Plant-regulated biological nitrification inhibition: nitrogen sensing, root exudate biochemistry, and nitrifier...

Nitrification is a major route of nitrogen loss from agricultural soils, yet it is increasingly clear that plants are important for microbial nitrogen cycling. Through nitrogen-form sensing, proton extrusion and selective root exudation, roots can actively remodel the chemical and biological environment in which nitrifiers operate. Biological nitrification inhibition (BNI), the suppression of nitrifier activity by root-derived compounds and associated rhizosphere processes, provides a compelling example of plant-mediated control over soil nitrogen cycling. Here, we critically synthesize BNI from a plant physiology and biochemistry perspective, focusing on how ammonium nutrition, rhizosphere acidification, plasma-membrane H⁺-ATPase activity, and exudate transport regulate BNI release. We integrate the chemistry of hydrophilic and hydrophobic BNIs with their mobility, persistence, and accessibility to ammonia-oxidizing bacteria, ammonia-oxidizing archaea, comammox Nitrospira, and nitrite-oxidizing bacteria. Characterized BNIs act through multiple mechanisms, including inhibition of nitrification enzymes (AMO, HAO, and NXR), redox modulation, reactive oxygen and nitrogen species interactions, and trace-metal interference. This diversity suggests that BNI is not a single-compound effect, but an inducible rhizosphere trait shaped by plant metabolism, root transport, and microbial target specificity. We propose a plant-centered framework linking nitrogen sensing, secondary metabolite biosynthesis, exudate delivery, and nitrifier responses. This review highlights the need to resolve the biosynthetic pathways, transporter systems, and regulatory signals that control BNI release, and to validate their functions under realistic plant-soil conditions. By framing BNI as a plant-regulated biochemical trait, we provide a basis for understanding how roots influence rhizosphere nitrogen availability and for developing crops with improved nitrogen acquisition and reduced nitrification losses.