Differential Responses of Soil Thermal Conductivity, Microbial Carbon Use Efficiency, and Soil Organic Carbon to Feedstock-Specific Biochar Under Alternate Drying–Wetting Cycles

Biochar can alter soil physical conditions, microbial carbon processing, and soil organic carbon (SOC) responses under fluctuating moisture, yet how these changes are coordinated remains insufficiently understood. We conducted a two-season greenhouse pot experiment to examine the initial (first-year) and residual (second-year) effects of wheat-straw biochar (WSB) and softwood biochar (SWB) under conventional deficit irrigation (CDI) and alternate drying–wetting cycles (DWC). Compared with unamended soil, biochar amendment improved water-dispersible microaggregate-size distribution, mean microaggregate size, and water-holding capacity, which contributed to reduced soil thermal conductivity (STC) by 6.0–14.2%. Biochar application also improved microbial carbon use efficiency (CUE) by 41.1–52.3%, with WSB generally showing stronger and more persistent effects than SWB. Relative to CDI, DWC increased soil respiration rate by 14.9–48.8% but decreased CUE by 7.4–10.2% and SOC by 3.0–10.3%, indicating a shift toward greater respiratory carbon loss under repeated moisture fluctuations. Biochar amendment increased SOC across both seasons, particularly under WSB, and partially alleviated the DWC-associated reductions in CUE and SOC. Correlation analyses showed that lower STC was associated with higher CUE and SOC, but these relationships should be interpreted as coordinated associations rather than direct evidence of a causal thermal-regulation mechanism. Principal component and random forest analyses further highlighted STC as a prominent variable associated with variation in CUE and SOC among the measured soil attributes. These findings indicate that biochar-mediated changes in soil physical conditions are closely associated with microbial CUE and SOC responses under drying–wetting cycles, wherein soil thermal properties may represent an important physical dimension of these carbon responses.