Influence of environmental effects on Acacia-rhizobial community structure and plant productivity
Genetic variation in rhizobial symbionts can play an important role in influencing the benefits plants receive from their mutualistic partners. This can be critical given environmental variation, and differences in host specificity and effectiveness of rhizobial genotypes to their plant hosts, resulting in genotype by environmental interactions can have net positive or negative effects on plant productivity. We were interested in understanding how environmental stress can affect plant productivity and rhizobial community structure, as well as how differences in rhizobial community composition can influence plant productivity. Individual host plants of Australian native Acacia salicina and A. stenophylla were inoculated with a community of six rhizobial symbionts and then subjected to a full factorial design of two levels each of nitrogen, phosphorus, and water. We recovered rhizobial strain type and relative abundance through amplicon sequencing of the rhizobial 16S gene from root nodules of the plants upon harvesting the aboveground biomass. Half of the experiment was harvested at four months and the other half was harvested at eight months to look at temporal differences in response.
The model that best explained variation in aboveground biomass of plants included the main effects (species, nitrogen, phosphorus, and water) and the interactions between phosphorus and species, water and species, and water and nitrogen. Productivity (aboveground biomass) was higher for plants that received greater concentration of nutrients and water. Rhizobial community diversity was best explained by interactions between species and nitrogen, species and phosphorus, and the three main effects, nitrogen, phosphorus, and water, but only at the final harvest. The proportion of individual strains present was best explained by species and the phosphorus by species interaction. Lastly, we found a negative relationship between plant productivity and the number of rhizobial genotypes detected in root nodules, but no relationship between rhizobial community diversity and plant productivity. Our data show that changes in the environment can have major effects on plant productivity, generally resulting in a higher biomass of plants. Although environment can play a role in determining rhizobial community structure and diversity, host species appears to be a bigger driver of rhizobial communities. Our data also support past studies that higher rhizobial diversity may not always be positively correlated with productivity for plant hosts, and there may be higher order effects (e.g. rhizobial competition, plant sanctions) that complicate the relationship between plant hosts and their rhizobial symbionts.