Nutrient supplementation of pinewood biochar for use as a bacterial inoculum carrier.

Biochar can function as an effective low-cost inoculum carrier for plant growth-promoting bacteria. Here, we examined the shelf life and inoculum potential of Pseudomonas putida UW4 (UW4) and the potential for further optimization by adding nutrient supplements to a pinewood biochar (PBC) that was used as an inoculum carrier. PBC-600 was produced by pyrolysis at 600 °C and modified with Luria-Bertani broth, a worm casting extract, or mixed with earthworm castings. All treatments were compared with peat as a standard reference material. Subsequent experiments examined survival and root colonization potential of UW4. Results showed that the addition of LB broth to PBC-600 increased the growth of UW4 in storage, but that both supplemented and non-supplemented biochar were similarly effective for establishing a high population density of UW4 in the rhizosphere of inoculated plants. The population density represented by16S ribosomal RNA (rRNA) gene copy numbers for UW4 was correlated with above ground plant biomass ( r = 0.6725), which had a positive effect on alleviating salinity stress for the host plant. Analysis of community compositions in the rhizosphere by high throughput sequencing and Kohonen self-organizing maps indicated that high population abundances of UW4 were positively correlated with the abundance of Arthrobacter, Sphingomonas, Rubellimicrobium, and Gematimonas, but were negatively correlated with other dominant taxa. The results suggest that organic nutrient supplementation of the carrier does not promote increased shelf life or inoculum efficacy, but suggests the need for further studies on how biochar and inoculants can be used to shape plant-associated microbial communities. [ABSTRACT FROM AUTHOR]