Nitrogen fixation assessment in a legume-dominant alpine community: comparison of different reference species using the 15N isotope dilution technique

Abstract

In arctic and alpine environments the highest biologically fixed nitrogen inputs are measured in the presence of leguminous plants, with a maximum annual nitrogen input recorded of about 10 kg ha-1 year-1. Among the high-elevation legumes, Trifolium alpinum spreads over many southern European mountain grasslands as a dominant or eo-dominant species, accounting for 30-40% or more of total above-ground phytomass. We estimated the amount of symbiotic nitrogen fixation in T. alpinum-dominant communities using the 15 N isotope dilution technique and comparing the calculations derived from different non-fixing reference species. The estimated percentage of nitrogen derived from the atmosphere varied significantly depending on the reference species used and they were comparable to the ones reported in literature. No differences were detected among years. In the community studied, the use of one reference species instead of another can yield differences of more than 10 kg ha-1 year-1 , which may represent a considerable difference in the estimation of symbiotic nitrogen fixation in alpine ecosystems. Taking a very precautionary approach and using the grass Nardus stricta as the reference species, we estimated 21.9 kg ha-1 year-1 of nitrogen fixation in T. alpinumdominant communities. This quantity of nitrogen is the highest ever estimated in a natural alpine grassland and it is related to the high annual above-ground phytomass of T. alpinum.

Ecological functions provided by dung beetles are interlinked across space and time: evidence from 15N isotope tracing

“…Mixed-species assemblages of tunnelers and dwellers enhanced herbage growth but this increase was observed only after 1 yr. At the same time, the cover of the oligotrophic species remained constant 1 yr after the treatments, probably due to their physiological limitation for N uptake (Fransen et al 1998, Lonati et al 2015. Apparently, our results cannot be extended to all vegetation types (e.g., Yamada et al 2007).…”

Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

“…We used the following equation to calculate N derived from the atmosphere (%Ndfa) for the legume species (Lonati et al, ; Yang et al, ). The calculations used the APE of the legume species and the reference plants in the same site.…”

“…We used the 15 N dilution technique to quantify N fixation by legume plants. An important assumption is that the reference plants accumulate N with the same amount of 15 N enrichment from soil as legume plants (Lonati, Probo, Gorlier, & Lombardi, 2015;Yang et al, 2011). In this study, we selected a graminoid S. purpurea as the reference plant.…”

Effects of traditional salt placement and strategically placed mineral mix supplements on cattle distribution in the Western Italian Alps

“…Grazing distribution was more influenced by MMS placements than by proximity to water sources, and it is known that distance to water is one of most important factors affecting cattle spatial distribution (Valentine, ; Mueggler, ; Bailey, ; Putfarken et al ., ). Furthermore, strategic placement of MMS increased the use of traditionally underused vegetation communities, which generally have a higher percentage of forb cover than more intensively managed areas (Krahulec et al ., ; Fischer and Wipf, ; Lonati et al ., ). This result can help to explain cattle preference for areas with greater forb cover.…”