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

摘要：

羟基自由基（∙OH）作为重要的氧化剂普遍存在于大气、水体及土壤等自然环境中。过去的研究表明自然环境中的∙OH主要来源于硝酸盐和亚硝酸盐的光解和可溶性有机物的光化学反应。近年对∙OH的深入研究，发现土壤暗环境也可产生大量∙OH，主要来源于其中还原性物质的氧化反应。稻田土壤由于作物的种植特性，经常处于干湿交替的状态，致使土壤中的氧化还原反应极其强烈，具有完备的∙OH形成条件。该文对稻田土壤∙OH的研究进展与趋势进行了系统总结，对稻田土壤中∙OH的产生过程和机制进行了剖析，并在此基础上探究外源物质（有机肥料等）的添加及不同农艺措施对∙OH形成过程的影响。同时，详述了稻田土壤∙OH的土壤环境效应，包括对土壤中元素循环的贡献，对稻田中有机农药污染物（吡虫啉和多环芳烃等）与重金属（镉和砷等）的转化效应及对农作物本身产生的影响。研究发现，稻田土壤∙OH的主要生成机理和芬顿（Fenton）反应类似，亚铁和氧气对反应起决定性的作用，而土壤中的游离态亚铁和低结晶态亚铁则是反应过程中最为关键的亚铁形态，添加的外源有机质会增强铁还原菌的丰度与活性，并进一步提升活性铁物种的含量。此外，稻田土壤∙OH能够加速土壤中有机碳的释放，对重金属及部分有机农药也有很强的氧化和降解作用。最后，该文认为未来对稻田土壤∙OH的研究重点应主要关注污染物降解产物的毒性及其生态风险和土壤活性组分对稻田甲烷、二氧化碳和氮氧化物等温室气体排放的影响。

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