Objectives  Soil organic carbon (SOC) content and mineralization potential under long-term fertilization in three regions and their influencing factors were compared. We aimed to determine SOC storage factors to provide theoretical support for SOC storage improvement and sustainable farmland utilization.

  Methods  The study was conducted in Heilongjiang, Henan and Jiangxi provinces. Black soil, fluvaquent soil, and red soil under no fertilization (CK), chemical fertilizer application only (CF), and combined application of organic and chemical fertilizer (MCF) were the treatments used to explore SOC, microbial biomass carbon content (MBC), SOC mineralization potential (C₀), and kinetic constant (K_c). According to the long-term changes in soil organic carbon, the carbon input was simulated by the RothC model, and the SOC sequestration rate was calculated.

  Results  SOC content and C₀ in MCF were the highest among the three treatments in the studied soils. In black soil, SOC content and C₀ in MCF (P<0.05) increased by 4.17% and 33.94% compared with CK treatment. SOC content did not differ (P>0.05) between MCF and CF. C₀ in MCF increased by 31.73% compared with CF (P<0.05). In fluvaquent soil, SOC content in MCF increased by 40.59% and 21.94% compared with CK and CF (P<0.05). C₀ was not different (P>0.05) between MCF and CF. However, the C₀ in MCF was 3.14 times that in CK. In red soil, SOC content in MCF (P<0.05) increased by 64.35% and 43.10% compared with CK and CF, and C₀ in MCF (P<0.05) increased by 22.20% and 15.69 % compared with CK and CF. In black soil and red soil, the microbial quotient (MBC∶SOC) and mineralization quotient (C₀∶SOC) in MCF were (P<0.05) higher than CK. In fluvaquent soil, the microbial quotient and mineralization quotient in CF were significantly (P<0.05) higher than MCF and CK. ANOVA results showed that soil type, fertilization practices, and their interactions significantly (P<0.01) affected the soil organic carbon content and mineralization parameters on the whole. Partial correlation analysis showed that soil organic carbon was (P<0.05) positively correlated with available nitrogen and phosphorus and there was a significant (P<0.05) positive correlation between mineralization potential and carbon input, total nitrogen (N) and available phosphorus (P), as well as significant (P<0.05) negative correlates between mineralization potential and kinetic constant under controlled temperature, precipitation and evaporation, respectively. SOC was positively correlated with C₀, C input, total N, available N and P under controlled temperature and precipitation. Stepwise regression analysis showed that increasing total N content and decreasing annual average temperature and evaporation could increase soil organic carbon content. Improving soil available P and K content and decreasing soil pH and annual average evaporation could increase soil carbon mineralization potential.

  Conclusions  In the studied three farmland soils, the long-term combination of organic and chemical fertilizer improves soil carbon sequestration and mineralization potential, and the increase in soil pH and evaporation can decrease the mineralization potential of SOC. Therefore, with increasing SOC under reasonable fertilization practices, the regulation of soil pH and moisture should be employed to increase the mineralization potential of SOC so that soil organic carbon storage further increases.