Biochemical Parameters of Fallow Light Soil Enriched with Sewage Sludge

One way to manage sewage sludge, which is consistent with the assumptions of the European Green Deal, is to use it in agriculture. The study focused on the possibility of using soil enzyme activity and the GMea index (the geometric mean of enzyme activities) in connection with the total organic carbon (TOC) and the total nitrogen (TN) content to assess the quality of fallow light soil after exogenous organic matter (EOM) fertilization. Exogenous organic matter in the form of stabilized municipal sewage sludge was introduced into the soil. The experiment included five variants: one control site and four sites with 30, 75, 150, and 300 Mg ha−1 of sewage sludge added to the soil. The contents of TOC, TN and heavy metals (Zn, Cu, Pb, Cd) in the soil material were assayed. In addition, the activity of soil enzymes, i.e., neutral phosphatase, urease, protease and dehydrogenase, was examined, and the geometric mean of the enzyme activities (GMea index) was calculated. Fertilization of light soil with sewage sludge resulted in an increase in TOC and TN proportionally to the EOM dose. The addition of sewage sludge increased the content of tested heavy metals in the soil and did not exceed the levels considered acceptable. The introduction of sewage sludge contributed to the stimulation of biological life in the soil. This was evidenced by an intensification of soil enzyme activity. However, individual enzymes showed a different response to EOM fertilization, while GMea showed a significant increase in the quality of the fallowed soils as the EOM rate increased to 150 Mg ha−1.