Green manuring, biofumigation and soil microbialecology in Tasmanian vegetable cropping

Improving agricultural efficiency and sustainability are critically important challenges for humanity in the 21^st century. Rapidly increasing global population and continuing degradation and destruction of agricultural soils necessitate the production of more food with increasingly degraded land. The primary drivers of soil degradation include loss of soil organic carbon, damage to soil structure and compaction from stock and machinery, suppression of the soil biome from pesticides, and topsoil erosion. All of which destabilize the soil ecosystem reducing ecosystem services like nutrient cycling, soil aggregation, nitrogen fixation and disease control, resulting in reduced agricultural outputs.
Many practices have been developed to prevent and/or ameliorate soil degradation including green manuring and biofumigation. Green manuring involves growing a crop specialized for incorporation into the soil, and green manuring improves soil health by protecting the soil surface from erosion, improved soil aggregation and permeability from root action, weed suppression, nourishment of the soil biota, and increased soil organic carbon. Biofumigants are specialized green manures, typically Brassica species, which release biocidal compounds during their growth and post incorporation decomposition which suppress soil borne pests and pathogens.
Most biofumigation studies have been conducted on sandier soil types with lower to moderate organic carbon levels, but clay and soil organic matter are known to sorb and deactivate the biocidal compounds released by biofumigants. Consequently, the efficacy of biofumigants on high carbon, clay rich soils is not well studied. The primary goal of this research was to closely examine the effects brassica biofumigation had on a clay rich Red Ferrosol used for intensive vegetable production relative to a non-biofumigant green manure and a fallow control, with a particular focus on the soil’s microbial communities. I aimed to determine how brassica biofumigants affected soil microbiota, if and how their effects differed from those of non-biofumigant green manures, and whether they could increase yields in intensive vegetable cropping, all in the context of Tasmanian conditions on a high organic carbon and clay rich soil type. I also aimed to assess disease control from biofumigation relative to non-biofumigant green manuring and the fallow control, but due to insignificant disease levels across the site I was unable to examine this.
The first experimental chapter examined the timing, magnitude and duration of changes in the soil microbiome during the biofumigation and green manuring processes. Soil microbiome samples were taken at key phases of the cropping cycle with particular focus on the period immediately before, during, and after incorporation. The study revealed that the microbiome responded to incorporation within days, that eukaryotes were more responsive than prokaryotes, and that the biofumigant altered the soil microbiome more than the green manure. This suggests that the biofumigant’s biocidal compounds were impactful on the microbiome of the Ferrosol despite its high clay and organic carbon content.
The second experimental chapter examined the long term (> 10 year) effects of repeated biofumigation and green manuring on the soil’s microbiome, chemical and physical properties and vegetable yields over three consecutive years. Aggregate stability was assessed at maturity of the green manure/biofumigant, and soil chemistry/microbiome samples were taken after the vegetable harvests. Taxonomic analysis of soil biota was used to infer how the soil ecosystem had changed due to the treatments. Long term green manuring and biofumigation were shown to increase the relative abundance of copiotrophic, organic matter degrading organisms which could result in improved nutrient cycling and greater competition for soil borne pathogens. Increases in soil carbon and aggregate stability were also observed and the treated plots yielded 20% higher than the fallows in one of the three years.
The third and final experimental chapter examines the protective effects of biofumigants and green manures on the soil during wet harvest which caused major soil degradation. Soil chemistry, total soil carbon, and soil microbiome samples were taken post vegetable harvest. Clear evidence of reduced soil degradation was observed on the steeper side of the trial site exclusively; both treatments reduced losses of soil carbon and microbial diversity relative to the fallow control. This indicates that biofumigants and green manures can improve soil degradation resistance on vulnerable areas of Red Ferrosols, even when they are in good condition, this improves the sustainability of intensive farming operations on this soil type.
The combined outcomes of this thesis demonstrate several benefits to soil health in Tasmanian vegetable cropping from green manuring and biofumigation. Significant increasesin aggregate stability were observed in all four years, a 20% increase in vegetable (carrot) yield was observed in one year, and the treatments increased soil carbon on the steeper side of the trial site. Green manures and biofumigants were particularly beneficial in the steeper areas during a soil degradation and water erosion event, and significantly reduced soil carbon and eukaryote diversity loss relative to the fallow plots. However, benefits to soil carbon and microbial diversity were not observed in the flatter areas.
Green manuring and biofumigation both produced short and long term effects on the soil’s microbial ecology, and eukaryotes were more affected than prokaryotes. In the short term, microbial communities of the treated plots diverged from those of the fallow and each other within days before partially re-converging over the 35-week experiment. Long term (~ 13 years) green manuring and biofumigation increased the relative abundance of copiotrophic and saprophytic organisms which may improve nutrient cycling and disease suppression. The treatments also increased the soil’s resilience to degradation in steeper areas. In combination, these results indicate that green manuring and biofumigation can improve soil health and sustainability in Tasmanian vegetable cropping.