Abstract: 【Objective】Biogas slurry is rich in N, which is the basis of its farmland utilization. However, it also contains a large amount of HCO₃^-. The conversion of HCO₃^- in biogas slurry and its effect on rice growth were investigated in order to provide a new theoretical basis for replacing chemical fertilizer with biogas slurry, reducing the amount of chemical fertilizer and whether biogas slurry can be utilized as resources. 【Method】Four treatments, biogas slurry (BS), biogas slurry (BS?B), deionized water plus HCO₃^- (W+B) and deionized water (W), were set up in this paper. Using ¹³C labeling technology, the characteristics of CO₂ release were observed through pot experiment at Rice Seedling Stage, and the photosynthetic rate, intercellular CO₂ concentration, dry and fresh weight, plant height, overlying water and soil pH, HCO₃^- and NH₄⁺ contents, were analyzed. 【Result】(1) Under BS treatment, CO₂ release rates ranged from 9.55-38.07 mg·kg^-1·h^-1, with a net cumulative release of 4654.06 mg·kg^-1. Under BS?B treatment, the CO₂ release rates ranged from 4.55-17.25 mg·kg^-1·h^-1, and the net cumulative release was 780.68 mg·kg^-1. Under W+B treatment, the CO₂ release rate was 3.93-26.33 mg·kg^-1·h^-1, and the net cumulative release was 1274.07 mg·kg^-1. Under W treatment, the CO₂ release rate was 3.22-11.90 mg·kg^-1·h^-1, and the cumulative release amount was 2265.20 mg·kg^-1. Under BS treatment, the average CO₂ release rate was 4.18 times and 2.44 times of BS?B and W+B, respectively, and the net cumulative CO₂ release was 5.96 times and 3.65 times of BS?B and W+B, respectively, which were significantly higher than those under BS?B and W+B treatments. At the same time, the net cumulative release under BS treatment was greater than the sum of the two treatments (BS?B) + (W+B), and HCO₃^- had a synergistic effect with other components in biogas slurry on CO₂ release. (2) The net cumulative release of ¹³CO₂ under BS treatment was 32.87 mg·kg^-1, accounted for 0.71% of the net cumulative release of CO₂ in soil-rice system. The net cumulative release of ¹³CO₂ under W+B treatment was 13.18 mg·kg^-1. In comparison, the net cumulative release amount of ¹³CO₂ under BS treatment was significantly higher than that under W+B treatment (P<0.05), indicated that other components in biogas slurry promoted the conversion of HCO₃^- to CO₂. (3) The net photosynthetic rate of BS and BS?B treatments in the first 12 h was significantly higher than that of W+B and W treatments. After the addition of culture medium, the net photosynthetic rate under BS treatment was significantly higher than that under BS?B treatment at 2-7 d, and was significantly higher than that under W+B treatment during the whole culture period (P<0.05). HCO₃^- in biogas slurry significantly improved the photosynthesis of rice leaves. In comparison, the intercellular CO₂ concentration was significantly lower than that of the other three treatments 5 days before BS treatment. Rice plant height and fresh weight under BS and BS-B treatments were significantly higher than those under W+B and W treatments (P<0.05), and there was no significant difference in dry weight among the four treatments. (4) The fixed ¹³CO₂ content of rice seedlings treated by BS was 4.05 g·kg^-1, and the utilization rate of marker HCO₃^- was 18.54%. The fixed amount of ¹³CO₂ in W+B treatment was 3.29 g·kg^-1 and the H¹³CO₃^- utilization rate was 14.20%. The ¹³CO₂ of H¹³CO₃^- promoted the photosynthesis of rice and was beneficial to the growth of rice. (5) The release rates of CO₂ and ¹³CO₂ under BS and W+B treatments were significantly correlated with overlying water and soil HCO₃^- content and pH. At the same time, the photosynthetic rate of rice under BS and W+B treatments was significantly positively correlated with HCO₃^- content in soil. 【Conclusion】When biogas slurry was returned to the field, a large amount of HCO₃^- transformation significantly promoted the release of CO₂, which was beneficial to rice photosynthesis. Soil HCO₃^- content and soil pH value are important factors affected CO₂ release and rice photosynthesis. At the same time, rice had a higher utilization rate of HCO₃^- in biogas slurry, and HCO₃^- in biogas slurry had obvious CO₂-like fertilization effect.

Key words: biogas slurry, HCO₃^-,  , ¹³C marking, rice, CO₂ , fertilization effect, photosynthesis