Biochar decreased rhizodeposits stabilization via opposite effects on bacteria and fungi: diminished fungi-promoted aggregation and enhanced bacterial mineralization

Ryegrass was pulse-labeled with enriched (CO2)-C-13 for 18 h, followed by dynamic photosynthetic-carbon (C-13) quantification in the plant (shoot, root), soil aggregates (three size classes), and microbial phospholipids fatty acids (PLFA-SIP) in soil amended with or without 700 degrees C-pyrolyzed biochar. We observed that biochar led to no difference of C-13 allocation in shoot or root but reduced 88.7% of total C-13 in soil, with decreased incorporation by 92.8% (macroaggregates), 94.5% (microaggregates), and 84.1% (silt-clays), respectively, compared to biochar-unamended soil. Meanwhile, biochar exerted negative effects on fungal relative abundance but led to positive impacts on that of bacteria, e.g., it reduced root-associated fungi (i.e., 16:1 omega 5c) and fungal-assimilated C-13 (from averagely 71.2 ng C g(-1) soil to 26.3 ng C g(-1) soil after biochar application). The enhanced bacteria/fungi could be driven by biochar-mediated pH increase that relieved acid stress to bacteria. Co-occurrence network confirmed that biochar addition favored bacteria to compete with fungi, leading to decreased aggregation and stability (indicated by reduced normalized mean weight diameter) due to less fungal entangling with aggregates, thus exposing the rhizodeposits to bacterial (i.e., actinomycetes) decomposition. The correlation analysis further evidenced that fungal abundance was associated with C-13 accumulation in soil aggregates, while bacterial relative abundance especially that of actinomycetes was negatively correlated with C-13 accumulation. Random forest modeling (RF) supported the contributions of fungi to C-13-sequestration compared to bacteria. Taken together, we concluded that less stabilization of rhizodeposits in the biochar-amended soil was due to changes in microbial community, particularly the balance of fungi-bacteria and their interactions with soil physicochemical properties, i.e., aggregation and pH.