** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** AGRICULTURAL MICROBIOMES CONFER MANY BENEFITS FOR CROP PRODUCTIVITY, PLANT-, AND SOIL HEALTH. HOWEVER, NITROGEN USE EFFICIENCY IN AGRICULTURAL CROPS CONTINUES TO BE LESS THAN OPTIMAL. MICROBIAL NITRIFICATION IS CONSIDERED AS A PRIMARY CAUSE FOR THIS AND LARGE KNOWLEDGE GAPS REMAIN ON HOW ORGANISMAL INTERACTIONS CONNECT CARBON AND NITROGEN MINERALIZATION, CROP NUTRITION, NITRIFICATION, AND NITROGEN LEACHING IN AGRICULTURAL SYSTEMS. THIS PROJECT AIMS TO ELUCIDATE HOW PLANTS AND NITRIFIERS INTERACT IN THE RHIZOSPHERE, AND WHICH CHEMICAL HANDOFFS OCCUR BETWEEN BOTH GROUPS. THE SPECIFIC OBJECTIVES ARE 1) TO ENRICH REPRESENTATIVES OF GLOBALLY-PREDOMINANT SOIL AMMONIA-OXIDIZING ARCHAEA (AOA) IN LABORATORY CULTURE, AND DETERMINE GENOME SEQUENCES AND GENE EXPRESSION PATTERNS, 2) TO IDENTIFY AND CHARACTERIZE ASSOCIATIONS OF AOA WITH THE RHIZOSPHERE AND MYCORRHIZAL FUNGI OF A DIVERSE RANGE OF AGRICULTURAL CROPS AND OTHER LAND PLANTS, AND 3) TO CHARACTERIZE PHYSICAL AND CHEMICAL INTERACTIONS BETWEEN NITRIFIERS, MYCORRHIZAL FUNGI, AND PLANTS IN LABORATORY MODEL SYSTEMS. THE PROJECT ADDRESSES AFRI PROGRAM AREA PRIORITY 1402 AGRICULTURAL MICROBIOMES IN PLANT SYSTEMS AND NATURAL RESOURCES BY CHARACTERIZING MOLECULAR MECHANISMS AND SIGNAL EXCHANGE INVOLVED IN MICROBIOME ASSEMBLY AND INTERACTIONS, AS WELL AS BY FUNCTIONALLY CHARACTERIZING MICROBIOMES AND MICROBIOME METABOLITES FOR OPTIMIZATION OF ENVIRONMENTAL PROCESS. THE PROJECT WILL PROVIDE A COMPREHENSIVE MODEL FOR N TRANSFER WITHIN THE SOIL-RHIZOSPHERE-PLANT ROOT CONTINUUM. IT WILL GENERATE SIGNIFICANT INSIGHT INTO ACTIVITIES OF ARCHAEAL NITRIFIERS WITH A POTENTIAL FOR IMPROVED MANAGEMENT PRACTICES FOR INCREASED NITROGEN USE EFFICIENCY AND SUSTAINABILITY. THE PROJECT WILL FURTHER TRAIN SOIL MICROBIOLOGISTS AT THE UNDERGRADUATE, GRADUATE, AND POSTGRADUATE LEVELS.