<b>Effect of Biochar on Soil Quality and Plant Growth in Hydrocarbon Contaminated Soils</b>

This work sought to investigate the effectiveness of biochar in the restoration of diesel contaminated soil in the area around the thermal power station of Al-Doura in Baghdad. An interdisciplinary study was carried out to coordinate the relationship between the properties of the soil, total petroleum hydrocarbon content, enzymatic activity, plant growth, productivity, and food security. Four locations were chosen for the study, including three diesel contaminated areas (M1-M3) and one pristine area (C) as a control, with emphasis on the top layers of the soil (top 20 cm). It was found that the presence of oil caused a great reduction in the water-holding capacity of the soil, organic matters, and essential nutrients, as well as enzymatic activities that were toxic to the growth of Swiss chard. Following the application of 5% biochar, there was a marked improvement (p < 0.05) in the overwhelming majority of the properties of the soil with enhanced water-holding capacities, organic contents, and essential nutrients with moderate reduction of pH closer to the value of healthy soil. Total petroleum hydrocarbons reduced by 45-49%, with better efficiency of aliphatic hydrocarbons over aromatics This occurred with substantial restoration of enzymatic activities, with catalase or superoxide dismutase exceeding 100%, indicating the improvement of biological properties of the soil. This suggested a restoration of the growth of the plant with enhanced heights by almost twice that of the original with significantly higher biomass production and contents of chlorophyll. A composite index that took into account the extent of pollution, enzymatic activities, and growth of plants clearly brought about a considerable restoration of the system from the polluted state before treatment to an almost healthy state of the soil following the treatment. These findings establish that biochar is a powerful tool in the restoration of oil polluted soils with enhanced agricultural production as well as ensuring the security of food.