Effects of pioneer N2-fixing plants on the resource status and establishment of neighboring non-N2-fixing plants in a newly formed glacier floodplain, eastern Tibetan Plateau

Abstract

The ecological roles of dinitrogen-fixing plants (N2-fixers) in the early stage of primary succession remain unclear. We aimed to examine the effects of pioneer N2-fixers on the resource status and establishment of neighboring non-N2-fixing plants (non-fixers) in a newly formed glacier floodplain, eastern Tibetan Plateau. We set up three plots in the floodplain and assessed the abundance, height, and coverage of two N2-fixing and three non-fixing species. We also collected plant and soil samples for the analyses of nutrient status and stable isotope signatures. The two N2-fixing species acquired > 80% of the nitrogen (N) from N2 fixation. The cushion-forming N2-fixer Astragalus mahoshanicus increased the soil nutrient availability compared with the non-fixers and improved the microclimate beneath the cushions. Natural 15N abundance indicated significant transfer of N from A. mahoshanicus to its neighboring non-fixers. Consequently, the neighboring non-fixers were significantly higher in N concentration, above-ground to below-ground biomass ratio and height and density than the non-fixers growing alone, but more deficient in some rock-derived nutrients. The other N2-fixer Hippophae rhamnoides (in seedling stage) generally did not increase the resource availability, and its neighboring non-fixers were similar in height and lower in density than the non-fixers growing alone. The effects of pioneer N2-fixers on the resource status and establishment of neighboring non-fixers in the subalpine floodplain were species dependent and were related to the life history traits of the N2-fixers. A. mahoshanicus has the potential to be used in the restoration of newly exposed land in subalpine regions.