初始pH值对碱性和酸性水稻土微生物铁还原过程的影响

Abstract

酸碱度（pH值）是水稻土铁还原过程的重要影响因素之一。通过模拟水稻土淹水厌氧培养，以Al₂（SO₄）₃和Na₂CO₃溶液分别调节碱性和酸性水稻土pH值至强酸性（pH值<5.0）、酸性（pH值5.0-6.5）、中性（pH值6.5-7.5）、碱性（pH值7.5-8.5）、强碱性（pH值 > 8.5），以此来研究5种初始pH值对水稻土泥浆铁还原过程的影响；通过微生物群落厌氧培养研究了2种水稻土菌悬液在6种pH值条件下的铁还原能力差异。结果表明，碱性水稻土铁还原潜势（<em>a</em>）、最大铁还原速率（<em>V</em>_max）随初始pH值的降低而下降，而达到最大铁还原速率所需的时间（<em>T_V</em>max）则延长。提高酸性水稻土初始pH值使铁还原<em>V</em>_max增加而<em>T_V</em>max缩短，但土壤中无定形氧化铁均能还原，初始pH值与<em>V</em>_max具有显著正相关关系。碱性和酸性水稻土的土壤菌悬液在试验pH值范围内厌氧培养，其铁还原能力在培养初期差异不显著，但培养后期的差异明显，且最终都能把培养液中氧化铁完全还原。随着初始pH值升高<em>T_V</em>max延长，<em>V</em>_max则降低，且均显著负相关，但碱性水稻土微生物群落的<em>V</em>_max在pH值6.00时最大。初始pH值和土壤类型对水稻土铁还原过程具有显著影响，且对土壤菌悬液微生物群的铁还原具有一定影响。;The pH value is one of the most important factors affecting iron reduction in paddy soils. The objective of this study was to determine how changes in initial pH affected iron reduction in paddy soils from Jilin (JL) and Jiangxi (JX) provinces. Anaerobic incubations were conducted using (1) soil slurries and (2) mixed microbial cultures that had been isolated from each soil type. The pH value of the alkaline JL samples was reduced with Al₂(SO₄)₃. The pH value of the acidic JX samples was increased using Na₂CO₃. The pH values of the soil slurries were strongly acidic (pH < 5.0), acidic (pH 5.0-6.5), neutral (pH 6.5-7.5), alkaline (pH 7.5-8.5), and strongly alkaline (pH > 8.5). The mixed microbial cultures were incubated in medium with pH ranging between 4.84-10.51. The Fe(Ⅱ) concentration and pH value of the samples were measured at regular intervals. A logistic model was used to analyze the characteristics of Fe(Ⅲ) reduction. The results showed that iron reduction potential and the maximum rate of iron reduction decreased as the initial pH value decreased. The time to reach the maximum rate of iron reduction increased as the initial pH value of the JL soil slurry declined. The maximum rate of iron reduction in the JL slurry decreased by nearly 50% as the pH value decreased, declining from (0.05±0.00) mg·g^-1·d^-1 at pH 10.51 to (0.02±0.00) mg·g^-1·d^-1 at pH 7.00. All amorphous iron oxides were reduced in the JX paddy soils, although the maximum rate of iron reduction increased and the time to reach the maximum rate of iron reduction decreased as the initial pH increased. The maximum rate of iron reduction in the JX soil was highest ((0.46±0.04) mg·g^-1·d^-1) and the time to reach the maximum rate of iron reduction was shortest ((3.65±0.21) d) at pH 8.19. There was a highly significant positive correlation between the maximum rate of iron reduction and the initial pH value of both soils (JL <em>r</em>=0.897, <em>P</em>=0.000; JX <em>r</em>=0.903, <em>P</em>=0.000). Mixed microbial cultures from both the JL and JX soil reduced all ferric iron, regardless of the initial pH value. It finally indicated that the iron reducing ability of microbial communities from both paddy soils were similar during the early stages of incubation but became different during the later stages. This observation provided support for the idea that facultative iron-reducing bacteria contribute to iron reduction in paddy soils. In the mixed cultures, the initial pH value increased. This was especially evident in the JX soil where the time to reach the maximum rate of iron reduction increased nearly six-fold, from (7.49±0.03) d at pH 4.84 to (45.2±0.3) d at pH 10.51. The maximum rate of iron reduction by the JL microbial community was negatively correlated with the initial pH value (<em>r</em>=-0.838, <em>P</em>=0.000). The maximum rate of iron reduction was highest at pH 6.00. The maximum rate of iron reduction by the JX microbial communities was highest at pH 4.84. There was a significantly negative correlation between the maximum rate of iron reduction and the initial pH value of the JX microbial community (<em>r</em>=-0.913, <em>P</em>=0.000). In conclusion, the effect of initial pH and soil type on iron reduction was significant in paddy soils. The initial pH has a certain influence on microbial communities and soil components.