Patterns of foliar and soil nitrogen isotope composition of Caragana microphylla, a leguminous shrub species in the semi-arid regions of northern China

Abstract

Variation in foliar and soil nitrogen (N) isotope composition (δ15N) across environmental gradients has been widely studied at various scales. However, much less is known about the patterns of N2-fixing species that are presumed to be independent of climatic factors due to their capacity to fix atmospheric N2. In this study, the foliar and soil δ15N in Caragana microphylla, a leguminous shrub important for its role in controlling soil erosion and land desertification in northern China, were determined from 23 sites along an east–west transect across the species distribution range. The soil δ15N tended to be significantly higher than the foliar δ15N and there was a positive correlation between them (r2 = 0.58, p < 0.0001), though substantial variability was found in foliar and soil δ15N across all sites. Both soil and foliar δ15N were negatively correlated with precipitation, temperature, dry N deposition (DND) and longitude, but positively correlated with foliar N, soil N and altitude. The structural equation modeling further suggested growing season precipitation indirectly influenced foliar δ15N by its direct impact on soil δ15N and by its indirect impact on soil δ15N via DND and total N deposition (TND), whereas growing season temperature affected foliar δ15N indirectly both by its effect on foliar N and by its effect on soil δ15N via WND, TND and foliar N. Altogether, our results indicated that foliar δ15N also shifted along environmental gradients in leguminous plant species and showed primary dependence on soil δ15N, confirming previous findings in non-leguminous plants. These results augment our common understanding of foliar and soil δ15N patterns across climatic gradients in plants.