Effects of Biochar on Soil Microbial Carbon Use Efficiency: A Meta-Analysis of Mechanisms, Controversies, and Frontiers

Soil Microbial Carbon Use Efficiency (CUE) is a key ecological parameter that quantifies the conversion of absorbed carbon by microorganisms into their biomass or respiratory consumption. It directly regulates the fixation and release of soil carbon. As a promising soil amendment, biochar is believed to effectively influence CUE; however, its specific effects, optimal conditions, and underlying mechanisms are not yet universally recognized. This study utilized "biochar" and "microbial carbon use efficiency" as keywords to gather 545 data-sets from 26 articles in the CNKI and Web of Science core collection databases for a meta-analysis. The results showed that the addition of biochar significantly increased soil CUE, although this increase was moderated by the properties and dosage of the biochar. When the biochar application rate was ≤5%, the addition of high-temperature prepared nutshell, wood, and straw biochar resulted in increases in soil CUE by 31.78%, 7.43%, and 6.94%, respectively, compared to the control treatment. However, excessive application or biochar prepared at temperatures lower than 500 ℃ let to negligible or even decreased in soil CUE. Following biochar application, the average pH value increased by 3.07%, bulk density decreased by 5.47%, and soil organic carbon and microbial biomass carbon significantly increased by 46.87% and 15.74%, respectively. Additionally, the structure of the soil microbial community changed significantly, with fungal numbers increasing by approximately 28.36%, bacterial numbers by 24.72%, and actinomycetes by 11.9%. Total extracellular enzyme activity in the soil increased by 38.32%. Clearly, biochar improves soil pore structure by regulating soil pH, optimizes microbial community structure, enriches high-functioning bacterial groups, and increases soil enzyme activity, leading to a significant increase in soil CUE. This study systematically quantified the impact of biochar application on soil CUE, revealed the multiple mechanisms through which biochar enhances soil CUE via physical, chemical, and biological factors, and provided a scientific basis for precise agricultural management aimed at enhancing soil carbon sink function.