Toward a functional integration of anaerobic digestion and pyrolysis for a sustainable resource management. Comparison between solid-digestate and its derived pyrochar as soil amendment

[Display omitted] •Coupling anaerobic digestion and pyrolysis is technologically efficient.•This technology integration is environmentally sound and boosts circular economy.•Digestate and pyrochar are soil amendments with different but complementary properties.•While pyrochar sequesters carbon, digestate could contribute to soil microbiological processes.•Their characteristics are fully consistent with those of agricultural soil amendments. The integration of different technologies acts as a leverage in boosting “circular economy” and improving resource use efficiency. In this respect, the coupling of anaerobic digestion with pyrolysis was the focus of this work. Solid-digestate obtained from anaerobic digestion was addressed to supply pyrolysis thus increasing the net energy gains and obtaining “biochar” (called “pyrochar” in our case) to be used as soil amendment alternatively to solid-digestate. The current interest on biochar is linked to its long-term soil carbon sequestration, thus contributing to global warming mitigation. A parallel detailed screening of the physical and chemical properties of both solid-digestate and pyrochar was performed, inferring their effects on soil quality. Results showed that while P and K are enriched in pyrochar, total N showed no significant differences. Heavy metals revealed higher concentrations in pyrochar, but always largely below the biochar quality thresholds. Pyrochar exhibited a higher surface area (49–88m2g−1), a greater water holding capacity (352–366%), and a more recalcitrant carbon structure. Both solid-digestate and pyrochar showed good soil amendments properties but with complementary effects. Although starting from the same biomass, being the original feedstock processed differently, their ability to improve the physical and chemical soil properties has proved to be different. While several other soil improvers of organic origin can substitute digestate, the important role played by biochar appears not-replaceable considering its precious “carbon negative” action.