Estimates of N₂-fixation from variation in the natural abundance of ¹⁵N in Sonoran desert ecosystems

Summary

The ¹⁵N 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. The ¹⁵N abundance of soil and of tissues of presumed non-N₂-fixing (control) plants was also measured. Prosopis tissues were significantly lower in ¹⁵N than either soil N or corresponding tissues of presumed non-N₂-fixing plants which derive their N entirely from soil. Soil N was also significantly higher in ¹⁵N than atmospheric N₂. We conclude that it is feasible to use variations in the natural abundance of ¹⁵N as an index of N₂-fixation in this kind of ecosystem, and that N₂-fixation is of considerable importance to Prosopis growing at this site.

We also determined the ¹⁵N abundance of leaf tissue of presumed N₂-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 ¹⁵N abundances of the pooled legume population and control plants at all sites, although not every legume specimen exhibited this difference. From ¹⁵N abundance data we estimated the fractional contribution of biologically fixed N to the N economy of desert legumes. We concluded that N₂-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 N₂, N₂-fixation was important only for legumes of the sub-family Papilionoideae, Lupinus, Dalea, Astragalus and Lotus.