Fulvic Acid and Mycorrhizal Inoculation Alter Root-Zone Metabolite Profiles and Nitrogen Uptake in Soilless Substrate-Grown Tomato under Controlled-Environment Conditions

Abstract

Improving nitrogen use efficiency (NUE) is essential for tomato production in soilless controlled-environment agriculture (CEA). We previously showed that arbuscular mycorrhizal fungi (AMF) and fulvic acid (FA) coapplication enhanced tomato biomass, nitrogen uptake efficiency (NUpE), and root-zone substrate β-glucosidase activity. To characterize the underlying root-zone metabolic responses, untargeted LC–MS/MS metabolomics was performed on substrate from tomato seedlings grown under control, FA, AMF, and AMF + FA at 10 and 100 mg L–1 N. Of 118 annotated compounds, organic acids and derivatives dominated. Treatment effects were pronounced at 100 mg L–1, where PLS-DA identified 30 VIP discriminants, mostly amino acids and peptides; N-acetylarginine, gamma-glutamylleucine, Ser-Val, and 3,4-dihydroxy-l-phenylalanine accumulated under AMF and AMF + FA. OPLS-DA models further indicated enrichment of l-glutamine and N-acetylarginine in AMF + FA over AMF alone. AMF inoculum-containing treatments were associated with altered root-zone substrate metabolite profiles, and coapplication with FA amplified these shifts; these changes accompanied the improved nitrogen uptake in soilless CEA-grown tomato.