Biochar and Hydrochar from Agricultural Residues for Soil Conditioning: Life Cycle Assessment and Microbially Mediated C and N Cycles

Value-added biochar or hydrochar converted from agricultural waste has the potential to improve soil performance. However, how biochar and hydrochar addition affect ecological responses is not clear. Here, we examined the physicochemical property of biochar and hydrochar from straw (RS) and manure (SM), identified relevant eco-friendly pretreatment methods through life cycle assessment (LCA) methodology, and investigated their influences on soil properties and C and N cycles. The results showed that biochar prepared at 500 and 700 °C and hydrochar prepared at 250 °C for 4, 6, and 8 h had better physicochemical properties. The LCA results revealed that RBC500, RS250-4, and SM250-4 had lower environmental burdens in the preparation process. Moreover, biochar and hydrochar addition could promote the soil C and N cycles to varying degrees, especially in the case of SM250-4, which could simultaneously promote the degradation of cellulose, lignin, and chitin and reduce the emission of CO2 and N2O. Overall, SM250-4 application had a beneficial effect on carbon neutralization. These novel results compensate for the deficiency of research on the phaeozem improvement effects of biochar and hydrochar.