Hydrochar Surpasses Pyrochar in Mitigating Soil N2O Emissions from Denitrification Due to Its Improved Electron Shuttle Function and Low Levels of Persistent Free Radicals

Hydrochar, a biochar variant produced through hydrothermal carbonization (HTC), is increasingly reported to exhibit superior performance in mitigating soil nitrous oxide (N₂O) emissions, relative to traditional biochar produced through high-temperature pyrolysis (pyrochar). However, the underlying mechanisms for this are still unclear. In this study, we conducted a comprehensive comparative analysis of hydrochar and pyrochar, examining the soil N₂O mitigation potential from denitrification, electron shuttle functionality, soil microbial composition, and denitrification genes dynamics. Our results conclusively establish that hydrochar outperforms pyrochar due to its exceptional electron transfer capacity, characterized by higher electron exchange capacity (EEC), abundance of electron-donating moieties and more prevalent electron transfer components. Notably, the lower concentration of persistent free radicals (PFRs) in hydrochar results in unimpeded expression of the nosZ gene, promoting complete denitrification and resulting in reduced N₂O emissions. These findings highlight hydrochar’s potential as an electron shuttle and underscore its promise as a superior soil amendment for mitigating N₂O emissions compared to pyrochar.