Manure and mineral fertilizer enhance soil organic carbon recovery in reclaimed cropland through aggregate-associated carbon accumulation and plant-derived inputs

Reclaimed croplands often have depleted soil organic carbon (SOC) and impaired soil structure, yet the microbial and plant mediated mechanisms governing SOC recovery under long-term fertilization remain poorly understood. We conducted an 8-year field fertilization experiment (2014–2021) in reclaimed cropland with four fertilization treatments, including no fertilization (CK), mineral fertilizer (NPK), manure alone (M), and combined manure and mineral fertilizer (MNPK), to investigate how fertilization regulates carbon inputs, aggregate-associated SOC, microbial transformation, and SOC recovery in reclaimed cropland. Compared with CK, manure amended treatments (M and MNPK) increased maize grain yield by approximately 250%. Manure treatments also increased SOC across aggregate-size fractions, with MNPK increasing SOC in > 2 mm and 0.25–2 mm aggregates by 69%–124% and 57%–62%, respectively. However, SOC accumulation occurred despite reduced aggregate stability, suggesting a decoupling between carbon retention and aggregate stability during the early recovery of structurally degraded reclaimed soils. Manure application increased lignin phenol contents by 24%–30% in 2020 and 13%–20% in 2021 relative to CK, with MNPK generally giving the highest values. Microbial necromass carbon showed contrasting responses between years, while its contribution to SOC declined under manure treatments, remaining below 30% of total SOC, indicating a dynamic but non-dominant role in SOC accumulation during the recovery process. Overall, SOC recovery in reclaimed cropland was associated with increased C inputs and aggregate-associated C accumulation, while microbial necromass represented a dynamic pathway of C transformation rather than a consistently increasing contributor to SOC, furthermore, the application of organic manure demonstrates the most significant combined benefits for enhancing crop productivity and promoting SOC accumulation. graphical abstract