Horizontal and Vertical Movement of Azospirillum brasilense Cd in the Soil and along the Rhizosphere of Wheat and Weeds in Controlled and Field Environments

SUMMARY: Horizontal movement of Azospirillum brasilense Cd in soil and its vertical movement in the plant rhizosphere were studied. No movement was detected in the absence of living plants. In a controlled environment the bacteria moved horizontally at least 30 cm from the inoculation point to the first growing plant. Once the first root system was colonized, all the neighbouring plants became inhabited. Horizontal movement under field conditions was at least 160 cm, and depended on the presence of live plant roots. Several weeds that grew in the passes between plots acted as efficient vectors. The numbers of A. brasilense Cd decreased with increasing distance from the inoculated plot. Vertical movement in soil columns in a controlled environment was up to 40 cm. Under field conditions, bacteria were detected as deep as 50 cm in the root systems of wheat plants in various types of soil. During the growing season bacteria were mostly found on and in young roots at a depth of 20–50 cm and near the soil surface. A map of depth distribution of A. brasilense Cd showed an uneven colonization pattern within the same root system or between adjacent plants. It was concluded that A. brasilense Cd moved horizontally and vertically in various soil types and that this movement was mainly dependent on the presence of plants.