The Application of Sewage Sludge-Derived Compost or Biochar as a Nature-Based Solution (NBS) for Healthier Soil

The present study highlights the possibility of using sewage sludge-derived compost (SSC) or biochar (SSB) as valuable organic amendments. Such utilization of sewage sludge fulfills the principles of a carbon farming and nature-based solution strategy (NBS). This study focused on a detailed analysis of quantitative and qualitative changes in soil C compounds (total carbon—TC, total organic carbon—TOC, humic substances—CHS, labile carbon—LC, and water extractable organic carbon—WEOC), which resulted from the application of SSC or SSB; an assessment of variability in total and available forms of N and S as biogenic components that are integrally related to the organic matter of the amendments used in the experiment; and an indication of the possible relationships between C compounds and available nutrients. The experiment was conducted under greenhouse conditions with terra rosa soil amended with SSC or SSB at different application rates (25, 50, 75, 100% by mass). Soil samples were analyzed for the abovementioned parameters using appropriate analytical methods. Regardless of the organic amendment, the values of tested parameters increased with the applied dose, with the differences being significantly greater in relation to the contents determined for the control soil. In general, the application of SSC was more favorable than SSB, which was manifested by 12–49-fold higher TOC, 6–24-fold higher total N, and 10–41-fold higher total S levels. An exception was found for the content of available sulfur, which was significantly higher in the soil fertilized with biochar. In addition, SSC contributed more humic acid carbon (12.5–24.15 g∙kg −1 ) and labile carbon (10.34–27.37 g∙kg −1 ). On the other hand, SSB had a greater effect on fulvic acid carbon levels (2.18–2.75 g∙kg −1 ), which were comparable to the levels of LC (3.44–6.86 g∙kg −1 ) and WEOC (2.56–6.28 g∙kg −1 ). The research results highlighted the validity of processing SS into compost or biochar for further use for agricultural/reclamation purposes. Despite their different impacts on the studied soil properties, both organic amendments are important for maintaining soil health and can play a significant role in carbon farming as NBS practices. The findings allow us to conclude that the strategy of increasing the amount of C through SSC or SSB fertilization is the advisable direction in sustainable soil management.