水淹条件下水稻土中砷的生物化学行为研究进展

水稻土中砷的氧化还原和甲基化等生物化学过程是影响水稻砷毒性的主要作用机制;同时,水淹厌氧条件是驱动水稻土中砷的生物化学过程关键环节,且是导致水稻对砷大量吸收累积的主要原因,对以水稻为主食的人们造成健康威胁。本文综述了水稻土中砷的氧化还原和甲基化现象、砷的生物化学作用机制以及影响水稻土砷迁移转化的关键因素,并探讨了水淹厌氧条件对水稻土砷代谢微生物群落、微生物基因表达水平以及对砷归趋的影响。最后,展望了未来的研究方向,以期识别不同水管理模式下土壤微环境对水稻土中砷代谢微生物群落结构与基因表达水平的影响机制,为深入理解砷的生物化学行为和降低水稻对砷的吸收累积提供科学的理论参考。

Advancement in study on Biochemical Behavior of Arsenic in flooded Paddy Soil

Oxidation, reduction and methylation of arsenic in paddy soil are the key factors regulating transportation, transformation, and crop uptake of the element. Flooding is a common farming practice in rice cultivation, forming an anaerobic environment in the paddy soil, which not only affects the biochemical behavior of arsenic significantly, but also is often associated with enhanced uptake of arsenic by rice, thus further posing a health risk to those who consume rice as staple food. Studies in the previous focused mainly on those behaviors of soil arsenic in flooded anaerobic paddy soil and their relevant mechanisms, but a comprehensive review of the studies is yet to be prepared. In this study, the biochemical behaviors of arsenic in paddy soil is summarized, and their relevant mechanisms and influential factors, including iron oxides, organic matter, redox potential (Eh) and pH are discussed. Besides, the paper also elaborates discussed how the anaerobic condition in the flooded paddy field during the paddy rice growing season affects those biochemical behaviors. Generally speaking, the iron and arsenic reducing microbes in the soil are mainly anaerobic microbes, e.g. Geobacter, Shewanella and Myxobacter, while the iron and arsenic oxidizing microbes are predominantly aerobic microbes. Therefore, the development of an anaerobic reducing condition in flooded paddy fields favors microbial iron and arsenic reduction, and what is more, as iron oxides are the most effective scavenger of arsenic in paddy soil, the flooded anaerobic environment also favors release of arsenic. It is noteworthy that arsenic desorbed from iron oxides is more prone to bioreduction. Studies in the past indicate that adsorption of arsenic by iron oxides like ferrihydrite, goethite and hematite, especially ferrihydrite, the most abundant amorphous iron oxide in paddy soil, retards bioreduction of arsenic. Another contributor to enhanced bioreduction and release of arsenic is organic matter, which serves as nutritional substance and electron donor for microbes in metabolism. In flooded anaerobic paddy soil, the addition of extraneous organic matter facilitates formation of a reducing environment, stimulates reductive iron dissolution, arsenic reduction and arsenic release in rate and extent. Besides, flooded anaerobic paddy soil is also favorable to arsenic methylation, which uses arsenite as potenital inorganic substrate. Although flooded anaerobic paddy soil is not good to microbial arsenic oxidation, anaerobic arsenic oxidation processes mediated by microbes harboring arxA gene in paddy soil was reported in studies in the past. In terms of genes in microbes responsible for arsenic metabolism, current researches focus mainly on the following ones: arxA, arsenic respiratory reduction gene; arsC, arsenic detoxification reduction gene; arxA, arsenic oxidation gene; arxA, anaerobic arsenic oxidation gene; and arsM, arsenic methylation gene. In the past studies, gene arsC was found in close relationship with arsM, which is related to the response of the microbes harboring these genes to the stress of arsenic toxicity. By studying changes in abundance, diversity and gene expression of the microbial community in flooded paddy soil, a clearer picture can then be plotted of the biochemical behavior of soil arsenic in paddy soil as affected changes in environment. At the end, the paper describes prospects of the research and holds that the researches may serve as references for prevention of arsenic contamination in paddy soil and for alleviation of uptake and accumulation of arsenic by rice. For future researches the following aspects should be covered: (1) effects of organic matter, relative to type, on diversity of arsenic metabolising microbes that are capable of mediating dissimilatory iron reduction, arsenic reduction and methylation, direct physciochemical interaction between organic matter and arsenic, and ternary interaction of organic matter-iron mineral-arsenic as affected by chelation, competition and coupling; (2) Response of arsenic metabolism related enzymes to variation of micro-environment and its relationship with arsenic transportation and transformation, and relationship between organic matter and arsenic methyltransferase in the microbes; (3) Influence of carbon and nitrogen recycling, particularly Feammox, on biochemical behaviors of iron and arsenic, and influences of nitrogen-iron recycling and carbon-iron recycling on arsenic redox, e.g. influences of the competition between dissimilatory iron reduction and Feammox on arsenic dynamics; (4) Systems research on dynamics of the microbial community involved in arsenic metabolism in rhizospheric soil and bulk soil and biochemical behaviors of arsenic at the soil interface and soil-solution interface in paddy fields subjected flooding and draining, long term flooding or sprinkler irrigation.