关键词: 稻田土壤, 干湿交替, 羟基自由基, 污染物降解, 农业改良剂, 铁循环

Abstract:

Hydroxyl radicals(∙OH), as crucial oxidants, are widely present in natural environments such as the atmosphere, water bodies, and soil. Previous studies have indicated that ∙OH in natural environments mainly originates from the photolysis of nitrates and nitrites as well as the photochemical reactions of soluble organic compounds. In recent years, with deeper investigations into ∙OH, it has been discovered that substantial amounts of ∙OH can also be generated in the soil under dark conditions, primarily stemming from the oxidation reactions of reducible substances within it. During the growth cycle of rice, owing to its unique planting characteristics, paddy soil remains in a state of frequent wet-dry alternation for prolonged periods, providing optimal conditions for intense redox reactions in the soil and facilitating ∙OH formation. Consequently, the study of ∙OH in paddy soils as an essential component of soil has attracted considerable attention from researchers. This review systematically summarizes the research progress and trends regarding ∙OH in paddy soils, analyzes the generation process and mechanism of ∙OH in paddy soils, and explores the effects of exogenous substances (such as organic fertilizers) and different agricultural measures on the formation of ∙OH based on these findings. In addition, it elaborates on the environmental effects of ∙OH in paddy soils, including its contributions to element cycling in soil, its transformation effects on organic pesticide contaminants (such as imidacloprid and polycyclic aromatic hydrocarbons) and heavy metals (such as cadmium and arsenic) in paddy fields, and its impact on crop growth. The findings of this study revealed that the main generation mechanism of ∙OH in paddy soil is similar to the Fenton reaction, in which ferrous ions and oxygen play decisive roles, with free ferrous ions and low-crystalline ferrous ions in the soil being crucial forms of iron during the reaction process. Additionally, the addition of exogenous organic matter enhances the abundance and activity of iron-reducing bacteria, further increasing the content of active iron species. Moreover, ∙OH in paddy soil can accelerate the release of organic carbon and exhibit strong oxidation and degradation effects on heavy metals and certain organic pesticides in the soil, while also being generated and protecting crops from pollutant damage in the microoxic zones of crop roots. Finally, future research on ∙OH in paddy soils should focus primarily on the toxicity of pollutant degradation products, their ecological risks, and the effects of soil active components on the emission of greenhouse gases, such as methane, carbon dioxide, and nitrogen oxides in paddy fields. This review provides insights into the research and application directions of ∙OH generation and its environmental effects in natural soils.

Key words: paddy soil, wet-dry alternation, hydroxyl radicals, pollutant degradation, agricultural amendments, iron cycling