Nitrapyrin reduced ammonia oxidation with different impacts on the abundance of bacterial and archaeal ammonia oxidisers in four agricultural soils

The compound nitrapyrin can act as a potential nitrification inhibitor (NI) when applied with ammonium (NH4+)-based N fertiliser. The efficacy of nitrapyrin applications may be soil dependent however, being influenced by soil texture, organic carbon (C), and moisture. The objective of this study was to assess the effects of nitrapyrin on the nitrification process in soils of different physicochemical characteristics and resident microbial communities. Four agricultural soils were incubated in microcosms for five weeks, after which nitrapyrin effects were assessed for soil NH4+, nitrate (NO3-), the microbial functional genes ammonia-monooxygenase (amoA) and nitrite oxidoreductase (nxr), and bacterial and archaeal nitrifier genera using 16S rRNA sequencing. Nitrapyrin co-applied with urea ammonium nitrate (UAN) fertiliser inhibited nitrification, shown by increased retention of NH4+ in all four soils and a lower accumulation of NO3- in three of the four soils compared to UAN treated soil. These results are most likely due to the inhibition of growth of ammonia oxidising bacteria (AOB) as nitrapyrin plus UAN reduced bacterial amoA abundance in three of the four soils compared to the UAN treatment. This study is the first to report increased archaeal amoA abundance in response to the co-application of nitrapyrin and UAN in four soils, possibly due to reduced competition from AOB. The bacterial nitrifier genera Nitrosospira and Nitrospira and archaeal nitrifier genus Nitrososphaera were detected in all four soils using 16S rRNA sequencing. The effects of nitrapyrin on downstream N transformations were assessed based on tar and Nitrospira abundance though no conclusive treatment effects could be discerned for the four soils. Our results provide evidence of the interplay of AOB, ammonia oxidising archaea (AOA) and nitrite-oxidising bacteria (NOB) in nitrapyrin and fertiliser treated soils and will be useful in developing strategies to improve the efficacy of nitrification inhibitors in future studies.