The potential of plant-derived triterpenoids as biological nitrification inhibitors

Biological nitrification inhibitors (BNIs) present an environmentally friendly approach to reduce nitrogen losses and enhance nitrogen use efficiency, with plant-derived triterpenoids emerging as promising candidates. We evaluated 18 triterpenoids as BNIs using in vitro assays with soil ammonia-oxidizing bacteria (AOB) (Nitrosospira multiformis, Nitrosomonas ureae) and archaea (AOA) (Nitrososphaera viennensis, Nitrosotalea sinensis) at high and low concentrations. A Graph Neural Network framework was applied to predict nitrification inhibition (NI) and identify structural features, including key functional groups, linked to inhibitory patterns. Triterpenoids were more active on AOA, demonstrating higher efficacy than sakuranetin (a known BNI), but did not inhibit AOB. Six triterpenoids showed inhibitory activity on AOA (29-100%), with 3-O-acetyl-11-keto-beta boswellic acid and 11-keto-beta boswellic acid as the most potent inhibitors (ammonia oxidation inhibition > 94%), followed by echinocystic acid (> 87%), ursolic acid (> 74%), asiatic acid (> 65%), and echinocystic acid-3-O-glucoside (29-94%). In silico analyses predicted accurately the activity of model inhibitors such as DMPP, MHPP, and ethoxyquin on AOB and AOA, respectively, and the limited activity of triterpenoids on AOB, but did not predict their strong inhibitory effects on AOA, underscoring the need for expanded datasets for model refinement. The selective activity of some triterpenoids on AOA is hypothesized to involve interference with 3-hydroxy-3-methylglutaryl-CoA reductase, a key enzyme in archaeal membrane biosynthesis, although this requires experimental validation. Still, strain-specific responses suggest the involvement of additional mechanisms. This study provides the first experimental evidence for the potential of plant-derived triterpenoids as BNIs, supporting their relevance for sustainable agriculture. KEY POINTS: • Triterpenoids strongly inhibited AOA but had no effect on AOB nitrification activity. • Six ursane/oleanane-type triterpenoids showed strong AOA inhibition beyond known BNIs. • Inhibition patterns suggest triterpenoid structure relates to AOA selectivity.