**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** SOILS RETAIN LARGE POOLS OF CARBON AND CYCLE NUTRIENTS THAT SUSTAIN FOREST PRODUCTIVITY; TOGETHER THESE PROCESSES CAN SLOW CLIMATE CHANGE. UNDERSTANDING OF THE CONTROLS ON SOIL CARBON STORAGE INCREASINGLY RECOGNIZES THE IMPORTANCE OF MICROBIAL TURNOVER, PLANT BELOWGROUND ALLOCATION, AND STABILIZATION IN MINERAL SOILS, AS WELL AS THE INTERACTION OF THESE PROCESSES WITH NUTRIENT AVAILABILITY. YET, BOTH EXPERIMENTS AND EARTH SYSTEM MODELS REGULARLY NEGLECT THE DUAL ROLES OF NITROGEN AS NOT ONLY A NUTRIENT BUT ALSO AN ACIDIFIER, DESPITE THE BIOGEOCHEMICAL IMPORTANCE OF PH. THE OVERALL GOAL OF THIS PROJECT IS TO TEST HYPOTHESIZED MECHANISMS OF HOW NITROGEN AVAILABILITY AND PH INDEPENDENTLY AND INTERACTIVELY ALTER SOIL CARBON STORAGE. THESE PROCESSES INCLUDE: 1) STIMULATION OR SUPPRESSION OF MICROBIAL GROWTH, CONSUMPTION OF PARTICULATE ORGANIC MATTER, AND FORMATION OF MINERAL-ASSOCIATED ORGANIC MATTER; (2) CHANGES TO ABOVEGROUND PLANT AND LITTER PRODUCTION; (3) ALTERATIONS OF PLANT BELOWGROUND CARBON ALLOCATION; AND (4) CHANGES IN SORPTION OF DISSOLVED ORGANIC MATTER ONTO MINERAL SURFACES. WE PROPOSE TO TEST THESE MECHANISMS WITH A SUITE OF NEW MEASUREMENTS OF SOIL PARTICULATE AND MINERAL-ASSOCIATED ORGANIC MATTER POOLS AS WELL AS THE FATE OF DUAL-LABELED (13C, 15N) LITTER; PLANT PRODUCTIVITY AND BELOWGROUND ALLOCATION TO ROOTS, MYCORRHIZAE, AND ROOT EXUDATES AS WELL AS EXUDATE COMPOSITION; AND DISSOLVED ORGANIC MATTER COMPOSITION AND SORPTION, ALL AT A LARGE-SCALE LONG-TERM NITROGEN BY PH MANIPULATION EXPERIMENT IN MIXED DECIDUOUS FORESTS. TOGETHER, THE PROPOSED RESEARCH SUPPORTS THE GOALS OF THE USDA-AFRI SOIL HEALTH PROGRAM AREA TO ADVANCE SCIENTIFIC UNDERSTANDING OF SOIL BIOGEOCHEMICAL INTERACTIONS AND ENABLE IMPROVEMENTS TO BOTH MODELS AND LONG-TERM FOREST MANAGEMENT TO STORE CARBON AND SUSTAIN PRODUCTION OF TIMBER AND BIOFUELS.