Long-term impact of sewage sludge, digestate and mineral fertilizers on plant yield and soil biological activity

The aim of the study was to assess a long-term impact of the cumulative application of sewage sludge on plant yield and soil microbial parameters. The study was conducted in 1 m2 lysimeter plots. In 2006 sewage sludge was applied to selected plots at the rate of 100 t of dry matter per hectare while the other plots were fertilized with mineral nitrogen. After 6 years, the same sludge rate was applied to selected plots in order to achieve single or cumulative sludge rates. The sludge plots were compared to the plots constantly fertilized with ammonia nitrate and the plots treated with digestate. Sludge stimulated an increase in plant biomass similar or higher than the effect of mineral nitrogen fertilization. The highest enzyme activities were recorded for the cumulative sludge rates but the soils treated with single sludge rate also exhibited a high activity of alkaline phosphatase and dehydrogenases. The count of culturable microorganisms reflected trends observed for the enzyme activities. The study revealed that sewage sludges that meet the quality standards for application in agriculture stimulate both the soil microbial activity and the number of microorganisms even when applied at high reclamation rates.