Symbiotic nitrogen fixation by legumes in two Chinese grasslands estimated with the 15N dilution technique

Symbiotic nitrogen (N) fixation by legumes was investigated using the N-15 dilution technique in two Chinese grasslands: one in the north-eastern Tibetan Plateau and the other in Inner Mongolia in China. A small amount (0.03 g N m(-2)) of N-15 labelled (NH4)(2)SO4 fertilizer was evenly distributed in two soils. One month after the N-15 addition, four legumes (Astragalus sp., Gueldenstaedtia diversifolia, Oxytropis ochrocephala and Trigonella ruthenica) in the alpine meadow and two legumes (Thermopsis lanceolata and Melissitus ruthenica) in the temperate steppe were collected. Several non-legume plant species were harvested as the reference. Above-ground biomass of legumes ranged from 8 to 24 g m(-2) in the alpine meadow and from 11 to 35 g m(-2) in the temperate steppe. The reference plants showed distinctly higher N-15 atom% excess than legumes (0.08% vs. 0.02% in the alpine meadow, 0.10% vs. 0.02% in the temperate steppe). The N derived from atmosphere (%Ndfa) ranged from 50 to 90% N in the alpine meadow, while it ranged from 85 to 92% in the temperate steppe. Based on the legume above-ground biomass, total symbiotic N-2-fixation rate was estimated to be 1.00 g N m(-2) year(-1) in the alpine meadow and 1.15 g N m(-2) year(-1) in the temperate steppe. These N inputs by legumes can account for 9% of the gap between the N demand and the seasonal N release by mineralization in the alpine Kobresia grassland and 20% in the temperate Leymus grassland, respectively. Considering additional contribution of the root biomass, we suggest that biological N-2-fixation by legumes plays an important role in the cycling of N in both Kobresia and Leymus grasslands on an annual scale.