Abstract:

【Objective】 The biogas slurry is rich not only in N, but also in $\mathrm{HCO}_{3}^{-}$. This study aimed to reveal the impact of $\mathrm{HCO}_{3}^{-}$ on the release of CO₂. 【Mothed】 when the biogas slurry was applied in farmlands, by arranging and adding four types of culture mediums, namely, the original biogas slurry, original liquid for removing $\mathrm{HCO}_{3}^{-}$, deionized water plus $\mathrm{HCO}_{3}^{-}$ and deionization water, designated as BS, BS-B, W+B and W treatments, respectively, to the paddy soil for a 30-day cultivation experiment. 【Result】 The soil CO₂ release rates were observed. The results indicated that under the BS treatment, the soil CO₂ release rates decreased rapidly from 36.6 mg/(kg·h) at 2 h to 2.7 mg/(kg·h) at 30 d, and those for the BS-B treatment decreased from the highest 9.3 mg/(kg·h) at 1 d to 1.9 mg/(kg·h) at 30 d. The CO₂ release rates in the BS treatment were significantly higher than those in the BS-B. Within 30 d, the cumulative CO₂ emission under the BS was 193.0 g/kg, while that in BS-B was only 97.8 g/kg. It was evident that the presence of $\mathrm{HCO}_{3}^{-}$ in the biogas slurry significantly increased the release of CO₂ from the soil. In comparison, the CO₂ release rate under the W+B treatment decreased from the highest value of 12.0 mg/(kg·h) to 0.6 mg/(kg·h) at 15 d, with a cumulative CO₂ emission of 54.5 g/kg during the culture period. The CO₂ release rate under W treatment varied from 0.2 to 1.6 mg/(kg·h) during the culture period, and the cumulative CO₂ emission was only 34.4 g/kg. It is evident that the CO₂ emission under the BS treatment was 1.6 times higher than that under BS-B and W+B treatments together. Further analysis indicated that the contribution of $\mathrm{HCO}_{3}^{-}$ to the CO₂ release in the paddy soils mainly occurred in the overlying water layer, which was dominated by $\mathrm{HCO}_{3}^{-}$ hydrolysis and the reaction between $\mathrm{HCO}_{3}^{-}$ and H⁺ from $\mathrm{NH}_{4}^{+}$ oxidation. 【Conclusion】 With biogas slurry applying, the addition of a large amount of $\mathrm{HCO}_{3}^{-}$ was one of the main reasons for the acceleration and increase of soil CO₂ release, which was not just a simple sum of contributions from HC and the other components of the biogas slurry, and there was a significant synergistic effect between the two.

Key words: biogas slurry, soil pH, $\mathrm{HCO}_{3}^{-}$, CO₂ release rate