Different Relationships of Fine Root Traits With Root Ammonium and Nitrate Uptake Rates in Conifer Forests

Abstract Purpose: Nitrogen (N) uptake by fine roots of trees is important for understanding the root physiological function in forest ecosystems, but a direct investigation of in situ rate of ammonium and nitrate uptake is limited. Thus, we aimed to clarify the inorganic N uptake rates among tree species and to determine the factors controlling N uptake through relationships with fine root traits in cool temperate forests.Methods: Using a solution depletion method for measuring N uptake, we observed the relationship of N uptake rate in the form of NH4+ and NO3– by an intact root system with root morphological traits, such as root diameter, specific root length (SRL), and root tissue density (RTD), and chemical traits, including root nitrogen (N) content. Results: The coniferous roots in this study preferred NH4+ form more than NO3– form. Across species, there were significant relationships between NH4+ uptake and diameter, SRL, and RTD, while these were significant only for RTD in NO3– form. Relationships between N uptake rates and root morphological traits differed between NH4+ and NO3–. Conclusions: We found that the relationship of inorganic N uptake with the morphological traits depends on the characteristics of the N form adsorbed through soil and tree N assimilation efficiency. An approach on the relationships of in situ N uptake with root traits will provide a breakthrough in our understanding of the root physiological function and the prediction of fundamental N acquisition strategies.