Fertiliser nitrogen use in Australia has increased from 35 Gg N in 1961 to 972 Gg N in 2002, and most of the nitrogen is used for growing cereals. However, the nitrogen is not used efficiently, and wheat plants, for example, assimilated only 41% of the nitrogen applied. This review confirms that the efficiency of fertiliser nitrogen can be improved through management practices which increase the crop’s ability to compete with loss processes. However, the results of the review suggest that management practices alone will not prevent all losses (e.g. by denitrification), and it may be necessary to use enhanced efficiency fertilisers, such as controlled release products, and urease and nitrification inhibitors, to obtain a marked improvement in efficiency. Some of these products (e.g. nitrification inhibitors) when used in Australian agriculture have increased yield or reduced nitrogen loss in irrigated wheat, maize and cotton, and flooded rice, but most of the information concerning the use of enhanced efficiency fertilisers to reduce nitrogen loss to the environment has come from other countries. The potential role of enhanced efficiency fertilisers to increase yield in the various agricultural industries and prevent contamination of the environment in Australia is discussed.

The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is a powerful tool that can be used to promote nitrogen (N) use efficiency and reduce N losses from agricultural systems by slowing nitrification. Mounting evidence has confirmed the functional importance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in nitrification and N ₂ O production; however, their responses to DMPP amendment and the microbial mechanisms underlying the variable efficiencies of DMPP across different soils remain largely unknown. Here we compared the impacts of DMPP on nitrification and the dynamics of ammonia oxidizers between an acidic pasture soil and an alkaline vegetable soil using a ¹⁵ N tracing and ¹³ CO ₂ -DNA–stable-isotope probing (SIP) technique. The results showed that DMPP significantly inhibited nitrification and N ₂ O production in the vegetable soil only, and the transient inhibition was coupled with a significant decrease in AOB abundance. No significant effects on the community structure of ammonia oxidizers or the abundances of total bacteria and denitrifiers were observed in either soil. The ¹⁵ N tracing experiment revealed that autotrophic nitrification was the predominant form of nitrification in both soils. The ¹³ CO ₂ -DNA–SIP results indicated the involvement of AOB in active nitrification in both soils, but DMPP inhibited the assimilation of ¹³ CO ₂ into AOB only in the vegetable soil. Our findings provide evidence that DMPP could effectively inhibit nitrification through impeding the abundance and metabolic activity of AOB in the alkaline vegetable soil but not in the acidic pasture soil, possibly due to the low AOB abundance or the adsorption of DMPP by organic matter. IMPORTANCE The combination of the ¹⁵ N tracing model and ¹³ CO ₂ -DNA–SIP technique provides important evidence that the nitrification inhibitor DMPP could effectively inhibit nitrification and nitrous oxide emission in an alkaline soil through influencing the abundance and metabolic activity of AOB. In contrast, DMPP amendment has no significant effect on nitrification or nitrifiers in an acidic soil, potentially owing to the low abundance of AOB and the possible adsorption of DMPP by organic matter. Our findings have direct implications for improved agricultural practices through utilizing the nitrification inhibitor DMPP in appropriate situations, and they emphasize the importance of microbial communities to the efficacy of DMPP.

Summary: Soil ecosystem represents the largest contributor to global nitrous oxide (N ₂ O) production, which is regulated by a wide variety of microbial communities in multiple biological pathways. A mechanistic understanding of these N ₂ O production biological pathways in complex soil environment is essential for improving model performance and developing innovative mitigation strategies. Here, combined approaches of the ¹⁵ N‐ ¹⁸ O labelling technique, transcriptome analysis, and Illumina MiSeq sequencing were used to identify the relative contributions of four N ₂ O pathways including nitrification, nitrifier‐induced denitrification (nitrifier denitrification and nitrification‐coupled denitrification) and heterotrophic denitrification in six soils (alkaline vs. acid soils). In alkaline soils, nitrification and nitrifier‐induced denitrification were the dominant pathways of N ₂ O production, and application of the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) significantly reduced the N ₂ O production from these pathways; this is probably due to the observed reduction in the expression of the amoA gene in ammonia‐oxidizing bacteria (AOB) in the DMPP‐amended treatments. In acid soils, however, heterotrophic denitrification was the main source for N ₂ O production, and was not impacted by the application of DMPP. Our results provide robust evidence that the nitrification inhibitor DMPP can inhibit the N ₂ O production from nitrifier‐induced denitrification, a potential significant source of N ₂ O production in agricultural soils.