关键词: 砷浓度, 水稻土, 氮素转化, 功能基因, 氮砷互作

Abstract:

To explore the effects of nitrogen migration and transformation in paddy soil-water systems that regulate As(Ⅲ), the current study used NaAsO₂ to introduce As at different concentrations [0 mg·kg⁻¹ (control), 5 mg·kg⁻¹ (low content), 15 mg·kg⁻¹ (medium content), and 30 mg·kg⁻¹ (high content)], and performed an experiment of 55 days to assess the nitrification, denitrification, mineralization, and arsenic transformation in paddy soil-water systems using soil collected from Liangping District, Chongqing City. The absolute abundance of key functional genes (amoA, nirS, nirK, narG, and napA) involved in nitrogen transformation was analyzed using quantitative PCR. The results showed that the addition of As(Ⅲ) promoted the mineralization of nitrogen in sediments, with the maximum mineralization efficiency of 7.5% that recorded upon 5 mg·kg⁻¹ As(Ⅲ) addition, associated with the aquatic total nitrogen concentration in the As(Ⅲ) added treatments increased from 13.8 mg·L⁻¹ to 38.5 mg·L⁻¹. After 55 days incubation, the aquatic total nitrogen concentration increased from 12.5 mg·L⁻¹ to 24.1 mg·L⁻¹ in medium content (15 mg·kg⁻¹), and from 13.1 mg·L⁻¹ to 26.4 mg·L⁻¹ in high content (30 mg·kg⁻¹). The aquatic ammonia nitrogen concentration displayed a 5-folds increase in the As(Ⅲ) amendment treatments during days 0−10 compared to the initial value, but an insignificant difference in nitrite concentration (higher than that of the control) was recorded. This implied that nitrification was inhibited during this period. Mineralization was promoted on days 10−20. This was revealed by the increase in the abundance of amoA genes in the 5 mg·kg⁻¹ As(Ⅲ)-added treatments, which increased by 300.81%, while it increased by 28.5% and 114.3% in the 15 mg·kg⁻¹ As(Ⅲ) and 30 mg·kg⁻¹ As(Ⅲ)treatments, respectively. At the end of the experiment (day 55), the absolute abundance of nirS and nirK genes followed the order low-, medium-, and high-content treatments, implying that an increase in As(Ⅲ) content inhibited the denitrification process. On days 20 and 55, the nitrate-nitrogen content in the soil of the treatment groups was significantly higher than that in the control, and the nitrate concentration in the low-content group was significantly higher than that in the medium- and high-content groups. In addition, it should be noted that the absolute abundance of the aioA gene decreased in the following order: low, medium, and high contents, which was significantly higher than that in the control, indicating that an increase in nitrate-nitrogen, which was promoted by the amendment of As(Ⅲ), was responsible for the conversion of As(III) to As(V). These results may provide new insights into the understanding of the influence of As on nitrogen behavior in paddy soils as well as the reduction in As toxicity through nitrogen and As interactions.

Key words: arsenic concentration, paddy soil, nitrogen transformation, functional genes, nitrogen and arsenic interaction