Effect of biochar produced from sewage sludge on tomato (Solanum lycopersicum L.) growth, soil chemical properties and heavy metal concentrations

The addition of biochar, as shown in the literature, improves significantly the chemical and physical soil properties and plant growth. This study examined the effect of biochar, compost and the combination of them on growth, nutrient and heavy metal concentrations of tomato. Biochar (BC) was produced from sewage sludge by pyrolysis at the temperature of 300 °C. The pot trials were carried out under an open-side greenhouse for a total of four months and under four treatments. The treatments applied were: Untreated soil (Control); soil with 2% w/w biochar (BC–SS); soil with 2% w/w compost (Compost); a mixture of biochar and compost at a 2% w/w level (BC–SS + Compost). The application of biochar exhibited substantial improvement on several soil properties. Total organic carbon (TOC) of soil increased (67%–85%), as did the nitrate nitrogen (55%) and ammonium nitrogen (145%). Additionally, available phosphorus significantly increased (45.5%–54.5%) by the application of biochar with/without compost. Dry weight of the aboveground (stems) and belowground (roots) plant tissues increased as well, although tomato yield was not increased significantly. Concentration of heavy metals and trace elements in tomato tissues was quite low. Traces of chromium (Cr), nickel (Ni), and cobalt (Co) were found only in roots of those treated, while silicon (Si) was present in the roots and stems. Arsenic (As), molybdenum (Mo) and lead (Pb) were detected in all plant tissues, but their concentrations did not exceed the permissible levels established for vegetables. Furthermore, the concentration of arsenic (As) and lead (Pb) in fruits decreased by the addition of the amendments (12%–65%). In conclusion, the addition of sewage sludge biochar improved soil characteristics and plant growth. Yet, prior to its general use, factors such as the type of biomass, soil, rate of application and crop must always be taken into consideration.