Vertical movement of A. brasilense Sp7 and strain Cd in soil under different inoculation regimes

Azospirillum brasilense Sp7 and Azospirillum brasilense Cd are two plant growth-promoting bacteria (PGPB). Traditional inoculation methods with PGPB are seed inoculation, seedling root inoculation and soil inoculation. Although these methods are simple to use, they are limited to apply at a specific plant growth stage. Therefore, PGPB inoculation by drip irrigation has been suggested as a means to deliver PGPB directly to the root zone during the plant growth stages. To quantify the intrinsic transport characteristics of two A. brasilense strains following point source inoculation, the properties of A. brasilense Sp7 and A. brasilense Cd (e.g., cell size, hydrophobicity, and zeta potential) and the adsorption characteristics on fine sand were measured. The transport and fate of the two strains were examined under transient water flow conditions with three soil inoculation regimes: (i) surface irrigation (ii) subsurface irrigation and (iii) soil premixing. The water content, bromide, and bacteria distribution in the soil profile were measured after 2 and 48 hours. The measured data were described using the attachment/detachment model using the Hydrus 2/3D code. The result showed that even though A. brasilense Sp7 and Cd exhibit similar hydrophilicity and zeta potential their adsorption and/or straining in the soil profile were differed. A. brasilense Cd has a smaller cell size, less adsorption and less straining than A. brasilense Sp7, thus its vertical movement is deeper. However, both strains accumulated at the vicinity of the water source. The results of this study will be presented and the pros and cons of three inoculation regimes will be discussed.