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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 meanN 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. TheN 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) hadN 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.

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Two field experiments in California rice paddies are reported, one with a single treatment of a research plot and the other with varied treatments in a typical commercial rice field. Small total methane emissions, only 11 g CH4/m2, were measured for the entire growing season in the first experiment. In the second experiment, the addition of exogenous organic matter (rice straw), the presence or absence of vegetation, and the nitrogen fertilizer amounts were examined for their influence on methane emissions. The total methane emission over the growing season varied from 1.2 g CH4/m2 (with no added organic matter) to 58.2 g CH4/m2 (with largest organic matter treatments). Added organic matter was the major factor affecting methane emissions. Vegetation did not greatly affect total methane fluxes, but it did influence the mode and timing of release. Nitrogen fertilizer did not greatly affect the amount of methane emitted, but it influenced slightly the time course of the process. A diurnal effect in methane emission was observed during the early ontogeny of the crop. The variation of methane emission with time during the course of the growing season was very unusual in this experiment; only one peak was observed, and it was early in the season. During the period of largest emissions, δ13C values of the methane were measured to be −55.7 ±1.8‰ in plots with added organic matter.

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