Different relationships of fine root traits with root ammonium and nitrate uptake rates in conifer forests

Nitrogen (N) uptake by fine roots of trees is important for understanding root physiological function in forest ecosystems. However, direct investigations of in situ ammonium and nitrate uptake rates are limited. Thus, we aimed to clarify inorganic N uptake rates among tree species and determine the factors controlling N uptake through relationships with fine root traits in cool temperate coniferous forests. 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). The coniferous roots in this study preferred NH4+ to NO3-. Across species, there were significant relationships between NH4+ uptake and diameter, SRL, and RTD; in contrast, only RTD had a significant impact on NO3- uptake. Relationships between N uptake rates and root morphological traits differed between NH4+ and NO3-. We found that the relationship of inorganic N uptake with morphological traits depended on the characteristics of the N form adsorbed through soil and on tree N assimilation efficiency. Our results make a breakthrough in the understanding of root physiological function and the prediction of fundamental N acquisition strategies.