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.