SWITCHGRASS, AN IMPORTANT BIOENERGY CROP, RAISED HIGH EXPECTATIONS IN ITS C ACCRUAL POTENTIAL DUE TO ITS ENORMOUS BELOWGROUND PRODUCTION. SURPRISINGLY, IN A NUMBER OF SOILS AND ENVIRONMENTS THESE EXPECTATIONS REMAINED UNREALIZED, POINTING TO AN IMPORTANT KNOWLEDGE GAP. THE GOAL OF THE PROPOSED PROJECT IS TO FILL THIS GAP BY ADVANCING PROCESS-BASED UNDERSTANDING OF MECHANISMS DRIVING SOIL C SEQUESTRATION AND BY PROVIDING THE THEORETICAL BACKGROUND FOR DEVELOPMENT OF SUSTAINABLE BIOENERGY AND ARABLE CROPPING SYSTEMS WITH ENHANCED C SEQUESTRATION CAPABILITIES. WE POSIT THAT THE HERETOFORE OVERLOOKED COMPONENTS OF THE SOIL C ACCRETION PROCESS ARE INTERACTIONS AMONG (I) PLANT ROOTS, (II) THEIR EFFECTS ON SOIL PHYSICAL MICRO-ENVIRONMENTS, AND (III) THE RESULTANT EFFECTS ON SPATIAL PATTERNS IN DISTRIBUTIONS AND ACTIVITIES OF SOIL MICROORGANISMS. OUR INTERDISCIPLINARY TEAM WILL EMPLOY A UNIQUE COMBINATION OF NEW AND ESTABLISHED TECHNIQUES FOR ADDRESSING THE PROJECT'S GOAL, INCLUDING X-RAY COMPUTED MICRO-TOMOGRAPHY, 13C FIELD AND LAB PLANT LABELING, 3D MICRO-SCALE SOIL ORGANIC MATTER MAPPING BY OS STAINING, STABLE ISOTOPE PROBING, MICRO-SCALE ZYMOGRAPHIC MAPPING OF EXTRACELLULAR ENZYMES, AND PLANT ROOT ANALYSES. THE KEY OUTCOMES WILL BE IDENTIFYING THE PLANT ROOT TRAITS THAT LEAD TO FORMATION OF OPTIMAL PORE SIZE DISTRIBUTIONS FOR MAXIMUM C SEQUESTRATION, DECIPHERING THE MECHANISMS BY WHICH THESE TRAITS ENHANCE C GAINS, AND SUGGESTING IMPROVEMENTS IN BIOENERGY CROPPING SYSTEMS, NAMELY, SWITCHGRASS, TO INCREASE ITS C SEQUESTRATION CAPABILITIES. THE PROJECT WILL DIRECTLY CONTRIBUTE TO AFRI PROGRAM'S GOALS OF PROVIDING NEW UNDERSTANDING OF PHYSICAL, CHEMICAL, AND BIOLOGICAL INTERACTIONS FOR DEVELOPMENT OF CROPPING SYSTEMS WITH REDUCED ENVIRONMENTAL FOOTPRINT AND ENHANCED NATURAL RESOURCE BASE.