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

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