Electronic supplementary material from Precipitation is the main factor affecting the variation of foliar nitrogen isotope composition in two leguminous shrub species of northwestern China

An increase in foliar nitrogen isotope composition (δ<sup>15</sup>N) with decreasing precipitation has been shown to occur widely in non-N<sub>2</sub>-fixing plant species. However, similar patterns have not been identified in N<sub>2</sub>-fixing species. Here, we tested the relationships of foliar δ<sup>15</sup>N with local environmental factors and leaf properties in two leguminous shrub species (<i>Caragana korshinskii</i> and <i>Caragana liouana</i>) sampled from 30 populations. Results indicated that the mean annual precipitation (MAP) primarily accounted for the variation of foliar δ<sup>15</sup>N in the two species. Further analysis revealed strong negative correlations between foliar δ<sup>15</sup>N and MAP within and across species. These results suggest that the foliar δ<sup>15</sup>N of leguminous shrub species also shift along precipitation gradients, which augments our understanding of the relationships between foliar δ<sup>15</sup>N and climatic factors.