Objectives The co-incorporation of rice straw (RS) with Chinese milk vetch (Astragalus sinicus L., CMV) to the field is an effective measure to reduce fertilizer input and improve fertilizer utilization efficiency. We conducted field experiments in grey mud paddy fields in Fujian, to verify the effectiveness of the measure.

Methods Field experiments were conducted simultaneously in Shaxian, Shunchang and Pucheng counties, Fujian Province, the test rice variety in Shaxian was rutoon rice, and in Shunchang and Pucheng were single-cropping rice. Five treatments were set up in all the experiments, included: blank control (CK), whole rate of chemical fertilizer (100%CF), 80% of chemical fertilizer (80%CF), CMV planting and 80% of chemical fertilizer (CMV+80%CF), rice straw returning with CMV planting and 80% of chemical fertilizer (RS+CMV+80%CF). The rice yield, yield traits, nutrient absorption were determined at the mature stages of rice.

Results Compared with the 100%CF, CMV+80%CF and RS+CMV+80%CF did not increase the average grain yield and the average rice straw yield in the three regions, but the rice straw yield was significantly increased by 33.7% and 26.1% in Shaxiang. The fertilizer contribution rate to yield in 100%CF treatment was 2.74 percent point higher than in 80%CF. And relative to 80%CF, the contribution rates to yield in CMV+80%CF and RS+CMV+80%CF treatments were increased by 7.73% and 11.30%, and the average yield of RS+CMV+80%CF treatment was 3.9% higher than that of CMV+80%CF. The variation coefficient of rice yield was in ascending order of RS+CMV+80%CF<CMV+80%CF<100%CF<80%CF, while the sustainable index was in descending order of RS+CMV+80%CF>CMV+80%CF>100%CF>80%CF, showing that both the CMV+80%CF and RS+CMV+80%CF treatments were in favor of stable and sustainable rice production, and RS+CMV+80%CF performed better than CMV+80%CF. CMV and straw incorporation were beneficial for the accumulation of N, P, and K nutrient. Compared with 100%CF, CMV+80%CF treatment increased rice N, P and K recovery rates by 19.1, 20.1, and 29.5 percent points, and RS+CMV+80%CF treatment increased those by 19.3, 8.8, and 30.6 percent points, the N, K nutrient recovery rate of RS+CMV+80%CF was higher than that of CMV+80%CF. In addition, RS+CMV+80%CF was recorded the highest agronomy efficiency and partial factor productivity of nitrogen fertilizer among the treatments.

Conclusions For both rutoon and single-cropping rice varieties under the condition of 20% reduction of chemical fertilizer input, co-incorporation of rice straw and Chinese milk vetch is conducive to maintain the stable yield of rice, and keep the sustainable state of rice production due to the increased fertilizer recovery rate, and obvious effect in saving fertilizer and increasing efficiency of rice production. Therefore, the co-incorporation of green manure and rice straw is a reliable measurement for the green and sustainable rice production in the grey mud field area.