Soil organic carbon (SOC) has long been recognized as a dominant reservoir of carbon in terrestrial ecosystems, though over the last century, SOC pools have declined dramatically with human disturbance, particularly in cultivated ecosystems. Multiple mechanisms contribute to the capacity of soil to stabilize and sequester organic carbon. Little is known, however, about the identities and functional capabilities of diverse microbial inhabitants of soil and soil aggregates, though it is clear that the soil physical environment constrains and enables microbial activity, and microbial activity shapes the soil physical environment. This interaction among biotic and abiotic components has the capacity to affect long-term SOC storage. As a first step toward exploring links between bacterial community composition and the extent of SOC storage, we have used massively parallel 454 tag sequencing of the V6 region of bacterial rRNA to characterize the bacterial community in a suite of soils of differing SOC content. The field soils were taken from long-term experimental plots in Storrs, CT, where soils are already poised at four different SOC contents after four decades of four management treatments in split-plot design: no-till and conventional till, each with silage removed or only grain removed each year. This system offers the statistical power of (1) triplicate plots for each treatment, and (2) a dramatic and consistently expressed gradient in soil aggregation and organic carbon content (20-40 gC kg^-1­_soil) across four treatments.  

Results/Conclusions

Data show clear shifts in frequencies of particular bacterial groups across treatments, and those shifts may be explained by co-varying soil characteristics including carbon content, aggregate size distribution, and gravimetric soil moisture.  For example, the frequency of organisms typically found in anoxic environments, such as Chlorobi, increased as SOC decreased, as expected due to the high water content of low-carbon soils collected in late autumn.  The frequency of Conexibacter woesei (Actinobacteria::Rubrobacterales) increased with increasing SOC, perhaps reflecting the presence of more microaggregates within macroaggregates in no-till soils (as observed by Six et al. 2000), since Rubrobacteria are known to be enriched in the microaggregate fraction of diverse soils (Mummey et al. 2006).