ORGANIC FARMERS IDENTIFY IMPROVED COVER CROP OPTIONS AS A HIGH PRIORITY AREA FOR RESEARCH, AND RATE SYMBIOTIC NITROGEN FIXATION AS THE MOST IMPORTANT TRAIT FOR LEGUMINOUS SPECIES. LEGUMINOUS COVER CROPS ARE THE PRIMARY SOURCE OF NEW NITROGEN FOR ORGANIC AGRICULTURE AND PROVIDE A RANGE OF ECOSYSTEM SERVICES VALUED BY ORGANIC GROWERS. THE PERFORMANCE OF VETCH COVER CROPS ACROSS FARMERS' FIELDS IS HIGHLY VARIABLE, MAKING IT DIFFICULT FOR ORGANIC FARMERS TO MANAGE THEIR N FERTILITY. THE OVERARCHING GOAL OF THIS PROJECT IS TO ELUCIDATE HOW INTERACTIONS AMONG SOIL FERTILITY, PLANT GENOTYPE, AND SYMBIONTS DRIVE THE N ACQUISITION STRATEGIES OF HAIRY VETCH (VICIA VILLOSA ROTH) IN ORGANIC SYSTEMS. WE FOCUS OUR RESEARCH ON HAIRY VETCH BECAUSE OF ITS IMPORTANCE AS A COVER CROP IN ORGANIC FARMING, PARTICULARLY IN COLDER CLIMATES WHERE COVER CROP SPECIES OPTIONS ARE MORE LIMITED. SYMBIOTIC NITROGEN FIXATION IS A COMPLEX PROCESS GOVERNED BY THE THREE-WAY INTERACTION INVOLVING THE PLANT HOST, SYMBIONTS (RHIZOBIA BACTERIA AND ARBUSCULAR MYCORRHIZAL FUNGI), AND THE SOIL ENVIRONMENT. WE HAVE FOUR OBJECTIVES. 1) QUANTIFY THE RELATIVE IMPORTANCE OF PLANT GENOTYPE, SOIL NITROGEN AVAILABILITY AND GENOTYPE X ENVIRONMENTAL INTERACTIONS IN GOVERNING THE CAPACITY AND PLASTICITY OF NITROGEN FIXATION IN HAIRY VETCH. 2) CHARACTERIZE RHIZOBIA POPULATIONS THAT REFLECT DIFFERENT SELECTION PRESSURES AND IDENTIFY STRAINS WITH IMPROVED PLANT GROWTH BENEFITS FOR HAIRY VETCH. 3) QUANTIFY DIFFERENCES IN AM FUNGAL COLONIZATION ACROSS HAIRY VETCH BREEDING LINES/POPULATIONS. 4) DEVELOP CURRICULA MODULES COVERING THE BASICS OF LEGUME BIOLOGY AND MANAGEMENT OF LEGUMES FOR NITROGEN SUPPLY TO CROPS IN ORGANIC SYSTEMS.