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

摘要： 为探究As(III)对水稻土-水体系中氮素迁移转化的影响，采集重庆市梁平区稻田土，以NaAsO₂为供试品构建周期为55 d的实验体系，研究不同As(III)添加量下（0 mg·kg⁻¹，空白对照；5 mg·kg⁻¹，低含量；15 mg·kg⁻¹，中含量；30 mg·kg⁻¹，高含量）水稻土-水体系中氮的硝化、反硝化、矿化、砷的形态转化等过程。并通过定量PCR对氮转化关键功能基因（amoA、nirS、nirK、narG、napA）的绝对丰度进行分析。结果表明，As(III)的加入会促进沉积物中氮矿化，其中5 mg·kg⁻¹含量下氮的矿化度最高（7.5%），实验55 d后低含量组水相总氮从13.8 mg·L⁻¹增至38.5 mg·L⁻¹，中含量处理组水相总氮从12.5 mg·L⁻¹增至24.1 mg·L⁻¹，高含量处理组水相总氮从13.1 mg·L⁻¹增至26.4 mg·L⁻¹。实验第0－10天低、中、高含量处理组水相的氨氮浓度相较于初始值增加了5倍，亚硝氮浓度无显著差异，表明此阶段硝化过程被抑制。10－20 d处于促进矿化阶段，处理组amoA基因绝对丰度均高于对照组，其中低含量处理组amoA基因丰度增加了300.81%，中、高含量组amoA基因丰度分别增加了28.5%和114.3%。实验至第55天时，nirS和nirK基因的绝对丰度表现为低含量>中含量>高含量，表明As(III)浓度越高，反硝化过程的抑制作用越强。实验第20、55天时，处理组土壤中硝氮含量均明显高于对照组，且低含量组硝氮含量远高于中、高含量组，aioA基因绝对丰度在低含量组高于中、高含量并显著高于对照组，表明由于As(III)对反硝化过程的抑制导致的硝氮的增加，驱动As(III)向As(V)的转化。研究结果为认知水稻土中As(III)对氮素行为的影响，以及通过氮砷互作驱动As的低毒化调控提供了新视角。

关键词: 砷浓度, 水稻土, 氮素转化, 功能基因, 氮砷互作