Evaluating the effects of sewage sludge compost applications on the microbial activity, the nutrient and heavy metal content of a Chernozem soil in a field survey

Sewage sludge contains organic matter and micro and macronutrients which are potentially useful for agricultural usage. However, it can be harmful when containing undesirable amounts of organic pollutants, heavy metals, or pathogens. Our study focused on examining the changes in the extractable nutrient, organic matter and heavy metal contents of a Chernozem soil and the alteration of the soil biological activity as a consequence of low-dose municipal sewage sludge compost applications (0.5 t/ha). Sampling campaigns were done in 2018 near Újkígyós (SE Hungary) during which composite soil samples (0–30 cm and 30–60 cm) and groundwater samples were collected for assessing changes in the nutrient and heavy metal concentrations as a result of compost amendments’ use. Additionally, upper soil (0–50 cm) and subsoil (50–80 cm) were sampled for assessing biological parameters, considered to be aerobic and anaerobic soil layers, respectively. Soil samples were analyzed for the basic pedological parameters (pH, organic matter, carbonates and texture), nutrients (K2O, P2O5, N-forms and organic matter) and heavy metal concentrations following standard extraction procedures. The microbial properties were characterized by colony-forming units (CFUs) and enzyme activity measurements. The results of the nutrient analyses show significantly increased soil-bound K2O, P2O5 and NO2− + NO3− contents linked to the sewage sludge treatments. However, neither the organic matter nor the heavy metal content varied significantly in the sludge-amended soil compared with a control site. The microbiological analyses revealed that the sewage sludge treatments tended to increase the aerobic CFUs, but not that of the anaerobic microbes. The average catalase enzyme activity in both the aerobic and anaerobic samples and the average dehydrogenase activity only in the aerobic layers showed a slight but not significant increase in the compost-amended soils. Overall, these results convincingly demonstrated that amending soils with low doses of municipal sewage sludge composts (lacking any industrial sources) can be a sustainable fertilizing practice taking advantage of their high N, P and K contents that are slowly converted to their bioavailable forms thus preventing their excessive leaching in the groundwater.