shifts in plant metabolomes.