Responses of ureolytic and nitrifying microbes to urease and nitrification inhibitors in selected agricultural soils in Victoria, Australia

Purpose: Urease inhibitors (UIs) such as N-(n-butyl)thiophosphoric triamide (NBPT) and nitrification inhibitors (NIs) such as 3,4-dimethylpyrazole phosphate (DMPP) have been reported to improve the efficiency of nitrogen (N) fertilizers. However, their effects on the ureolytic and ammonia-oxidizing microbes in agricultural soils are uncertain. This study aimed to investigate the effects of DMPP and NBPT on the abundance and community composition of ureolytic and nitrifying microbes in selected agricultural soils in Australia. Materials and methods: Soil was collected from five agricultural farms and used to establish an incubation experiment. Urea, urea + NBPT, urea + DMPP, and urea + NBPT + DMPP were applied on the soils which were incubated under 25 °C and 60% water-filled pore space for 28 days. Sampling was done on different days for DNA extraction and measurement of ammonium (NH4+) and nitrate (NO3−) concentration. Results and discussion: NBPT inhibited NH4+ accumulation in all the soils but had no significant effect on nitrification in any soil. DMPP alone or DMPP + NBPT significantly inhibited nitrification. The abundances of ammonia-oxidizing bacteria (AOB) and complete ammonia oxidizers (comammox Nitrospira), but not ammonia-oxidizing archaea (AOA), were significantly influenced by the application of NBPT, DMPP, or DMPP + NBPT. AOA, AOB, and comammox Nitrospira clade B might be significant contributors to nitrification in the studied soils. Conclusions: These findings suggest that NBPT and DMPP can reduce N losses and improve N fertilizer efficiency by directly inhibiting the growth of AOB and comammox organisms in the soils, with implications for our mechanistic understanding of the molecular mechanisms of urease and nitrification inhibitors