Independence of alpine meadow plants from soil organic matter for nitrogen supply: evidence from stable nitrogen isotopes

Stable nitrogen isotope signatures of major sources of mineral nitrogen (mineralization of soil organic nitrogen, biological N2 fixation by legumes, annual precipitation and plant litter decomposition) were measured to relatively define their individual contribution to grass assimilation at the Haibei Alpine Meadow Ecosystem, Qinghai, China. The results indicated that δ15N values (−2.40 ‰ to 0.97 ‰) of all grasses were much lower than those of soil organic matter (3.4±0.18 ‰) and mineral nitrogen (ammonium and nitrate together, 7.8±0.57 ‰). Based on the patterns of stable nitrogen isotopes, soil organic matter (3.4±0.18 ‰), biological N2 fixation (0 ‰), and precipitation (−6.34±0.24 ‰) only contributed to a small fraction of nitrogen requirements of grasses, but plant litter decomposition (−1.31±1.01 ‰) accounted for 67 %.