Feedstock-dependent biochar properties shape short-term soil CO2 emissions through microbial enzyme-associated processes beyond the effect of soil pH in black soil.

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.