MICROBIAL AND MOLECULAR INSIGHTS INTO HOW SHIFTS IN HYDROLOGIC PORE CONNECTIVITY FROM DROUGHT OR FLOODS REGULATE THE ROLE OF SOIL IN THE TERRESTRIAL CARBON CYCLE (Invited Presentation)

The three-dimensional arrangement of particles and pore spaces that make up the soil matrix ultimately governs the fate and transport of water, nutrients, microorganisms and soluble organic carbon (OC). Shifts in soil water content interact with soil pore architecture thereby influencing hydrologic pore connectivity and the ability of microorganisms to access and mineralize OC, therefore, shaping soil greenhouse gas emissions. Linking microbial structure, and function with pore-scale biogeochemical processes to core-scale greenhouse production is often confounded by complex geochemical, hydrological and physical interactions occurring in the soil matrix. This is especially relevant for understanding the source and sink potential of soils in response to extreme hydrologic events, such as droughts and floods. Our objective was to identify how droughts and floods influence (1) the molecular composition of OC at pore-scale, (2) the genetic and functional potential of soil microorganisms, with (3) core-scale greenhouse gas production across three sites/soil types. Our results show that soil type was a more important predictor than drought or flood treatment on the abundance and molecular composition of OC in pore waters retained by fine (6-20 µm) or coarse (>200 µm) pore throats. The functional potential of microbes (i.e., metagenomes) also varied by soil type and not treatment, whereas microbial gene expression (i.e., metatranscriptomes) was influenced by soil type and treatment, albeit there were many soil-specific responses to the drought and flood treatments. Similarly, core-scale greenhouse gas emissions (CO₂ and CH₄) showed varied responses across soil types and treatments. However, all soils subjected to drought experienced significant increases in CO₂ when rewet. Most notably, differences in pore size distribution and water-filled pore space at set soil moisture contents differed for each soil. These site-specific responses suggest that the impact of extreme climate events is not uniform for all soils, and may be linked to differences in pore structure, and hydrologic connectivity.