Production of a granular compost carrier for bacterial inoculants

Bacterial inoculants play an important role in both agriculture and bioremediation. The long-term use of Rhizobium inoculants in agriculture has resulted in increased demands on natural peat reserves (the primary carrier material for these inoculants) and environmental concerns regarding the sustainable use of our natural resources. Methods of application of other bacterial inoculants, such as those used in the bioremediation of petroleum oil-contaminated soil, have been relatively unsuccessful. It is with consideration of these concerns that this study was undertaken to identify and develop compost as an alternative carrier material for Rhizobium and as an alternative to liquid bacterial inoculants for bioremediation technologies. Composted manure was selected as the carrier material in both studies. The survival of Rhizobium and petroleum-hydrocarbon (phenanthrene )-degrading bacteria on granular compost was examined. In addition, the ability of these newly developed inoculants to fulfill their expected functions was assessed. For example, delivery of Rhizobium to leguminous roots and the degradation of phenanthrene in soil by phenanthrene-degrading bacteria. Rhizobium leguminosarum bv. viciae bacteria were not detected via standard plating of non-sterile compost granules after ~30 days of storage for the composted Saskatoon Zoo waste (SZ) and ~70 days for the composted cattle manures (designated as EA, PA) at 4°C. However, survival of R. leguminosarum bv. viciae (9.28 x 10⁷ CFU g⁻¹ compost inoculant) inoculated onto sterile granular composted zoo waste was 5.36 x 10⁵ CFU g⁻¹ compost inoculant after 224 days of storage at 4°C. The populations of rhizobia present on the sterile granular composted zoo waste after 224 days of storage consistently nodulated field pea (Pisum sativum) roots under sterile conditions. However, variable nodulation occurred when either non-sterile or sterile, freshly inoculated compost granules were added to soil prior to seeding pots with peas. The sterile granulated composts in this study, due to their many similarities to inoculant-quality peat and their capability of promoting the survival of R. leguminosarum bv. viciae for an extended period of time, should be further examined as potential carriers for Rhizobium. The survival of phenanthrene-degrading strains of Arthrobacter globiformis and Rahnella aquatilis, inoculated onto granular composted manure, was generally one log unit higher when stored at 4°C as opposed to 22°C after 240 days of storage. Survival was relatively consistent between the different composts. When the A. globiformis- inoculated compost granules were added to 1.0% phenanthrene-spiked soil no significant phenanthrene degradation occurred, possibly in part due to the lack of bacterial movement from the granules into the spiked soil environment. However, when crushed granules inoculated with phenanthrene-degrading A. globiformis, or a liquid broth of the A. globiformis were added to phenanthrene-spiked soil, a decrease in measurable volatile phenanthrene was seen. The inoculant/contaminant contact is a very important aspect of bioremediation technologies. The use of PAR-degrading bacteria inoculated onto crushed granules by way of direct incorporation into contaminated soil or application by coating seeds used to promote phytoremediation should be investigated further