N-206. Specific Microbial Populations Thrive under Fluctuating Redox Conditions in Tropical Soils

K. M. DeAngelis1, W. L. Silver2, A. W. Thompson2, M. K. Firestone2;
1Lawrence Berkeley Natl. Lab., Berkeley, CA, 2Univ. of California, Berkeley, CA.

The highly weathered soils of upland humid tropical forests are characterized by rapidly fluctuating redox conditions, dominated by Fe-oxide mineralogy, and have relatively low sulfate availability. To assess how fluctuating redox conditions and accompanying biogeochemistry impact microbial community structure and function, we collected soil cores from the Luquillo LTER forest in Puerto Rico and incubated them for 32 days under one of three redox regimes: static oxic, static anoxic, and 4-day fluctuating redox. Over this time course we measured CO2, CH4, and N2O production, amorphous iron and Fe(II), and microbial community structure by high density microarray (PhyloChip) analysis. Static oxic, anoxic, and fluctuating redox soils all had statistically indistinguishable respiration rates over the course of the experiment. Fluctuating redox conditions permitted simultaneous methanogenesis, N2O production, and iron reduction, all accompanied by steady CO2 production. We analyzed the standing and active microbial community using the 16S ribosomal DNA and RNA biomarkers, identifying 2489 taxa in these soils. Ordination analysis showed significant separation between the active (RNA-based) and standing (DNA-based) communities, with much more variation in the active community compared to the standing community. Fluctuating redox conditions maintained a microbial community structure similar to that of the pre-incubation samples, while static anaerobic conditions had the most profound effect on the communities. Finally, there was considerable overlap between the taxa that were the most highly correlated with production of CH4 and Fe(II). Association of groups of taxa with specific biogeochemical processes begins to identify organisms potentially responsible for field biogeochemical processing.