Microbes on the redox ladder: Community and trace gas dynamics (H2, O2, CO2, N2O, CH4, CO) in a highly variable wet tropical soil

Microbial inhabitants of wet tropical and temperate rainforest soils actively produce and consume atmospheric trace gases, yet little is known about in situ dynamics of trace gases and microbial populations in such systems.  This is particularly true in soils where redox fluctuates rapidly, creating a mosaic of spatial and temporal gradients.  Of particular interest are the mechanisms regulating soil fluxes of molecular hydrogen (H₂), as there is significant uncertainty in the global atmospheric H₂ budget. Using soils from a wet tropical forest, we assayed microbial community composition and soil O₂ along with other redox-active chemical species (H₂, N₂O, CH₄, CO₂, CO) in both field and laboratory settings.  Under controlled conditions, increasing the labile organic carbon pool by 5% increased the rate of O₂ consumption tenfold, dramatically accelerated the timescale of redox depletion, and enhanced CO₂, H₂, and CH₄ production.  Under oxic conditions, we saw significant production of N₂O.  Immediately after O₂ disappeared, a pulse of H₂ production occurred, but was quickly consumed by methanogens. Under highly reduced conditions, we measured high concentrations of CH₄, followed by anaerobic consumption.  In situ field measurements of the same gases were highly variable, yet also showed sequential redox processing in both space and time.  With daily sampling, we show that belowground O₂ depletion and N₂O, CH₄, and H₂ production are correlated with rainfall from the previous two days.  These gases are also correlated with variations in microbial community composition that occur along a spatial redox gradient.  Using universal 16S rRNA microarrays, we identified over 2000 distinct operational taxonomic groups (OTUs) in these soils and analyzed spatial patterns for several phylogenetic/functional groups.  While diversity (richness) does not vary along our redox gradient, the DNA copy number (a proxy for abundance) of methanogenic archaea and clostridia is significantly higher in the low-redox end of our gradient.