Rapidly fluctuating redox regimes frame the ecology of microbial communities and their biogeochemical function in a humid tropical soil

Soil oxygen availability and redox potential play a fundamental role in terrestrial biogeochemical cycling by regulating microbial metabolism and fluxes of inorganic nitrogen and trace gases. In upland humid tropical forest soils, fluctuating redox conditions result from high biological O 2 demand, warm temperatures, and fine textured soils that limit diffusive transport. In my dissertation research I explored microbial adaptation to fluctuating redox in tropical soils, comparing communities from dynamic and static redox environments. Stochastic environmental conditions engendered high diversity and spatial heterogeneity amongst soil bacteria, selecting for highly responsive organisms that tolerate unfavorable redox periods. Indigenous bacterial communities retained higher phylogenetic diversity and biomass when incubated under variable redox conditions. Microbial community complexity and redox biogeochemistry was spatially distinct at sub-meter scales, hence sampling small volumes of soil gases yielded a tractable bioreport of soil redox conditions.; Fluctuating redox conditions allowed for alternating anoxic and oxic N processing. In the field, these reactions occur simultaneously due to high spatial heterogeneity and reductant availability. Indigenous microorganisms soils have evolved capacities for rapid N cycling and trace gas production/consumption, allowing utilization of resource pulses prior to leaching (labile C, NO 3-), plant uptake (NO3-, NH 4+), or diffusion (H2, O2) loss. Nitrifiers and organisms using dissimilatory nitrate reduction to ammonium (DNRA) were particularly tolerant of physiologically unfavorable redox periods. Nitrifying communities regained capacity rapidly following 3-6 week anoxic periods and had the highest autotrophic nitrification capacity (Vmax ) when incubated under 4-6 day oxic/anoxic regimes. DNRA occurred in both fluctuating and static redox soils and was more strongly regulated by NO3- availability than bulk soil redox, indicating the organisms mediating this reductive process tolerate oxic conditions. Multivariate statistics allowed correlation of DNA-based fingerprints with process data from 15N experiments and trace gas measurements, thus linking biogeochemistry and community composition. Microbial community structure was closely linked to processes that regulate NO3- production and consumption (nitrification, DNRA, denitrification). In soils where O 2 is frequently depleted and resupplied, characteristics of microbial tolerance and resilience thus frame N cycling patterns. These results illustrate how soil physiochemical characteristics shape microbial community composition and function and ultimately constrain ecosystem function.