Kanellopoulos, A. E.; Malits, A.; Ribeiro, H.; Kerou, M.; Ghatak, A.; Chaturvedi, P.; Weckwerth, W.; Karpouzas, D. G.; Schleper, C.; Papadopoulou, E. S.

Nitrogen cycling is critical for ecosystem functioning, with nitrification playing a central role. Excessive nitrification, triggered by heavy nitrogen fertilisation, contributes to reduced nitrogen use efficiency, nitrate leaching, and nitrous oxide emissions. While synthetic nitrification inhibitors (SNIs) help reduce these impacts, their erratic performance and environmental risks have shifted focus to biological nitrification inhibitors (BNIs) as sustainable alternatives. In vitro bioassays with ammonia-oxidising microorganisms (AOM) are valuable for BNI discovery and research, but often have low throughput and rely on a limited number of mostly non-soil-relevant or genetically modified ammonia-oxidizing bacteria (AOB) strains, lacking validation with established BNIs. We present a refined fast-track, high-throughput assay for BNI screening, utilizing soil-relevant, ecophysiologically and phylogenetically diverse AOB (Nitrosospira multiformis, Nitrosomonas ureae, Nitrosomonas communis) and ammonia-oxidizing archaea (AOA) strains (Nitrososphaera viennensis, \"Ca. Nitrosocosmicus franklandianus\"), achieving consistent cellular activity and density. The assay was validated with established SNIs and BNIs, showing differences in inhibition efficacy and strain sensitivity, consistent with literature. As a proof of concept, root exudates from diverse wheat genotypes were screened, demonstrating distinct inhibition profiles. The proposed system advances previously available screening systems and, when integrated with realistic soil tests, will facilitate the discovery of novel BNIs and BNI-producing plant genotypes.