Significance of organic nitrogen acquisition for dominant plant species in an alpine meadow on the Tibet plateau, China

Though the potential of plants to take up organic N (e.g., amino acids) is well established, the true significance of organic N acquisition to plant N nutrition has not yet been quantified under field conditions. Here we demonstrate that organic N contributes significantly to the annual N uptake of three dominant plant species (Kobresia humilis, Saussurea superba and Stipa aliena) of alpine meadows on the Tibet Plateau, China. This was achieved by using double-labelled (¹⁴C and ¹⁵N) algae as a source for slow and continuous release of amino acids, and tracing both labels in the above- and below-ground plant biomass. Four months after addition of algae, between 0.5% and 2.6% of ¹⁴C and 5% and 14% of ¹⁵N from added algae were recovered in the plants, which translate into an uptake of organic N between 0.3 mg N m⁻² and 1.5 mg N m⁻². The calculated contribution of organic N to total N uptake was estimated to range between 21% and 35% for K. humilis, and between 13% and 21% for S. aliena and S. superba, respectively, implying that organic N uptake by grassland plants is quantitatively significant under field conditions in the studied alpine meadows. This finding has important ecological implications with regard to competition for organic N between microorganisms and plant roots.