Interactions among nitrogen fixation and soil phosphorus acquisition strategies in lowland tropical rain forests

Paradoxically, symbiotic dinitrogen (N-2) fixers are abundant in nitrogen (N)-rich, phosphorus (P)poor lowland tropical rain forests. One hypothesis to explain this pattern states that N-2 fixers have an advantage in acquiring soil P by producing more N-rich enzymes (phosphatases) that mineralise organic P than non-N-2 fixers. We assessed soil and root phosphatase activity between fixers and non-fixers in two lowland tropical rain forest sites, but also addressed the hypothesis that arbuscular mycorrhizal (AM) colonisation (another P acquisition strategy) is greater on fixers than non-fixers. Root phosphatase activity and AM colonisation were higher for fixers than non-fixers, and strong correlations between AM colonisation and N-2 fixation at both sites suggest that the N-P interactions mediated by fixers may generally apply across tropical forests. We suggest that phosphatase enzymes and AM fungi enhance the capacity of N-2 fixers to acquire soil P, thus contributing to their high abundance in tropical forests.