Natural 15N abundance of presumed N2-fixing and non-N2-fixing plants from selected ecosystems.

The 15N/14N ratios of plant and soil samples from Northern California ecosystems were determined by mass spectrometry. The 15N abundance of 176 plant foliar samples averaged 0.0008 atom % 15N excess relative to atmospheric N2 and ranged from-0.0028 to 0.0064 atom % 15N excess relative to atmospheric N2. Foliage from reported N2-fixing species had significantly lower mean 15N abundance (relative to atmospheric N2 and total soil N) and significantly higher N concentration (% N dry wt.) than did presumed non-N2-fixing plants growing on the same sites. The mean difference between N2-fixing species and other plants was 0.0007 atom % 15N. N2-fixing species had lower 15N abundance than the other plants on most sites examined despite large differences between sites in vegetation, soil, and climate. The mean 15N abundance of N2-fixing plants varied little between sites and was close to that of atmospheric N2. The 15N abundance of presumed non-N2-fixing species was highest at coastal sites and may reflect an input of marine spray N having relatively high 15N abundance. The 15N abundance of N2-fixing species was not related to growth form but was for other plants. Annual herbaceous plants had highest 15N abundance followed in decreasing order by perennial herbs, shrubs, and trees. Several terrestrial ferns (Pteridaceae) had 15N abundances comparable to N2-fixing legumes suggesting N2-fixation by these ferns. On sites where the 15N abundance of soil N differs from that of the atmosphere, N2-fixing plants can be identified by the natural 15N abundance of their foliage. This approach can be useful in detecting and perhaps measuring N2-fixation on sites where direct recovery of nodules is not possible.