Assessing the long-term impact of urease and nitrification inhibitor use on microbial community composition, diversity and function in grassland soil

Reductions in ammonia (NH₃) and nitrous oxide (N₂O) emissions from agricultural systems are critical for achievement of sustainability targets that underpin international efforts on climate and biodiversity. Urease inhibitors (UI) such as N-(n-butyl) thiophosphoric triamide (NBPT) and nitrification inhibitors (NI) such as dicyandiamide (DCD) slow down microbial and chemical N transformation rates in soil, resulting in decreased environmental N losses. To date there has been minimal assessment of the long-term non-target impacts of UI and NI on soil microbial communities and biological function in grasslands. Utilising a temperate grassland field experiment where fertilisers (with or without inhibitors) were repeatedly applied over a five year period, we assessed the impact of individual or combined inhibitor use on microbial community composition, abundance and function via a combination of functional assays, quantitative polymerase chain reaction (qPCR) assays and amplicon sequence analysis. We also investigated the effect of N inhibitor use on the N functional community, and whether the form of applied N fertiliser (i.e. calcium ammonium nitrate (CAN) or Urea) affected microbial community composition and function. Treatments included a Control (no N); CAN; Urea; Urea + NBPT (UI); Urea + DCD (NI); and Urea + DCD + NBPT (NI & UI). There was no impact of either UI or NI use on non-target microbial community composition or abundance. Function and the abundance of N cycling communities were mainly unaffected by fertilisation or the use of inhibitors. The observed effect of NI was primarily on the nitrification process. There was a significant reduction in nitrification potential associated with the use of NI, and in the case of the Urea + DCD treatment a reduction in COMAMMOX nitrifier abundance, and an increase in potential N mineralisation and N₂O emissions. Finally, there was a significant impact of fertilisation and fertiliser type (i.e. CAN or Urea) on the fungal community structure but no impact on bacterial community structure. These results provide a knowledge base that will inform policy regarding the utilisation of N inhibitors as a mitigation measure for reducing gaseous N losses in grasslands.