Root traits and tree functional groups determine variability in root exudation and N uptake rates in mature temperate trees

Root exudation and nutrient uptake have profound impacts on soil biogeochemistry, yet quantification of these fluxes in mature trees is rare.

We measured exudation and nitrogen (N) uptake rates of 11 tree species - gymnosperms and angiosperms associating with arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi – at the Morton Arboretum, USA, to explore relationships between root traits, exudation and nitrogen uptake.

Exudation rates were affected by phylogenetic group and mycorrhizal type, as EcM-gymnosperms exuded more carbon than the other three groups. Exudation correlated positively with specific root area and negatively with root tissue density. Nitrogen uptake did not differ between phylogenetic groups or mycorrhizal types. Root traits predicting N uptake varied with N form: root tissue density and phylogenetic group predicted ammonium uptake, whereas exudation and phylogenetic group predicted nitrate uptake. In root economic space, exudation and uptake aligned weakly with the fast side of the conservation gradient, though the two processes were not correlated.

Our results indicate that exudation and N uptake are dynamic processes not predicted easily by species identity, plant functional groups or root traits. As such, incorporating these processes into trait frameworks and large-scale models may require ancillary information about the soil environment and whole-tree physiology.