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

Abstract

Nitrogen fixation by symbiotic bacteria associated with different plant species is a key process of natural ecosystems. To better understand the role of native N2-fixing species in the N economy of riparian ecosystems in northwest Patagonia (Argentina), we evaluated: 1) foliar δ15N 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 δ15N and N concentration with mean annual precipitation (MAP). Although soil N concentrations and δ15N did not vary among plant species within any of the sites, foliar N concentration and δ15N differed among species. In general, N2-fixing species had higher foliar N concentrations and δ15N values closer to 0, the atmospheric value. Both variables separated the groups of N2-fixing and non-N2-fixing plant species. Foliar and soil δ15N correlated positively for non-N2-fixing species but not for N2-fixing species. Across all sites, the Ndfa of C. ruscifolia and C. hystrix was ~100%, ~75% for D. chacaye, and ~50% for G. chilensis. For all species, foliar N concentration and soil δ15N was negatively correlated with MAP, but only non-N2-fixing species showed a significant correlation of foliar δ15N with MAP. These data suggest that plant available N decreases as MAP increases but with no effect on N2-fixing species.