Lack of efficacy of reported plant metabolites to inhibit soil biological nitrification

Adding plant-derived metabolites to soils has been proposed to inhibit microbial nitrification and, in turn, to improve crop production. However, the efficiency of the proposed inhibitors remains debated. Here we developed a miniaturized high-throughput soil assay to assess the ammonium retention and nitrification inhibition potential of 18 representative inhibitors including isothiocyanates; phenylpropanoids such as vanillin, caffeic acid, methyl ferulate, syringic acid, and umbelliferone; linoleic acid and linolenic acid; 1,9-decanediol; limonene; a benzoxazinoid; and the synthetic nitrification inhibitor 3,4-dimethyl-1H-pyrazole. Potential non-target effects were studied using the Microtox assay. Results show that none of the tested compounds showed strong nitrification inhibition in soil. Only isothiocyanates and 6-methoxy-2(3H)-benzoxazolone induced clear effects, but these were transient, required unrealistically high application rates and were associated with non-target effects. These results challenge current assumptions regarding the relevance and societal value of currently proposed biological nitrification inhibitors. Our findings suggest that genuine biological nitrification inhibition activity may be driven by other, yet unidentified metabolites.