Greater root phosphatase activity in nitrogen‐fixing rhizobial but not actinorhizal plants with declining phosphorus availability

Summary The abundance of nitrogen (N)‐fixing plants in ecosystems where phosphorus (P) limits plant productivity poses a paradox because N fixation entails a high P cost. One explanation for this paradox is that the N‐fixing strategy allows greater root phosphatase activity to enhance P acquisition from organic sources, but evidence to support this contention is limited. We measured root phosphomonoesterase ( PME ) activity of 10 N‐fixing species, including rhizobial legumes and actinorhizal Allocasuarina species, and eight non‐N‐fixing species across a retrogressive soil chronosequence showing a clear shift from N to P limitation of plant growth and representing a strong natural gradient in P availability. Legumes showed greater root PME activity than non‐legumes, with the difference between these two groups increasing markedly as soil P availability declined. By contrast, root PME activity of actinorhizal species was always lower than that of co‐occurring legumes and not different from non‐N‐fixing plants. The difference in root PME activity between legumes and actinorhizal plants was not reflected in a greater or similar reliance on N fixation for N acquisition by actinorhizal species compared to co‐occurring legumes. Synthesis . Our results support the idea that N‐fixing legumes show high root phosphatase activity, especially at low soil P availability, but suggest that this is a phylogenetically conserved trait rather than one directly linked to their N‐fixation capacity.