Relationships Between Arbuscular Mycorrhizal Fungi, Soil Aggregate Turnover and Bacterial Communities.

We conducted a series of experiments to examine relationships between arbuscular mycorrhizal fungi (AMF), soil structure, aggregate turnover dynamics and phylotype-specific bacterial activities. A system for imposing AMF and no-AMF treatments to small volumes of soil, while keeping all other factors (e.g. water potential) equal, was developed. Soil structure was examined based on stable aggregate size distributions. Aggregate turnover was examined using reconstituted soil in which each aggregate size fraction was labeled with a different rare earth oxide (REE). Aggregate turnover rates were calculated based on transfers of REE labels between different aggregate size classes after treatments were imposed. To evaluate which bacterial groups are active in the presence or absence of AMF, we used a massive parallel sequencing approach in which thousands of cDNA sequences derived from 16S rRNA of each sample were examined.

Our results indicate that our experimental system allows for very rapid AMF hyphal infiltration into treatment soils.  Large differences in aggregate turnover dynamics between soils of AMF and no-AMF treatments were detected. Soils of the no-AMF treatment exhibited substantial turnover, with multiple transfers from different size fractions apparent. In contrast, small macroaggregate, microaggregate and silt/clay soil fractions of AMF treated soils were rapidly incorporated (< 14 days) into large macroaggregate fractions where they stably maintained for at least 35 days. These results highlight the importance of AMF to soil aggregate stabilization and suggest significant alteration of microbial habitats can occur due to AMF influences. Specific changes in activities of specific bacterial groups in relation to these changes will be discussed.