Effects of Co-Applications of Biochar and Solid Digestate on Enzyme Activities and Heavy Metals Bioavailability in Cd-Polluted Greenhouse Soil

To deal with the problems of increasing the heavy metal (HM) bioavailability and declining the soil biological properties resulting from a direct application of solid digestate (SD). A low-temperature fruit biochar and pig-SD (BSD-0, BSD-1, BSD-2, BSD-4, BSD-8) co-application experiment was performed to evaluate enzyme activities and HM bioavailability in Cd-polluted greenhouse soil. The advantage of BSD co-applications compared to SD application was maintained the stable of pH and electrical conductivity (EC) in soil and was more effective to improve soil organic matter (OM). BSD-8 treatment significantly promoted the uptake of available nitrogen, available phosphorus, and available potassium by plants. The immobilization effect of BSD co-applications on Cu, Zn, and Cd was better than SD application. BSD-8 treatment has the best immobilization effect on Cd and the contents of bioavailable Cd was 0.167 mg kg⁻¹. The optimal enzyme activities of invertase, urease, and alkaline phosphatase were shown in BSD-8 treatment, which were 0.027 mg glucose g⁻¹ soil h⁻¹, 88.654 mg NH₃-N g⁻¹ soil h⁻¹, and 15.766 μmol PNP g⁻¹ soil h⁻¹, respectively. The activities of enzymes also were influenced by soil physicochemical properties and HM bioavailability. BSD-8 treatment was suggested as an appropriate mixing proportion to alleviate soil acidification and salinization, decreasing HM bioavailability and stimulating enzyme activities in Cd-polluted soil. Statement of Novelty Solid digestate (SD) is commonly used for agricultural crops as organic fertilizer because of its abundant nutrients and long-term soil fertility maintenance. However, excessive application of SD may cause heavy metal bioavailability to increase and soil biological properties to decline, while the effects of biochar and SD (BSD) co-applications on greenhouse soil enzyme activities and heavy metal bioavailability are often neglected. To deal with these problems, a greenhouse experiment was established comparing the effects of co-applications of different SD application rates with low-temperature fruit biochar on HM bioavailability and biological properties. Our research uses the weight of SD per square meter as the fertilization basis for the rational utilization of SD and gives evidence for the safety and effectivity of BSD co-applications in agriculture.