Nitrogen fixation by riparian plants belonging to Coriariaceae, Rhamnaceae, and Gunneraceae in Northwest Patagonia

Nitrogen fixation by symbiotic bacteria associated with different plant species is a key process of natural ecosystems. To better understand the role of native N-2-fixing species in the N economy of riparian ecosystems in northwest Patagonia (Argentina), we evaluated: 1) foliar N-15 and N concentrations of actinorhizal Coriaria ruscifolia, Discaria chacaye, and Colletia hystrix, several non-actinorhizal plants (including Gunnera chilensis), and associated soils in riparian forest sites; 2) the proportion of N derived from the atmosphere (Ndfa) of the actinorhizal plant species from riparian forest and of D. chacaye and Ochetophila trinervis from steppe sites; and 3) trends of foliar and soil N-15 and N concentration with mean annual precipitation (MAP). Although soil N concentrations and N-15 did not vary among plant species within any of the sites, foliar N concentration and N-15 differed among species. In general, N-2-fixing species had higher foliar N concentrations and N-15 values closer to 0, the atmospheric value. Both variables separated the groups of N-2-fixing and non-N-2-fixing plant species. Foliar and soil N-15 correlated positively for non-N-2-fixing species but not for N-2-fixing species. Across all sites, the Ndfa of C. ruscifolia and C. hystrix was similar to 100%, similar to 75% for D. chacaye, and similar to 50% for G. chilensis. For all species, foliar N concentration and soil N-15 was negatively correlated with MAP, but only non-N-2-fixing species showed a significant correlation of foliar N-15 with MAP. These data suggest that plant available N decreases as MAP increases but with no effect on N-2-fixing species.