Collembola as mediators of microbial symbiont effects upon soybean

The hypothesis that collembola affect rhizobia and mycorrhizas of soybean (Glycine max) and thus indirectly change leaf tissue nutrient concentration was studied in pot and field experiments. When a high density of the collembolan species Folsomia candida, was added to pots, the number of nodules per plant increased 52%. When moderate densities of two collembolan species, Folsomia candida and Tullbergia granulata, were added in factorial combinations to cylinders sunk in the soil around soybean in fields, the following responses were observed: 40% greater mycorrhizal root length, and 5% higher leaf tissue N, but no changes in leaf P, nodule number or root mass. Collembola density in the field was too low to increase nodule number per plant as observed in pot experiments: there was no mechanism to explain the 5% increase in leaf tissue N associated with collembola in the field. In the field, intermediate densities of collembola were associated with greater mycorrhizal root length, but since available soil P concentrations were high, longer mycorrhizal root length was not associated with higher leaf tissue P. A path model showed that if mycorrhizas had been positively associated with higher leaf P, the indirect effect of collembola would have been significantly higher leaf tissue P. This study showed that both available soil P and collembola density determine mycorrhizal benefits. In natural habitats, intermediate collembola density and low soil P are expected to maximize benefits of mycorrhizas to plants.