Studies of NH ₄ ⁺ and NO ₃ ^- uptake ability of subalpine plants and resource-use strategy identified by their functional traits

The leaf economics spectrum (LES) is based on a suite of leaf traits related to plant functioning and ranges from resource-conservative to resource-acquisitive strategies. However, the relationships with root traits, and the associated belowground plant functioning such as N uptake, including nitrate (NO ₃ ^- ) and ammonium (NH ₄ ⁺ ), is still poorly known. Additionally, environmental variations occurring both in time and in space could uncouple LES from root traits. We explored, in subalpine grasslands, the relationships between leaf and root morphological traits for 3 dominant perennial grass species, and to what extent they contribute to the whole-plant economics spectrum. We also investigated the link between this spectrum and NO ₃ ^- and NH ₄ ⁺ uptake rates, as well as the variations of uptake across four grasslands differing by the land-use history at peak biomass and in autumn. Although poorly correlated with leaf traits, root traits contributed to an economic spectrum at the whole plant level. Higher NH ₄ ⁺ and NO ₃ ^- uptake abilities were associated with the resource-acquisitive strategy.

Nonetheless, NH ₄ ⁺ and NO ₃ ^- uptake within species varied between land-uses and with sampling time, suggesting that LES and plant traits are good, but still incomplete, descriptors of plant functioning. Although the NH ₄ ⁺ : NO ₃ ^- uptake ratio was different between plant species in our study, they all showed a preference for NH ₄ ⁺ , and particularly the most conservative species. Soil environmental variations between grasslands and sampling times may also drive to some extent the NH ₄ ⁺ and NO ₃ ^- uptake ability of species. Our results support the current efforts to build a more general framework including above- and below-ground processes when studying plant community functioning.