Rapid assessment of nitrification inhibitor efficacy, competitiveness, and specificity using microrespirometry

Nitrification inhibitors are applied to reduce nitrogen losses and greenhouse gas emissions from fertilized agricultural ecosystems. However, their characterization is typically focused on determining effective inhibitor concentrations from growth or substrate conversion assays that are time-intensive and provide limited mechanistic resolution. Here, we present a microrespirometry (MR)-based workflow for rapid mechanistic characterization of nitrification inhibitors using oxygen consumption as a real-time readout for metabolic activity. The workflow enables the simultaneous assessment of inhibitor efficacy, competitiveness, and enzyme specificity within a single experimental setup, as sequential substrate and inhibitor additions enable direct discrimination between competitive and non-competitive inhibition and between ammonia monooxygenase-specific and broader respiratory inhibition. As a proof of concept, we evaluated three known nitrification inhibitors phenylacetylene (PA), nitrapyrin (NP), and dicyandiamide (DCD) using the ammonia-oxidizing bacteria Nitrosomonas europaea and Nitrosospira multiformis, the complete ammonia oxidizer Nitrospira inopinata, and the nitrite oxidizer Nitrospira moscoviensis. We also compared the results from the MR-based inhibition workflow with those from a conventional growth-based approach and observed a poor correlation between results for inhibitors that are not fully enzyme specific. In conclusion, this work establishes MR as a rapid and versatile platform for the mechanistic screening of novel potential nitrification inhibitors. MR assays reproduce known inhibitory responses while substantially reducing experimental time and increasing mechanistic resolution compared to other assays types. Additionally, we provide the first pure-culture characterization of PA, NP, and DCD efficacy and inhibition mechanisms in a complete ammonia oxidizer, N. inopinata.