Bioaugmentation for reduction of diffuse pesticide contamination

Pesticides and their residues frequently contaminate surface waters and groundwater so consequently there is a great need to identify methods and practices that reduce such contamination. This thesis examined the potential of a 'bioprophylactic' concept based on the hypothesis that diffuse contamination after application in the field can be significantly reduced if pesticides are degraded as rapidly as possible after the intended effect has been achieved. This involves adding pesticide degraders together with the pesticide at the time of application. Such enhanced degradation is particularly important for pesticides used on non-agricultural soils with low degradation and high leaching potential (e.g. roadsides, railway embankments, paths, farmyards, urban land). The herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) was used as a model herbicide in this thesis, together with its degrader Sphingobium sp. T51. Soil inoculation with Sphingobium sp. T51 in microcosms significantly enhanced the degradation of MCPA compared with an uninoculated soil, without losing the herbicidal effect. In sand, 105 cells g⁻¹ dry weight of soil reduced the MCPA concentration by 79-86% within one day, while only 0-31% reduction was observed in the uninoculated control after three days. However, Sphingobium sp. T51 needs to be formulated to retain its viability and degradation activity during prolonged storage and also for protection from the harsh environmental stresses associated with its application and functioning in the field. Both freeze drying and fluidised bed drying of Sphingobium sp. T51 resulted in high initial cell survival rates, of 67-85%. The storage stability of formulated Sphingobium sp. T51 was dependent on the formulation excipient/carrier used, storage temperature and atmospheric conditions. Sphingobium sp. T51 demonstrated approximately 50% survival in a freeze-dried sucrose formulation after six months of storage under partial vacuum at 25 °C. A fluidised bed-dried formulation with cottonseed flour as carrier resulted in the highest MCPA degradation in microcosms, but the storage stability was insufficient. Formulated and long-term stored Sphingobium sp. T51 reduced pesticide leaching by 85-94% of the total amount added in sand columns. Future research needs regarding practical implementation of the concept are discussed.