Transport of plant growth promoting bacteria (Azospirillum brasilense) in sand under transient water flow: effect of inoculation regime

Azospirillum brasilense strains Sp7 and Cd are commonly employed plant growth-promoting bacteria (PGPB) that produce phytohormones and fix nitrogen. The two basic methods of PGPB soil inoculation are direct mixing with the soil and irrigation with liquid inoculants. The integration of drip irrigation with delivering plant growth-promoting bacteria in desert areas with sandy soil is becoming more common. With the drip irrigation system, the mobility of PGPB in the sandy soil determines the range of root zone inoculation. Therefore, we examined the transport and fate of PGPB under transient water flow conditions in a 30-cm high segmented sand column with three sand-inoculation regimes: (1) surface irrigation, (2) subsurface irrigation, and (3) sand premixing. The water, bromide, and bacterial distribution in the sand profile was measured at 2 and 48 h after irrigation. The measured data were described using the attachment/detachment numerical model using the HYDRUS 2/3D code. Results showed that even though A. brasilense Sp7 and Cd exhibit similar hydrophilicities and zeta potentials, their deposition in the sand profile differs. Strain Cd consists of smaller cells that undergo less adsorption and less straining than strain Sp7, and the former's vertical movement therefore reaches greater depths under surface- and subsurface-inoculation regimes. Nevertheless, most of the PGPB accumulated near the water source. In the sand-premixing inoculation regime, the bacteria barely moved at all. Overall, when the target root zone was deep, subsurface-irrigation inoculation worked better than the other two inoculation regimes, because it was more likely to deliver large amounts of PGPB to the root zone. Furthermore, the first-order attachment model optimized two parameters (first-order attachment coefficient and die-off rate) and well simulated the bacterial vertical movement in the surface- and subsurface-inoculation regimes (R2 > 0.91).