Evaluating the effects of management practices on the soil organic carbon (SOC), total nitrogen (TN) and grain yields would be valuable to explain field-level variability in crop production. A 33-year field experiment on the fluvo-aquic soil of North China with six treatments in a wheat (Triticum aestivium L.)-maize (Zea mays L.) rotation was evaluated. The six treatments were: non-fertilization (CK), nitrogen (N), nitrogen-phosphorus fertilization (NP), nitrogen-phosphorus-potassium fertilization (NPK), manure and nitrogen fertilization (NM), and straw returned with nitrogen fertilizers (NS). The results showed that the content of SOC and TN significantly increased in NM treatment. Application of inorganic fertilizers had small influence on SOC, but SOC and TN increased significantly in NM treatment over the long-term experiment. Compared to control, grain yield of wheat and maize increased two times under all treatments. The highest grain yield was detected in NM and NPK treatments. However, wheat yield was not significantly different (P > 0.05) between control and N treatment. Grain yields were more than doubled under fertilization for both wheat and maize, with the highest yield under the NM and NPK treatments and the lowest under CK treatment for maize and N treatment for wheat. The NP fertilization had little effect on maize yield in long-term, suggesting that potassium was not the primary limiting nutrients in the study site. Statistical analysis indicated that maize yield was significantly correlated with SOC and TN, and wheat yield was significantly correlated with SOC only. However, the relationships were stronger with TN (r = 0.26-0.42) than SOC (r = 0.12-0.37), indicating the importance of maintaining TN in agricultural soils. There was a strong positive linear correlation between carbon sequestered and carbon input (r = 0.828, P < 0.01) in the study site, indicating that the conversion rate of carbon input to SOC was 8.5 %. SOC did not reach the saturation in fluvo-aquic soil and have the potential to sequester more carbon.
