The influence of soil properties on nitrifying organisms and nitrification inhibition efficacy of 3,4-dimethylpyrazole phosphate (DMPP)

    Research output: ThesisDoctoral Thesis

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    Autotrophic nitrification can be performed by either ammonia oxidising bacteria (AOB) or ammonia oxidising archaea (AOA). The nitrification inhibitor 3,4-dimethlypyrazole phosphate (DMPP) can be used to suppress AOB nitrifier activity to mitigate potential leaching and denitrification N losses incurred as a result of nitrification. Much of the research regarding DMPP efficacy is based on European and United States soil and farming systems that are very different to Australian conditions. The practice of dry sowing in response to variable autumn break rainfall and the implementation of crop-pasture rotations to preserve organic carbon (OC) is widespread in south eastern Australian agriculture. In addition, widespread pH stratification within the topsoil is a common issue. The impact of these conditions on the effectiveness of DMPP to inhibit nitrification was investigated by a series of incubation experiments and validated in the field.

    The application of DMPP in conjunction with four different NH4+-based fertilisers occurred in an incubation trial to soils with moisture contents equivalent to dry sowing and post-seasonal break sowing, Measurements of soil mineral N (NH4+ and NO3-) and urea concentrations indicated the ability of DMPP to suppress the oxidation of NH4+ for all fertiliser types. However, fertilisers applied onto dry soil incurred increased net immobilisation of applied N in response to wetting.

    The effects of management was investigated by comparing cropping and pasture phases on DMPP efficacy. These were investigated in an incubation trial where fertiliser and DMPP was applied to soils of two different moisture contents, dry sowing and post-seasonal break sowing. Increased nitrification rates of fertilisers when dry-sown amplified the inhibitory effects of DMPP. Net immobilisation of mineral N occurred at week 1 for dry sown soils whilst a priming effect that increased mineralisation occurred simultaneously for N treated soils with moisture contents equivalent of sowing after a post seasonal break. Management practices affected DMPP behaviour with greater inhibition efficiency occurring in cropped soils compared to pastures, although the efficacy of DMPP declined regardless of the management practice as soil acidity increased. Soil pH also influenced nitrifier abundance with increased AOB amoA gene copies recorded in relatively neutral soils and higher AOA amoA gene abundance in acidic soils. A shift to archaeal nitrification in acidic soils
    potentially contributed to poor DMPP inhibition.

    The effects of pH on DMPP efficacy were further investigated in an incubation trial that used a pH stratified soil to examine DMPP efficacy and nitrifier distribution in stratified layers, and the consequential effects of ameliorating soil acidity by lime application. AOB were concentrated in the surface (0-2 cm) layer and their abundance decreased with depth, with detection in the 8–10 cm layer only occurring upon amelioration of soil acidity with lime application. By contrast, AOA abundance was highest in the 4–8 cm layers. Liming increased AOB abundance below 2 cm and significantly improved the inhibition efficiency of DMPP, which demonstrates the benefits of amending an acidic soil with lime before DMPP application.

    A field trial was conducted to verify laboratory results in a field situation in addition to testing the effect of surface applied or incorporated lime on the inhibitory efficiency of DMPP. Surface applied lime exacerbated pH stratification and as a result, applied DMPP did not inhibit nitrification. DMPP was only effective when lime was incorporated, possibly due to a shift from archaeal to bacterial nitrification which can be inhibited by DMPP.

    This thesis shows that cropping and pasture rotations can alter the nitrification inhibition efficacy of DMPP. In neutral soils, DMPP will inhibit more effectively on cropped soils rather than pasture soils, although the effect of management is reduced as soil acidity increases. DMPP inhibited nitrification regardless of the soil moisture at application or NH4+ fertiliser used, however increased nitrification rates of fertilisers applied in dry sowing scenarios enhanced the benefits of DMPP application. Increasing the soil pH with lime addition significantly improved the efficacy of DMPP by shifting nitrification from AOA to AOB. Therefore, it is recommended that lime application be used to ameliorate soil acidity before DMPP is used a method to inhibit nitrification and improve nitrogen use efficiency.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • Charles Sturt University
    • Condon, Jason, Principal Supervisor
    • Doran, Gregory, Co-Supervisor
    Place of PublicationAustralia
    Publication statusPublished - 2021

    Grant Number

    • 101798


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