Symbiotic nitrogen fixation contributes to divergent patterns of foliar and soil nitrogen isotopic composition in an alpine legume shrub species

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Legumes that possess the capacity to fix atmospheric nitrogen (N) via symbiotic N2 fixation are widely considered to have the potential to regulate ecosystem N cycling. However, the N dynamics of those N2-fixing species in response to changes of climatic, edaphic and biotic factors are not well-understood, especially for alpine plants that are particularly susceptible to climate change. In this work, foliar and soil N isotopic composition (δ15N) of Caragana jubata, a legume shrub species mainly growing in alpine regions, was measured from 35 sites across a 2666 km east–west transect. Results showed that there were significant differences between foliar δ15N (ranging from − 0.67 to 0.60‰) and soil δ15N (ranging from 0.60 to 5.71‰), but no correlation was observed between them. Foliar δ15N increased with growing season temperature, foliar N concentration, and decreased with altitude, foliar carbon (C) concentration, foliar carbon to nitrogen ratio (C/N), and soil pH, whereas soil δ15N increased with altitude and decreased with soil C, soil N, soil C/N, and soil electrical conductivity. A principal component analysis indicated that the significant influential factors on foliar δ15N were foliar N, foliar C/N, growing season precipitation, growing season temperature and soil pH, whereas those on soil δ15N were soil electrical conductivity, soil C, soil N, soil C/N, foliar C, growing season precipitation and growing season temperature. Structural equation modeling indicated that the temperature and precipitation during the growing season affected foliar δ15N mainly via their effects on soil pH and foliar N, while affected soil δ15N mainly by their effects on soil electrical conductivity and soil C/N. This study provides strong evidence that C. jubata in the alpine regions obtains N mainly by symbiotic N2 fixation, which has resulted in the decoupling of N dynamics between plant and soil, and in different patterns and controls of foliar and soil δ15N across climatic, edaphic and biotic gradients.