Rehabilitation of soil microbial communities in Queensland Pasture Dieback

The beef cattle industry in Australia is a substantial source of income, especially in Queensland, which has almost half of the national herd. This industry relies mainly on grazing pasture. Therefore, a healthy pasture system is of the utmost importance as it provides the livestock with the nutrition needed to generate a good quality product. However, pasture dieback, a syndrome or disease of unknown origin affecting grasses across Queensland and New South Wales, has been a significant issue for graziers. The affected grass presents red or yellowing of the tip of older leaves, spreading to the rest of the plant and eventually killing the grass. This severely affects the quality of the pasture composition and nutritional properties and reduces the paddock's livestock carrying capacity. Currently, millions of hectares of pastureland have been affected by this problem. The livestock industry faces many challenges in reconciling the pressure to increase production efficiency with the growing demand for ethical, natural and sustainable livestock production. This thesis examines environmentally sustainable solutions for pasture dieback by studying economically viable natural products that can help restore soil microbial communities and enhance grass growth in affected areas. This was accomplished by running an on-farm experimental study in a pasture dieback-affected paddock, applying treatments and evaluating dieback recovery by analysing the improvements in soil and plant health parameters. Following chapter 1, the introduction and literature review, where the pasture dieback problem and the efforts to pinpoint the possible causes are introduced, chapter 2 presents the first investigated treatment – a single application of the Sea Minerals solution. This chapter was published in the Sustainability (journal impact factor: 3.889 (2021)). The use of sea minerals has been reported as capable of improving plant health, taste, quality, nutritional value and yield. However, sea minerals have never have been used in conventional practice as an alternative to chemical amendments. A single application of diluted sea minerals significantly increased total dry matter twenty weeks after application, producing 967 kg/ha more dry matter than the control; this benefit was still noticeable eleven months after the application, with about 500 kg/ha more dry matter production than control. Sea minerals application also improved microbial communities by boosting the beneficial bacterial species and reducing bacterial species linked to toxic and arid soils. A single sea mineral application also improved plant root morphometric measures eleven months after a single application. This indicates the benefits of sea minerals as an alternative to chemical amendments in poor, unproductive or damaged soils. The subsequent treatment presented in Chapter 3 investigated an application of a mix of potassium humate and TMT (trimercapto-S-triazine), commonly used in soil remediation, available commercially as a product called RC3. This chapter was published in Agronomy (journal impact factor: 3.949 (2021)). Potassium humate is a well-known plant growth stimulator, while TMT is primarily used to remediate heavy metal-contaminated soils and wastewater. Eleven months after a single application of RC3, treated plots presented more grass than dicot species compared to the control plots. Plant morphometric measures show a minor improvement in the dicot plant at week twenty post- application. The effect extended to eleven months post-application with more branches and seed heads in treated plots. Microbial alpha diversity did not show any changes in RC3-treated plots. However, a temporal correlation analysis demonstrated that RC3 reduced the presence of bacterial genera associated with harsh environments and poor soil quality over time, helping to prevent the decline of beneficial bacterial genera such as Actinomadura, mle1-7, Marmoricola and Dactylosporangium. The final pasture dieback treatment investigated in this thesis was potassium humate, presented in Chapter 4. This manuscript was published in Heliyon (journal impact factor 3.776 (2021)). Potassium humate is a well-recognised plant-growth stimulant with a hormone-such as influence on plants. This product did not change the mineral composition of the soil nor the alpha and beta diversity in soil microbial communities. However, the application of humate did change the temporal interactions of mineral-microbiota and introduced subtle changes in the microbial community that can improve pasture dieback syndrome. The application of humate significantly increased the plant biomass twenty weeks after application. Eleven months after a single application, plant root morphometrics showed significant improvement and an improvement in grass/weed ratio compared to the control. Lastly, Chapter 5 compares the products in terms of their ability to provide long-term improvements to the soil, plants and pasture health and productivity. The results show that potassium humate had the best results in the short term by producing 2500 kg/ha more than the control and producing taller grass with more roots than other treatments. Sea minerals, contrarily, produced the best results in the long term, as eleven months after application, it yielded 498 kg/ha more plant biomass than control plots. Plant morphometrics reflected this improvement as grass with more tillers and roots than any other treatment. Finally, RC3 showed the best results in dicotyledon plants, showing more seed heads and branches and producing 248kg/ha more grass plants than the control. Even though pasture dieback has been a severe problem in Queensland for the last two decades and millions of dollars have been invested, only one paper was published in a peer-reviewed scientific journal before this study. Interestingly, most of the material found online or from conferences was focused on insects as probable causative agents or attempts at pasture and soil remediation using, for example, slashing, ploughing and resowing, burning or fertilising. This thesis is the first investigation focused on soil microbial community restoration and consequent pasture yield remediation, aiming to provide farmers with immediate and long-term improvements. This work presents a new approach to dealing with pasture dieback. It provides evidence of cost-effective paddock “boosters” that will benefit cattle and farmers and propel a new approach towards pasture dieback in farming practice and future research.