旱作褐土中氧化铁的厌氧还原与光合型亚铁氧化特征

土壤中铁的氧还过程与碳氮转化及自净能力关系密切,已还原亚铁的氧化受土壤性质的影响。采用室内恒温培养试验研究了旱作褐土中铁还原氧化过程、及其与水溶性碳、NO3-、SO42-的关系。结果表明旱作褐土中铁氧化物在厌氧光照条件下可先被还原后被再次氧化,其再氧化量介于1.46~3.00 mg g-1之间,平均2.09 mg g-1;再氧化速率常数介于0.23~0.80 d-1之间,平均0.48 d-1。再氧化量与土壤无定形铁、水溶性硫酸盐含量、阳离子交换量显著负相关,与土壤总氮、总磷显著正相关;再氧化速率常数与土壤有机碳显著负相关,与黏粒含量极显著正相关。厌氧光照培养可使旱作褐土水溶性无机碳平均降低52.74%,水溶性NO3-降低92.15%,水溶性SO42-增加55.38%。研究结果为深入理解旱作土壤潜在的微生物铁循环转化方式提供理论支持。

Anaerobic redox of iron oxides and photosynthetic oxidation of ferrous iron in upland cinnamon soils

Iron redox cycle is an important microbial process of the degradation of organic matter in anaerobic soil and sediment environment, and it is closely related to transformation of carbon and nitrogen and soil self-purification capacity. As Fe(II) produced anaerobically by dissimilatory iron reduction process could be oxidized by nitrate-reducing/iron-oxidizing bacteria, anoxygenic photosynthetic bacteria or oxygen produced by oxygenic photosynthetic bacteria in wetland soil anaerobically. Furthermore, risk of emission of greenhouse gases, such as CO2 and biological availability of heavy metals decreases in the process at the same time. Whether oxidation of ferrous generated by dissimilatory iron reduction would occurs or not depends on soil properties, and ferrous oxidation characteristics, including capacity and rate constant are governed by soil pH and contents of water, nitrate, inorganic carbon and other co-substrates. Though upland cinnamon soils belong to oxisol, dissimilatory iron reduction of iron oxide may occur in the semi-luvisol cinnamon soils with adequate water content under non-illuminated incubation as the preliminary study shows, whether the Fe(II) produced anaerobically under non illuminated incubation could be re-oxidized in upland cinnamon soils under illumination incubation is not still clear and neither, are characteristics of the re-oxidation in these soils, not to mention relationship between oxidation characteristics and soil properties. To learn further in-depth about potential microbial iron redox cycle in upland soils, oxidation of ferrous generated by iron reduction, and relationship between ferrous oxidation in upland soils under illuminated incubation and soil properties, soil samples collected from upland farmlands at 7 different locations, and form nearby paddy fields, in which ferrous oxidation occurs under illumination anaerobically, were both prepared into slurry and then put under illuminated anaerobic incubation, to study characteristics of the anaerobic redox of iron oxide in these soils and its influence on contents of water soluble carbon, nitrate and sulfate. Results of the incubation show that iron oxide in upland cinnamon soils could be reduced first and then re-oxidized under light. The amount of iron oxide re-oxidized in these soils ranged from 1.46 to 3.00 mg g-1, with an average of 2.09 mg g-1. And the re-oxidation rate constant ranged from 0.23 to 0.80 d-1, with an average of 0.48 d-1. The amount of re-oxidation is closely and negatively related to the contents of amorphous iron, water soluble sulphate and cation exchange capacity, and closely but positively related to the contents of total nitrogen and total phosphors, while re-oxidation rate constant is significantly and negatively related to the content of soil organic carbon, but positively to the content of clay. These findings suggest that anaerobic oxidation of ferrous in upland cinnamon soils under illumination is mainly caused by oxygen generated by oxygenic photosynthetic bacteria, and may decrease the content of water soluble inorganic carbon by 52.74%, and the content of water soluble nitrate by 92.15%, but may increase the content of water soluble sulfate by 55.38% in the soil under illuminated anaerobic incubation. The findings may help understand further in-depth the potential microbial iron redox cycle in upland cinnamon soils.