Influence of the Application of Sewage Sludge and Presence of Pesticides on the Development of the Microbial Population of the Soil and on the Transformation of Organic Carbon and Nutrient Elements

The laboratory trial consisted in incubating samples of soil and soil treated with sewage sludge, with the application of organophosphate pesticides with different active ingredients under controlled conditions of temperature and moisture. On the basis of a previous study of the influence of the application of sludge on the degradation of pesticides in the soil, a kinetic study is included of the degradation process and we concentrate on its effects on the development of the microbial population and the mineralization of organic carbon, together with the transformation of the main nutritive elements for plants: nitrogen and phosphorus. Three different active ingredients were used: fenitrothion, diazinon and dimethoate, all of them organophosphates with different chemical structures. From the results, it is to be observed that for all the conditions studied, degradation followed first-order kinetics. The presence of pesticides in the soil produces an increase in micro-organism populations in comparison with the control sample in the different matrices assayed, favouring the mineralization of organic carbon. As for available nitrogen, the predominant form, either ammonia or nitrates, depends on the active ingredient applied. On the other hand, the use of pesticides favours the process of mineralization/solubilization of phosphorus.