The symbiotic nitrogen fixation by legumes in a legume-companion and a legume-dominant alpine steppe on the central Tibetan Plateau

摘要

Symbiotic N-2-fixing plant species occur in many stressful environments, including the central Tibetan Plateau, where the legume species Oxytropis microphylla, Oxytropis glacialis, Astragalus arnoldii occur in the alpine steppe at elevations as high as 4700 m above sea level, either as companion or as dominant species, accounting for 13.91-60.25% of the total aboveground steppe biomass. However, the degree to which they are nutritionally reliant upon symbiotic N-2 fixation is still unclear. We measured the symbiotic nitrogen fixation rate in a legume-companion (LC) and a legume-dominant (LD) alpine steppe community using the N-15 isotope dilution technique. The N derived from the atmosphere (%Ndfa) in the three legume species ranged from 17% to 61% in aboveground tissues and from 70% to 88% in belowground tissues. The N-2 fixation rate was estimated to be 0.25 +/- 0.04 and 1.39 +/- 0.20 g N m(-2) year(-1) for LC and LD based on the aboveground biomass. It was 0.86 +/- 0.04 and 2.77 +/- 0.22 gNm(-2) year(-1) for LC and LD, respectively, when both above and belowground tissues were considered. The total biological nitrogen fixation (BNF) of the alpine steppe on the Tibetan Plateau (ca. 0.7 million km(2)) can then be conservatively estimated at approximately 0.6 T g N year(-1) using the BNF estimate from the LC community. Our findings indicate that estimates based solely on aboveground biomass and LC communities might drastically underestimate the N inputs arising from BNF by legumes to the alpine ecosystem N budget.