COS 204-2 - Root traits and plant functional groups determine variability in root exudation and N uptake rates in mature temperate trees

Abstract: Root traits are increasingly viewed as essential to plant function, yet most measurements are morphological traits, which only indirectly relate to plant physiology. In this study, we measured two critically important but poorly-quantified physiological functions - root exudation of carbon and root uptake of nitrogen (N) - to explore the drivers of these fluxes. Given that plant phylogeny, mycorrhizal associations, and root traits can all influence root function, we implemented a targeted sampling campaign to measure root morphological traits, root exudation and root N uptake in 11 tree species with diverse evolutionary histories and mycorrhizal associations (i.e., arbuscular mycorrhizal and ectomycorrhizal- associating gymnosperms and angiosperms). All trees were mature, field grown trees occurring in monospecific plots at The Morton Arboretum, Lisle, IL, USA. We used an in situ cuvette- based method to measure exudation and N uptake rates on multiple individuals of each species, and related rates to morphological traits and plant functional groups. The best combinations of traits and plant functional types for predicting exudation and N uptake were determined using a mixed-effects model.

Overall, intraspecific variation in exudation and N uptake exceeded interspecific variation, reflecting the dynamic nature of these physiological processes. Additionally, exudation rates differed significantly among species and functional groups. EcM gymnosperms exhibited 235% greater exudation rates than AM gymnosperms, whereas there were no significant differences between EcM and AM angiosperms (significant mycorrhizal x phylogenetic group interaction; p = 0.006). The projected root economic space for the 11 species indicated that exudation and N uptake rate mapped weakly onto the fast side of the conservation gradient (root tissue density-root[N]) rather than the collaboration gradient (specific root length-root diameter). The best model for predicting exudation contained plant phylogenetic group, mycorrhizal type, specific root length and root [N] (p < 0.01). Surprisingly, exudation rates were independent from N uptake rates, signifying that the two functions were modulated by different drivers. Root traits that predicted N uptake differed between N forms; root tissue density and phylogenetic group predicted NH4+ uptake (p < 0.05), whereas exudation rate and phylogenetic group were the weak predictors of NO3- uptake (p < 0.1). Collectively, our results showed neither plant functional groups nor morphological traits alone can sufficiently explain variation in exudation and N uptake. Given the high degree of variation within species, our study reveals that root physiological traits might be mediated by other factors such as soil microhabitat and whole tree physiology.