Evaluating lignite as a microbial inoculant carrier

Victoria holds a large proportion of the world's lignite resource, being generally easy to access at low cost. Interest in the use of lignite as an organic amendment is increasing because it is a rich source of humic acids and can reduce the bioavailability of heavy metals. To date, there is no information about the effect of this amendment on soil biological properties. The work presented in this thesis is based on a microcosm experiment to determine the influence of Victorian lignite on soil microbial communities and nitrogen dynamics and an incubation experiment to evaluate the use of lignite as a carrier material for beneficial microbial inoculants. In the first experiment, I added raw lignite to an agricultural soil, with or without additional N-fertiliser, and measured a number of chemical and microbial soil properties. The results indicated that the application of lignite either alone or in combination with urea had a minimal effect on the availability of inorganic nitrogen (NH/ and N03-), the microbial biomass, and the activity of three different enzymes involved in C-cycling. Shifts in the soil microbial community caused by lignite application were detected by phospholipid fatty acid analysis, and mainly occurred among microbial groups rather than as change to total microbial biomass. However, I found strong evidence that lignite addition could suppress the heterotrophic respiration and it's likely that the soil microbial community could not assimilate the organic C in lignite even with the supplementation of nitrogen fertilizer. In chapter 2 of this thesis, I present results of a study in which the use of lignite as an alternative microbial carrier in comparison with peat as a conventional carrier was explored. The physicochemical properties of lignite and peat were characterized by elemental analysis, solid-state NMR (Nuclear Magnetic Resonance) and SEM (Scanning Electron Microscopy). I found that lignite had a lower available nutrient content than commercial peat. However when the lignite was treated with 5% CaC03 and sterilized by gamma irradiation, it supported the growth and the survival of nitrogen fixing bacteria during a 3-month storage period to almost the same level as the conventional peat carrier. In addition, a glasshouse experiment showed that the addition of a lignite-based inoculants significantly increased the shoot dry weight (SDW) and root dry weight (RDW) of rice in comparison with a peat-based inoculant and untreated controls. These results confirm that lignite is a suitable carrier material for microbial inoculants used in agriculture.