Which root traits determine nitrogen uptake by alpine plant species on the Tibetan Plateau?

Nitrogen (N) is one of the most important limiting factors influencing plant growth and reproduction in alpine and tundra ecosystems. However, in situ observations of the effects of root traits on N absorption by alpine plant species are still lacking. We investigated the rates of N uptake and the effect of root characteristics in ten common herbaceous alpine plant species using a N-15 isotope tracer technique and the root systems of plants growing in a semi-arid steppe environment on the Tibetan Plateau. Our objective was to determine the root traits (root biomass, volume, surface area, average diameter, length, specific root length and specific root area) that make the largest contribution to the total uptake of N (N-15-NO3 (-), N-15-NH4 (+) or N-15-glycine) by alpine plant species. Monocotyledonous species had higher absorption rates for N-15-NH4 (+), N-15-NO3 (-), N-15-glycine and total N-15 than dicotyledonous species (P < 0.05). The root biomass, volume, surface area and average diameter were negatively correlated with the absorption capacity for N-15-NH4 (+), N-15-NO3 (-) and total N-15 across the ten alpine plant species. However, the specific root length and the specific root area had significantly positive effects on the uptake of N. In contrast with traditional views on the uptake of N, the N uptake rate was not improved by a larger root volume or root surface area for these alpine plant species in a high-altitude ecosystem. Root morphological traits had greater impacts on N absorption than traits related to the root system size in alpine herbaceous plants.