Research examining biochar (pyrolyzed biomass) as a microbial inoculant carrier may enable broader use of inoculant microbes and elucidate relationships between non-spore forming bacteria, such as rhizobia, and their microhabitats in carriers and soils. We tested 32 biochars as habitat for Rhizobium tropici (CIAT 899) to quantify the effects of pore size distribution, chemical characteristics and clay addition on bacterial abundance, in both in sixmonth storage incubations at 27°C, and under drying conditions. Pressure plate measurements and micrographic analysis yielded correlated estimates of mean macropore (0.3-30 μm) size in the different biochar carriers (r=0.80, p<0.0001). Macropore size was assigned to the first principal component of variation in biochar properties, along with mineral content derived from plant feedstocks. Under moist storage conditions, a number of biochars were equivalent to peat as microbial carriers. Rhizobium tropici abundance in these storage incubations exhibited a quadratic dependence on biochar pore size (p<0.001) with maximal abundance at a macropore size of 13.6 μm (pressure plate) or 10.1 μm (micrographs). Abundance was lower for biochars with higher ASTM volatile content (p<0.001) and was increased by plant feedstock derived mineral content in the biochars (p<0.01). Goethite and Montmorillonite additions to biochar before pyrolysis increased macropores of size <0.3 μm. Added Goethite reduced bacterial survival, while montmorillonite increased R. tropici abundance in a large-pored pine biochar by 10 times (p<0.05), and improved its survival between two and 11 times (p<0.001) in four biochars after drying for 10 days. We conclude that optimizing pore size distribution and chemical properties of biochars is a promising strategy to produce carrier materials that are as effective as mined irradiated peat for non-spore forming bacteria such as R. tropici.