Soil-specific protection of organic carbon by biochar-derived hydroxyl radicals associated with enzyme suppression

The application of biochar to soils introduces persistent free radicals (PFRs), which can generate reactive oxygen species (ROS), notably hydroxyl radicals (·OH). While ·OH is known to damage extracellular enzymes involved in soil organic carbon (SOC) decomposition, its net effect on SOC mineralization and soil respiration remains complex and context-dependent. This study investigates the hypothesis that ·OH produced from biochar-derived PFRs reduces SOC mineralization by suppressing the activities of extracellular enzymes responsible for carbon decomposition. We amended three distinct soils with biochar, comparing untreated biochar (WBC) against biochar with its PFRs chemically quenched (TBC). A separate treatment quenched the soil ·OH directly. A suite of response variables, including CO2 flux, ·OH content, and key enzyme activities, was then measured. PFR quenching reduced ·OH yield, confirming PFRs as a key ·OH source. In Black and Red soils, WBC suppressed CO2 emissions by 6.8–12.9%, whereas TBC stimulated them. Conversely, in the Fluvo-aquic soil, both WBC and TBC increased CO2 emissions, though TBC (with lower ·OH) caused a greater increase. Across all soils, quenching soil ·OH led to higher CO2 emissions and DOC concentrations, alongside a significant drop in the SOC. Enzyme activities increased with PFR quenching and were the highest upon ·OH scavenging, particularly in Red soil. Our results demonstrate that biochar-derived ·OH can inhibit SOC mineralization, primarily by suppressing hydrolase activities. This protective effect is soil-specific, being overridden by positive priming in calcareous soil but dominant in acidic soils. This study underscores the soil-specific role of biochar-derived ROS in carbon cycling and highlights the need for tailored biochar management strategies.