Feedstock-dependent biochar properties shape short-term soil CO₂ emissions through microbial enzyme-associated...

Black soils represent globally important reservoirs of soil organic carbon (SOC), yet the mechanisms regulating short-term carbon dioxide (CO₂) emissions following biochar amendment remain poorly understood. Although biochar-induced alkalization has been widely proposed as a key mechanism influencing soil carbon mineralization, whether increased soil pH alone can explain feedstock-dependent CO₂ responses remain unresolved. Here, we investigated the effects of biochars derived from six feedstocks on CO₂ emissions, soil chemical properties, microbial functional traits, and extracellular enzyme activities in a short-term incubation experiment using black soil. Biochar application increased cumulative CO₂ emissions by 9.1%-44.6%, with substantial variation among feedstocks. Biochars increased soil pH, nutrient availability, and the activities of carbon-, nitrogen-, and phosphorus-acquiring enzymes, but the magnitude of these responses varied substantially with feedstock-dependent biochar properties. Cumulative CO₂ emissions were positively associated with β-glucosidase (BG), leucine aminopeptidase (LAP), and acid phosphatase (ACP) activities. Random forest and regression analyses identified soil pH and extracellular enzyme activities as major predictors of CO₂ emissions, with extracellular enzyme activities collectively contributing 52.9% of the relative importance and exhibiting stronger relationships with CO₂ emissions (R = 0.68-0.72) than soil chemical properties alone. Importantly, pH manipulation using NaOH in the absence of biochar failed to reproduce the enhanced enzyme activities and CO₂ emissions observed following biochar amendment, demonstrating that alkalization alone was insufficient to explain the contrasting CO₂ responses among different biochar treatments. These findings indicate that short-term CO₂ emissions following biochar application are shaped by feedstock-dependent biochar properties in association with microbial enzyme-mediated carbon turnover beyond the effect of soil pH. Our results highlight the importance of integrating biochar properties and microbial functional responses when evaluating short-term soil carbon mineralization and the environmental implications of biochar amendment.