Biochar application increased soil carbon sequestration by altering organic carbon components in aggregates

Biochar application critically influences soil organic carbon (SOC) dynamics through aggregate stabilization, but their long-term effect on the chemical components and properties of aggregate-organic carbon (OC) under crop planting remains unclear. We conducted an experiment using two biochar application methods (homogeneous application (HA), bottom-concentrated application (CA)) combined with four application amounts (0 (CK), 10 (10B), 20 (20B), 40 Mg ha⁻¹(40B)) to investigate their effect on SOC, easily-oxidized organic carbon (EOC), carbon fraction in soil aggregate, function groups of aggregate-OC and aggregate carbon preservation capacity (CPC) under maize (Zea mays) cropping after 6 yrs’ biochar application. Results showed that compared with CK, (1) 20B and 40B increased the proportion of > 2 mm aggregates in HA while decreased it in CA but not significantly; (2) 10B increased the OC and EOC of aggregates in CA (except for 0.25–2 mm) by 13–22 % and 5–18 %, respectively, while 40B increased them in HA by 11 %-27 % and 8–28 %, respectively; (3) CA increased hydrophobicity of < 0.25 mm aggregate-OC by 2–19 %, while HA enhanced aromatic band stretching by 0.8–5 %, improving SOC resistance to microbial degradation; (4) 20B and 40B improved SOC and CPC of total water-stable aggregates but decreased EOC in HA and CA. Meanwhile, HA increased SOC by affecting aggregate-OC and its hydrophobic and aromatic functional groups, while CA increased SOC by affecting dissolved OC and aggregate-OC. Overall, a low amount of bottom-concentrated application and a higher amount of homogeneous application are promising practices to enhance the anti-microbial decomposition ability of SOC and long-term SOC sequestration.