Abstract:

The mobility and bioavailability of arsenic (As) significantly increases in soils under flooded conditions, which facilitates its uptake by rice. However, the effects of As transformation on paddy soils during flooding remain poorly understood. In this study, low-arsenic (15.5 mg·kg^-1) and high-arsenic (52.1 mg·kg^-1) paddy soils were selected for soil incubation experiments with and without rice plants under flooded conditions. Soil pore water and rhizosphere soil samples were collected on days 7, 14, 28, 42, and 56 during the flooded incubation period and various parts of the rice plants were sampled on day 110 at maturity. Dynamic changes in different arsenic species, soil arsenic fractions, ferrous ions, and molecular components of dissolved organic matter in soils were analyzed to elucidate the mechanism of rice in arsenic transformation in flooded soils. Compared to the treatment without rice, the treatment with rice showed lower proportions of available arsenic and amorphous iron-aluminum oxide-bound arsenic fractions in soils, but higher humic acid-bound and fulvic acid-bound arsenic fractions during the flooded period. The presence of rice decreased the concentrations of dissolved Fe(II) in pore water from 43.6‒51.3 mg·L^-1 to 11.9‒19.4 mg·L^-1, and increased the concentrations of adsorbed Fe(II) on soils from 1.14‒1.21 g·kg^-1 to 3.73‒4.28 g·kg^-1 on day 28. Three-dimensional fluorescence spectroscopy analysis showed that the presence of rice significantly increased the fluorescence intensities of fulvic-like, humic-like, tyrosine-like, and tryptophan-like substances in the soil dissolved organic matter. In addition, these organic fractions were significantly positively correlated with the organic matter-bound arsenic fraction in soils (p≤0.05, p≤0.01), but significantly negatively correlated with the available arsenic fraction in soils (p≤0.01 or p≤0.001). Rice grown in high-arsenic soil had straighthead disease, which might be associated with its higher abundance of CHOS and protein/lipid compounds (16.4% and 12.0%, respectively) than in low-arsenic soil (12.8% and 6.62%), as revealed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis. These results indicate that rice cultivation can increase dissolved organic matter in the soil and form more organic matter-bound arsenic fractions, leading to a decrease in available arsenic in the soil, likely via rhizosphere response and root uptake. The finding that rice promotes As immobilization by regulating soil organic matter under flooded conditions provides new insights into the environmental effects of crops on As behavior in soils.

Key words: paddy soil, arsenic, anoxic, immobilization, organic matter

摘要：

淹水条件下土壤砷的移动性和生物有效性显著提高，易被水稻吸收；然而水稻种植过程对土壤砷转化的影响机制尚不明确。选取了含低砷（15.5 mg·kg^-1）和高砷（52.1 mg·kg^-1）两种稻田土壤，每种土壤在淹水条件下分别设置种植和不种植水稻两种处理，在淹水培养期间的第7、14、28、42、56天采集土壤孔隙水、根际土壤，并在水稻成熟时采集水稻植株各部位样品。分析了不同砷形态、土壤砷组分、亚铁浓度、土壤溶解性有机质分子组分等的动态变化，探究了水稻对淹水土壤砷转化的影响机制。结果表明，与不种植水稻处理相比，种植水稻降低了淹水期土壤有效态砷和无定型铁铝氧化物结合态砷的比例，但提高了土壤腐殖酸结合态和富里酸结合态砷的比例。在第28天时种植水稻使孔隙水中溶解态Fe(II)质量浓度从43.6－51.3 mg·L^-1降低至11.9－19.4 mg·L^-1，使土壤吸附态Fe(II)质量分数从1.14－1.21 g·kg^-1提高至3.73－4.28 g·kg^-1。三维荧光光谱分析表明，种植水稻显著提高了土壤溶解性有机质中类富里酸、类腐殖酸、类酪氨酸和类色氨酸等组分的丰度，且相关性分析结果表明，四者均与土壤有机质结合态砷呈显著正相关（p≤0.05或p≤0.01），但与土壤可利用态砷呈显著负相关（p≤0.01或p≤0.001）。此外，在高砷土壤种植的水稻呈现直穗病，这可能跟高砷土壤溶解性有机质中CHOS和蛋白质/脂质化合物的相对丰度（16.4%和12.0%）比低砷土壤（12.8%和6.62%）高有关。水稻植株可能通过根际响应和根系吸收等作用，降低土壤有效态砷，提高土壤溶解性有机质的含量，并使更多砷与土壤有机质结合，有利于砷的固定。

关键词: 稻田土壤, 砷（As）, 厌氧, 固定, 有机质