The N abundance of tissues of five Prosopis specimens at our primary study site (a Prosopis woodland at Harper's Well in the Sonoran desert of Southern California) was determined over two growing seasons 1980 and 1981. TheN abundance of soil and of tissues of presumed non-N-fixing (control) plants was also measured. Prosopis tissues were significantly lower in N than either soil N or corresponding tissues of presumed non-N-fixing plants which derive their N entirely from soil. Soil N was also significantly higher in N than atmospheric N. We conclude that it is feasible to use variations in the natural abundance of N as an index of N-fixation in this kind of ecosystem, and that N-fixation is of considerable importance to Prosopis growing at this site.We also determined the N abundance of leaf tissue of presumed N-fixing and control plants growing at the same site at six additional sites (five in the Sonoran desert of southern California and one in Baja California, Mexico near the town of Catavina). Four of these additional sites were dominated by Prosopis and two were mixed communities. There were statistically significant differences between the N abundances of the pooled legume population and control plants at all sites, although not every legume specimen exhibited this difference. FromN abundance data we estimated the fractional contribution of biologically fixed N to the N economy of desert legumes. We concluded that N-fixation is very important to Prosopis at six of seven sites in the Sonoran Desert. At the site where Prosopis did not appear to be fixing N, N-fixation was important only for legumes of the sub-family Papilionoideae, Lupinus, Dalea, Astragalus and Lotus.

The expolysaccharides produced by Klebsiella sp. strain K32 and Acinetobacter calcoaceticus BD4 under different growth conditions have been analyzed for sugar composition. The first use of ion chromatography for the quantitative determination of microbial exopolysaccharide composition is reported. Klebsiella sp. strain K32 produced a polymer composed of rhamnose, galactose, and mannose early in its fermentation. The composition of the polymer varied markedly depending on the growth stage of the organism. Klebsiella sp. strain K32 grown in a fermentor produced a polymer which was rich in mannose during early exponential growth in a complex medium, but in the late stationary phase it did not contain detectable levels of mannose. The rhamnose present in the polymer increased from 12 to 55% over the course of growth, whereas galactose decreased from 63 to 45%. A. calcoaceticus BD4 produced a polymer containing rhamnose, glucose, mannose throughout its growth and stationary phase. Klebsiella sp. strain K32 and A. calcoaceticus BD4 were grown on various carbon sources in shake flasks. The polymer yield and composition from both organisms were found to vary with the carbon source. The exopolysaccharide with the highest mannose composition was obtained by using rhamnose as a carbon source for both organisms. These and other data suggest that regulatory changes caused by growth on different substrates result in either the production of a different distribution of polymers or a change in exopolysaccharide structure.

This paper expands upon previous reports of '5N elevation in nodules (compared to other tissues) of N2-fixing plants. N2-Fixing nodules of Glycine max (soybeans), Vigna unguiculata (cowpea), Phaseolus vulgaris (common bean), Phaseolus coccineus (scarlet runner bean), Prosopis glandulosa (mesquite), and Olneya tesota (desert ironwood) were The N of whole soybean plants which are grown with atmospheric N2 as the sole source of N has an isotopic composition within approx. 2 6'5N units2 of atmospheric N (2, 8). In contrast, we have consistently observed that soybean nodules are significantly enriched in 1 N (7,14). This observation has been recently confirmed by Turner and Bergersen (15). The difference in 6'5N between soybean nodules and whole plants ranged from +2.8 to +12.8, with an average (for 59 observations) of +8.3. The 15N abundance of other plant parts (roots, stems, foliage, pods, seeds) was much more uniform (7,14). The largest difference between any of the other plant parts was only about 2 815N units, a modest difference in comparison with the usually quite large difference between nodules and other plant parts. The homogeneity of isotopic composition of nonnodular tissue persisted throughout the growing season, even during times of massive mobilization and transport of N from vegetative to reproductive tissues, a time at which isotopic fractionation might be expected to result in the alteration of isotopic abundances. By measuring the '5N abun- strongly correlated with N2-fixing efficiency (9).Kohl et al. (9) previously reported that the 15N abundance of nodules from nonleguminous N2-fixing plants was not elevated. Certainly this difference in nodular 15N abundance between soybeans, with its rhizobial symbiont, and nonlegumes, which are infected with actinomycetes, is a reflection of differences in the metabolism of fixed N. We report here results which contribute to answering the question: How widespread is the elevation of 15N abundance in nodules and what metabolic characteristics distinguish those plants which do and which do not have nodules with elevated 15N abundance? On the basis of previously reported results Kohl et al. (9) rejected the hypothesis that elevated 15N in nodules is the result of the denitrifying capability of the symbiont. Instead they proposed that the 5N abundance in nodules is a result of the difference between isotopic fractionation associated with the synthesis of nodule tissue and that which accompanies the concurrent synthesis of the main form of N transported from the nodule to the rest of the plant. If this hypothesis is correct, then one might expect the "5N abundance of nodules (compared to the rest of the plant) to vary among plant species, depending on the form of the major transport compound. METHODSWe analyzed the concentration and 15N abundance of the total N of various tissues of a number of N2-fixing plants (both legumes and nonlegumes) by methods previously described (12,13

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