Objective  Effects of long-term application of organic waste, such as Chinese milk vetch, pig manure, and/or straws, on the organic carbon components and microbial biomass in the soil, as well as the yield of rice cultivated two consecutive seasons in a year on the land were studied with the aid of a 40-year research project on red soil of rice paddy fields.

  Methods  Six treatments designed for the study included (1) CK of no added fertilizer, (2) NPK of chemical fertilizer application, (3) M1 of Chinese milk vetch fertilization on early rice, (4) M2 of Chinese milk vetch plus pig manure applications on early rice, (5) M3 of Chinese milk vetch applied on early rice and pig manure on late rice, and (6) M4 of Chinese milk vetch application on early rice and straw returning on late rice. Soil samples were collected at a depth of 0–20 cm before the late rice was harvested in 2020 for fertility determinations on organic carbons, microbial biomass, nitrogen, etc.

  Result  (1) Long-term organic fertilization could increase rice yield. As shown in the study, the yields of the early rice under M2 and M3 increased 1.4 and 1.25 times, respectively, and those of the late rice 0.59 and 0.65 times, respectively, over CK. In comparison with NPK, M2 and M3 raised the early rice yield by 18.1% and 10.6%, respectively, and the late rice yield by 15.7% and 20.0%, respectively. (2) Long-term organic fertilization could enhance the organic carbon content in soil. As compared to CK, the contents of permanganate oxidative organic carbon, free particulate organic carbon, and dissolved organic carbon on the early rice field under M2 significantly increased (P＜0.05). The free particulate organic carbon content of 0.97 g·kg⁻¹ was significantly higher than that under NPK treatment at 0.68 g·kg⁻¹. The microbial biomass carbon in the soil under NPK, M1, M2, M3, or M4 was 22.1% to 58.9% higher than CK with the early rice field rendering the most significant effect at 231.2 mg·kg⁻¹ (P＜0.05). (3) Long-term organic fertilization could heighten the ratio of free particulate carbon and dissolved organic carbon distributions in soil. The most significant results were observed on the early rice lots under M2, and the main form of carbon was permanganate oxidative organic carbon. In the soil, organic carbon correlated with permanganate oxidative organic carbon, free particulate carbon, and dissolved organic carbon (P＜0.01). And (4) long-term organic fertilization could also increase the total nitrogen, alkali-hydrolyzed nitrogen, and other nutrients in the soil. The rice yield was found significantly correlate with the available phosphorus, organic carbon, total nitrogen, available nitrogen, and dissolved organic carbon in soil at P＜0.01, as well as with the total phosphorus, free particulate organic carbon, and permanganate oxidative organic carbon at P＜0.05.

  Conclusion   Long-term organic fertilization could materially improve the fertility and adequately regulate the dissolved organic carbon in the red soil promoting the yield of rice cultivated on the fertilized land. The application of Chinese milk vetch and pig manure presented a promising potential for sustainable agricultural development.