Microbial dynamics of tropical nitrogen fixation: Relaxed selectivity for nodule symbionts under reduced N demand

Abstract

Symbiotic nitrogen fixing plants supply tropical forests with much of the nitrogen needed to recover from land use change, respond to disturbance, and to potentially serve as vast carbon sinks under anthropogenic change. However, fixation and the associated rapid plant growth taper off in characteristic ways as forests progress through successional time, relative N supply becomes unproblematic, and active fixation loses its competitive advantage. Although actual N fixation is carried out by microbial symbionts housed inside plants’ root nodules, little is known about how the dynamics of the plant-microbe mutualism map onto broad-scale fixation patterns and perhaps contribute to tropical forest growth limitations. In order to investigate potential microbial controls on fixation patterns, this metagenomic study compares partial 16S rRNA data from the bulk soil, the rhizosphere, and the root nodules of a representative N fixing tree at two key successional time points in the tropical forests of Panama—in early succession, when pressure to fix N is high; and in mid-succession, when pressure to fix N is low. Doing so reveals decreased selectivity for nodule community structure under reduced pressure for N fixation, as well as intriguing looks into the give-and-take of this essential mutualism.