Biochar alters soil aggregate organic carbon component distribution and stability under nitrogen fertilization

Biochar application to soil can increase carbon sequestration; however, the long-term mechanisms through which different feedstocks and nitrogen (N) fertilization jointly regulate the distribution and stability of soil organic carbon components within aggregates remain unclear. We used a 12-year paddy field experiment in Yuhang County, Zhejiang Province, China, to compare rice straw and bamboo biochars with and without N fertilization, evaluating black carbon (BC, the refractory polycyclic aromatic carbon fraction derived from biochar) and dissolved black carbon (DBC) content, and aromaticity across aggregate size classes. Rice straw biochar preferentially accumulated in microaggregates (0.053−0.25 mm), increasing BC amount by 181−189%, while bamboo biochar predominantly enriched macroaggregates (0.25−2 mm), enhancing BC storage by 683−729% and significantly increasing BC aromatic condensation (B6CA/B5CA BC increased by 19−22%) relative to the control. Although total DBC concentrations did not change significantly, biochar application enriched the condensed aromatic structures of DBC. Annual N fertilization reduced the aromatic condensation of BC and DBC, with stronger effects under rice straw biochar amendment. Structural equation modeling suggested that black carbon (BC) accumulation closely covaried with the bulk soil organic carbon (SOC) pool, highlighting the importance of its physical integration and physical protection within the broader organic matrix. Conversely, DBC retention was governed by the interaction of dissolved organic matter (DOM) components and metal oxides. ​Our results emphasize that biochar characteristics determine its geochemical fate and stabilization pathways within soil aggregates, and that physical preservation of highly aromatic biochar within aggregates is crucial for achieving long-term carbon sequestration. We reveal that N fertilization can compromise aromatic stability, exposing a critical trade-off between maximizing crop productivity and maintaining long-term soil carbon sinks in biochar-amended paddy systems.