Biochar and Plant Growth Promoting Rhizobacteria as Soil Amendments

Abstract

The notion of introducing biochar into soil for carbon sequestration provides the impetus for studying these materials as inoculum carriers. For this goal I assessed the survival of plant growth promoting rhizobacteria (PGPR) introduced into natural soils after pre-inoculation onto biochar materials. First, molecular and culture-based methods, which involved the use of a green fluorescent protein (GFP) marker, were developed to track the introduced bacterial strain. The key findings of a case study indicated that cucumber root and shoot development were improved with biochar treatment but were unresponsive to treatment with auxin-producing strain, Enterobacter cloacae UW. Also, UW5 survival slightly increased when biochar was used as a carrier. This indicated that there are positive benefits of using biochar as a carrier, but did not provide an optimal formulation of biochar and PGPR. Next, 10 types of biochar were characterized and assessed as inoculum carriers. Survival of UW5 was determined by enumeration of the GFP marker by quantitative PCR (qPCR) and was statistically related to biochar characteristics. From this study it became apparent that biochar chemical characteristics, such as C:N ratio and pH, have a strong influence on inoculum density post inoculation. However, after 4-week incubations in natural soils characteristics, like specific surface area, mean pore-opening diameter, and water holding capacity, had greater impacts on inoculum survival. Biochar made from pinewood at 600°C had a significantly greater outcome on inoculum survival compared to others and was equivalent to traditionally-used carrier, peat moss. Lastly, the influence of biochar on PGPR activity was assessed. Specifically, I assayed mineral phosphate solubilization, auxin production, and an enzyme, 1-amino-cyclopropane-1-carboxylate (ACC) deaminase, which reduces rhizosphere ethylene concentrations that could alternatively trigger plant stunting responses. Soils from collaborative projects were used to source strains and mixed consortia to examine the effects of biochar on native soil communities. The influence of biochar on the expression of an enzyme involved in auxin production and ACC deaminase was evaluated using spectrophotometric assays, promoter-reporter systems, and reverse transcriptase qPCR. Biochar had no negative impact on PGPR activity in most cases.

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