Spatial distribution of roots and nodules, and δ15N evidence of nitrogen fixation in Adesmia volckmanni, a Patagonian leguminous shrub

Descriptions of the spatial distribution of nodules and their relationship with the distribution of roots are extremely rare in leguminous shrubs of arid ecosystems. In this paper we show that in the Patagonian arid steppe the spatial distribution of nodules of Adesmia volkmanni Philippi (ex- A. campestris) paralleled that of their roots. Nodules and roots of A. volkmanni were concentrated in intermediate (70–90 cm) soil layers, but did not appear to be biologically active (no visual signs of leg-haemoglobin). In addition, leaves showed the same natural 15N abundance (δ15N) than those of another native shrub species, Mulinum spinosum, a known non-fixer with the same rooting and phenological patterns. However, high leaf N content, presence of nodules, and clear isotopic differences between the two species in a more humid site suggest that nitrogen fixation could occur in some favourable years in Adesmia volkmanni plants of xeric sites.