Effect of Microwave Soil Treatment for Weed Management on Cereal Productivity

The evolution of herbicide-resistant weeds has encouraged the development of new non-herbicidal weed management technologies for sustainable food production. Agricultural pests are enormously reducing the sustainability of farmland and the traditional usage of chemicals for pest control has shifted the pests’ adaptability to resist this widely adopted control measure. This adaptability of pest organisms (i.e., pesticide and herbicide resistance) can impose a significant challenge to sustainable food production. In addition, chemicals are deteriorating the cropping environment and soil health, which is a significant threat to the environment and is one of the potential factors to climate change. Therefore, an environment-friendly non-chemical weed control approach is needed to ensure sustainability of agricultural systems and food security. In pursuit of this, numerous thermal weed management technologies have been proposed and developed. However, a novel and effective solution to herbicide-resistant weeds is still in question. Therefore, this thesis evaluated the effect of microwave (MW) soil treatment to minimize the regeneration capacity of the weed seedbanks as a novel tool for weed management in temperate rice and dryland wheat production systems in Australia. During the initial field trials at Dookie, Victoria, an open structure horn antenna assembled with a waveguide and a 750 W magnetron was employed to channel the MW energy for soil treatments. MW energy was assigned in a randomized complete block design with five replicates under field conditions. Soils exposed to 120 s of MW energy rapidly increased in temperature, with increases of up to 75 – 80 °C in top soil horizons of 0 – 8 cm. This temperature range significantly inhibited (40 – 60% reduction) the germination of weeds relative to untreated control soils for both temperate rice and dryland wheat crops. In addition to weed suppression, soil microwaving substantially increased the grain yield of both crops. Subsequently, a 2-kW MW weed killer was tested for pre-emergence soil treatment to reduce seedbank viability in a rice crop under field conditions, in two agroecological zones of Australia. This transient soil heating resulted in an average of 50 – 60% reduction in weed establishment compared to untreated control plots; however, there was no statistically significant difference detected between MW and untreated control plots at both study locations. Irrespective of this pattern for weed control, the MW soil treatment gave 20 – 52% increases in rice grain yields. Furthermore, under the MW soil treatment, the agronomic efficiency of applied nitrogen (kg grain kg–1 N) was 10 times higher and irrigation water use efficiency (t ML–1) was 50 – 100% higher compared with untreated control plots at both study locations because of lower weed-crop competition for resources and/or soil heating impact on the indigenous N supply for crop yields. In summary, investment in this non-chemical weed control technique can potentially reduce weed establishment and enhance rice productivity and thus this study clearly supports the practicability of this technology for sustainable weed management. Furthermore, a series of pot trials were conducted to evaluate the impact of MW soil treatment on crop growth and nitrogen use efficiency in a wheat crop using an isotope dilution technique over two seasons. Pre-cropping soil heating at up to 75 – 80 °C did not significantly increase the N% in wheat dry matter and N derived from fertilizers (15N atom% recovery) relative to untreated soils; however, despite this, there was a substantial increase in crop growth and development. Additionally, the total N accumulation in the wheat dry biomass significantly increased with soil heating intensities compared with untreated control soils. The increased growth effect in response to a once-off MW soil heating continued for up to two seasons (residual effect; 440 and 740 days after MW soil treatment), confirming the positive long-lasting impact of soil heating on soil fertility and productivity. Finally, in order to fully understand the potential effects of the MW weed management treatment and its role in shaping the soil microbiota, the soil microcosms exposed to the MW weed disinfestation treatment at three MW durations and monitored the bacterial community composition (16S rRNA amplicon sequencing and 16S rRNA qPCR) as well as the total abundance of ammonia oxidising archaea (AOA’s) and bacteria (AOB’s). The microcosms were sampled immediately after treatment and 28 days later. Because the treatments were expected to be more extreme on the upper soil strata, the soil was sampled at 5 cm and 10 cm separately to evaluate the effectiveness of the MW treatment in regard to penetration depth for treatment efficacy. Therefore, these results demonstrate that MW soil heating (75 – 80 °C) intensities proposed for weed management had a strong rearrangement impact on bacterial community composition without significantly influencing the ammonia oxidisers abundance. These results also indicate that the survival of ammonia-oxidisers after MW soil heating may favor the fertilizer nitrogen use efficiency in cropping systems and could potentially support the organic production of high-value crops. Additionally, the triggering of biologically important taxa following heat disturbance could cope with the yield increase behaviour of crops because of ample availability of decomposed organic carbon to release the numerous yield-limiting nutrients. Overall, MW soil heating appeared to be effective for reducing the viability of soil weed seedbanks and increasing crop yields. In addition, the heating intensities proposed for deactivation of the soil weed seedbanks also modified soil bacterial communities - activating important dormant taxa - and enhancing soil health and crop productivity. Therefore, MW technology can be adopted as an environmentally-sound weed control technique. Further, it can be integrated into the existing weed control approaches to improve the sustainability of farming systems