Objective Based on long-term located experiments, this study examined the variations in soil organic carbon (SOC) content and its active fractions under different long-term nitrogen and phosphorus fertilizer application levels and varying rates of maize straw return in the wheat-maize rotation region of North China. This research aimed to provide a scientific foundation for enhancing soil fertility and increasing agricultural productivity in this area.

Method Utilizing a 42-year long-term fixed-site experiment, a two-factor design was adopted with fertilization levels as the main factor and maize straw return rates as the sub-factor to study the changes in active fractions of SOC. The fertilization levels were: no chemical fertilizer (T1), N 90 kg hm⁻² + P₂O₅ 60 kg hm⁻² (T2), N 180 kg hm⁻² + P₂O₅ 120 kg hm⁻² (T3), and N 360 kg hm⁻² + P₂O₅ 240 kg hm⁻² (T4). The maize straw return rates included no straw return (S1), 2250 kg hm⁻² (S2), 4500 kg hm⁻² (S3), and 9000 kg hm⁻² (S4).

Result Various fertilizer and straw return rates significantly affected the SOC content and its active fractions in the 0 ~ 20 and 20 ~ 40 cm, with an interaction (P < 0.05). In the 0 ~ 40 cm, the SOC content and its active fractions increased by 9.89% to 36.51% and 14.87% to125.03% respectively, compared to no fertilizer treatment. The SOC content and its active fractions increased by 12.17% to 23.71% and 17.90% to 51.52% respectively, compared to no straw return treatment. Under the T1 and T2 fertilizer levels, with the increase of straw return rates, the SOC, particulate organic carbon, and readily oxidizable organic carbon in the 0 ~ 20 cm showed an upward trend. However, under the T3 and T4 fertilizer levels, there was no significant difference between the S3 and S4 treatments. The SOC and microbial biomass carbon contents showed a significant positive correlation with the rate straw carbon input. However, the contents of particulate organic carbon and readily oxidizable organic carbon increased first and then decreased or remained unchanged with the increase of straw input. Additionally, fertilizer and straw return significantly increased the soil carbon management index in the 0 ~ 20 cm, but medium and high fertilizer and high straw return were not conducive to the improvement of the carbon management index in the 20 ~ 40 cm soil layer.

Conclusion Long-term fertilization and straw return both significantly increase the SOC content and its active fractions, and the carbon pool management index in the 0 ~ 20 cm. Under no fertilization or low fertilization conditions, high straw return can improve soil carbon sequestration. However, with medium and high fertilization levels, high straw return can significantly increase the proportion of active organic carbon fractions, which is unfavorable for SOC accumulation.