COLLEMBOLA AS MEDIATORS OF MICROBIAL SYMBIONT EFFECTS UPON SOYBEAN

The hypothesis that collembola affect rhizobia and mycorrhizas of soyb ean (Glycine max) and thus indirectly change leaf tissue nutrient conc entration was studied in pot and field experiments. When a high densit y 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, we re added in factorial combinations to cylinders sunk in the soil aroun d soybean in fields, the following responses were observed: 40% greate r mycorrhizal root length, and 5% higher leaf tissue N, but no changes in leaf P, nodule number or root mass. Collembola density in the fiel d was too low to increase nodule number per plant as observed in pot e xperiments: there was no mechanism to explain the 5% increase in leaf tissue N associated with collembola in the field. In the field, interm ediate densities of collembola were associated with greater mycorrhiza l root length, bur since available soil P concentrations were high, lo nger mycorrhizal root length was not associated with higher leaf tissu e P. A path model showed that if mycorrhizas had been positively assoc iated 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.