Effects of soil moisture on organic and inorganic nitrogen uptake by dominant plant species in Zoige alpine wetlands

Differences in nitrogen (N) form preference among species may lead to niche differentiation that facilitates species coexistence. In alpine wetlands, N niche differentiation may exist between dominant plant species, and such differentiation may be modified by soil moisture. To test these hypotheses, we conducted a 15N labeling experiment along three soil moisture gradients (low vs. medium vs. high, i.e. 30.6% vs. 52.1% vs. 69.9% of volumetric water content) in an alpine wetland dominated by Carex muliensis Hand-Mazz. and Potentilla anserina L. on the eastern Qinghai-Tibetan Plateau. We labeled plants and soils with ammonium-15N, nitrate-15N or glycine-15N. N uptake rate was significantly higher in P. anserina than in C. muliensis, irrespective of soil moisture. Under medium soil moisture, C. muliensis absorbed more ammonium-N than nitrate-N and glycine-N as the soil contained more ammonium-N. Under high soil moisture, however, C. muliensis took up more nitrate-N than ammonium-N and glycine-N because the soil contained more nitrate-N. Potentilla anserina preferred ammoniumN to nitrate-N and glycine-N and such preference did not change with soil moisture. These results suggest that dominant species in alpine wetlands prefer to take up inorganic N rather than organic N and that soil moisture may alter inorganic N form preference of alpine wetland plant species.