Change in Microbial Metabolic Quotient Under Biochar Amendment Was Associated with Soil Organic Carbon Quality, Microbial Community Composition, and Enzyme Activity in Bulk and Rhizosphere Soils in an Acid Rice Paddy

Decreases in soil microbial metabolic quotient ( q CO 2 ) have been frequently observed under biochar amendment; however, the potential driving factors are not fully understood. This study aimed to investigate whether changes in q CO 2 under biochar amendment could be closely linked with the alterations in soil carbon and nitrogen quality, microbial community composition and enzyme activity in both bulk and rhizosphere soils. Both bulk and rhizosphere soils were collected from a field experiment after a 2-year biochar application (at 0, 20, and 40 t ha −1 ) in an acid rice paddy in subtropical China. Changes in q CO 2 , the sizes of soil labile and recalcitrant carbon and nitrogen, enzyme activities, and bacterial community composition were investigated. Biochar amendment induced a lower q CO 2 in the bulk soil but had little effect on q CO 2 in the rhizosphere compared with the non-amended control. The sizes of soil labile carbon and nitrogen pools significantly declined in the bulk soil whereas increased in the rhizosphere under biochar. Biochar amendment at 40 t ha −1 significantly changed bacterial community composition and increased their co-occurrence network complexity. In addition, biochar enhanced substrate induced respiration and the activities of β-glucosidase, phenol oxidase and peroxidase in both bulk and rhizosphere soils. The q CO 2 in the bulk soil rather than in the rhizosphere had significant correlations with soil pH, soil labile and recalcitrant carbon pools, bacterial community composition, and enzyme activities. Structural equation modeling suggested that biochar decreased q CO 2 in the bulk soil mainly by increasing soil pH, C/N ratio, microbial substrate utilization and enzyme activities. This study suggests that biochar amendment in rice paddy induced a lower q CO 2 in the bulk soil rather than in the rhizosphere. Soil organic carbon lability, and carbon-degrading enzyme activities and changes in bacterial community composition were key drivers of q CO 2 in the bulk soil.