Effects of combined applying Chinese milk vetch and chemical fertilizers on CH₄ and N₂O emissions from double cropping paddy fields

Objectives Winter planting of Chinese milk vetch (Astragalus sinicus L.) for returning to fields is of significance for promoting paddy nutrient cycle and improving nitrogen use efficiency. The purpose of this paper was to investigate the greenhouse gas emissions in paddy fields of Astragalus sinicus straw and nitrogen fertilizers.

Methods A pot experiment was conducted, and there were 6 Chinese milk vetch and fertilizer treatments, no fertilizer (CK), single urea (CF), single application of Chinese milk vetch (MV), 1/4 Chinese milk vetch + 3/4 urea (1/4 MV + 3/4 CF), 1/2 Chinese milk vetch +1/2 urea (1/2 MV + 1/2 CF) and 3/4 Chinese milk vetch +1/4 urea (3/4 MV +1/4 CF). Except for the CK, the nitrogen rate of other treatments was 111.4 mg/kg dry soil. Chromatographic determination of CH₄ and N₂O emission characteristics in double cropping rice season paddy field, global warming potential (GWP) and greenhouse gas emissions intensity per unit grain yield (GHGI) were measured using the static chamber gas method.

Results 1) The CH₄ emission in different rice seasons was basically consistent in early rice and late rice growing seasons, and the CH₄ emissions were concentrated in the tillering stage and the heading stage of early rice and late rice. The early rice season CH₄ emission has no obvious emission peak, with the maximum value of 5.69 mg/(m²·h), and the late rice season emission had two obvious emission peaks, appeared at the early rice transplanting and field drying, with the maximum peak values of 13.33 mg/(m²·h) and 8.83 mg/(m²·h). The cumulative CH₄ emission was increased with the increase of milk vetch ratio. 2) Under different fertilization treatments, the N₂O emission flux had obvious seasonal variation pattern. The maximum peak value of N₂O at the early season appeared in the third day after the sowing, and was 1092.2 μg/(m²·h). The late rice season N₂O emissions mainly concentrated in the tillering stage and the late stage of alternative drying and wetting, with the peak value of 795.7 μg/(m²·h), the cumulative N₂O emission was decreased with the increase of the with milk vetch ratio, and the N₂O cumulative emission of MV was negative. 3) The double rice total yield of the CF treatment was the highest, and significantly higher than those of the CK, 1/4 MV+3/4 CF and MV, and the double rice total yield of the 1/2 MV+1/2 CF was significantly higher than those of the CK and 1/4MV+3/4 CF. The effects of treatments on GWP and GHGI in paddy fields were not significant.

Conclusions Compared with the CF, different ratios of Chinese milk vetch and nitrogen fertilizer had no significant effects on GWP and GHGI in paddy fields.