Variation of N natural abundance in leaves and nodules of actinorhizal shrubs in Northwest Patagonia

This study was performed to assess the N2-fixing capability of the native actinorhizal species Ochetophila trinervis (sin. Discaria trinervis) and Discaria chacaye (Rhamnaceae) in Northwest Patagonia. We measured the N concentration and N natural abundance in leaves and nodules of O. trinervis and D. chacaye, in leaves of associated non-actinorhizal vegetation, and in the soils under each sampled plant. O. trinervis and D. chacaye had foliar N concentrations that were about twice that of non-actinorhizal shrubs growing at the same sites, even though soils varied four-fold in total N across the sites. Leaves of both actinorhizal plants had a similar δN at any site and were close to atmospheric values. The foliar δN of non-actinorhizal plants and soil δN were strongly correlated across the sites. Nodules were depleted in δN relative to the foliage of the respective actinorhizal species. In conjunction with the uniformly high foliage N concentration of these actinorhizal plants and the universal presence of vesicles observed in root nodules, these data strongly suggest that O. trinervis and D. chacaye obtain a significant amount of their N from N2 fixation. To calculate the proportion of N derived from atmosphere, theoretical B-values were estimated. In all cases where the δN of fixing and reference foliage were significantly different, O. trinervis and D. chacaye obtained almost all of their N from N2 fixation. These results are the first to demonstrate N2 fixation by O. trinervis and D. chacaye in the field and therefore suggest an important role for these actinorhizal plants in the N economy of ecosystems in northwest Patagonia as well as their potential use for restoration of degraded lands in this region.