Chinese milk vetch (Astragalus sinicus L.) is a more effective inhibitor of methane production and emission than straw under equal nutrient condition in paddy field

Objectives The inhibitory effects of Chinese milk vetch (Mv) and rice straw on production and emission of methane in paddy fields were compared.

Methods Field trials were conducted at Gaoqiao experimental base of Hunan Academy of Agricultural Sciences, under one-season super-hybrid rice planting system. The soil samples were collected in six treatment plots, including no-fertilization control (CK) and five equal-nutrient fertilization treatments, i.e., chemical fertilizer only (CF), CF plus rice straw incorporation (S), CF plus Mv incorporation (M), CF plus both rice straw and Mv incorporation (MS), CF plus rice straw and Mv incorporation and slaked lime (MSC). At the early stage of rice tillering (June 21th, 2021), CH₄ flux was monitored using closed-static-chamber method, and the surface water samples were collected below the closed-static-chamber for determination of dissolved methane. At the same time, soil samples (0–20 cm) were taken for the test of physicochemical properties and the laboratory incubation experiment.

Results 1) Soil CH₄ daily fluxes fluctuated between 5.70–26.65 mg/(m²·h), fertilized soils recorded higher CH₄ emission than CK, and M treatment elicited the lowest CH₄ emission among fertilizer treatments. Compared to S treatment, M treatment reduced 13.78% of chemical N input, and reduced CH₄ flux by 12.50%. The dissolved CH₄ concentration in surface water ranged between 70.02–163.58 mg/kg, the surface water in MSC treatment had 30.68% higher CH₄ concentration than others. 2) Compared to S treatment, M treatment increased SOM and total carbon content by 7.60% and 7.55%, but decreased DOC by 25.99%. M treatment exhibited the lowest CH₄ production and oxidation potentials, which were 61.04% and 7.56% lower than the other five treatments, and 83.16% and 5.36% lower than S treatment, respectively. Compared with the other five treatments, M treatment markedly decreased the CH₄ production through aceticlastic methanogenesis pathway, M treatment decreased the contribution of CH₄ to total by 52.52%, and reduced the contribution of CH₄ to total by 53.49% in comparison with S treatment.

Conclusions Compared with rice straw incorporation, Chinese milk vetch incorporation not only reduces soil available N, but also reduces soil dissolved organic carbon (DOC) concentration during the early growth stage of rice, it is therefore beneficial to the storage of carbon in soil and decrease of CH₄ emission. Milk vetch incorporation mainly inhibits the production of CH₄ via aceticlastic methanogenesis.