Effect Of Sulfate On Dissimilatory Iron Reduction In Paddy Soil

In the system of anaerobic paddy soil, there are several oxidation-reduction processes exist simultaneously, such as reduction of nitrate, sulfate-reducing and Fe(III) reduction. By adding valence-variable elements into the system, it can affect the redox potentials of the soil on one hand. On the other hand, it can change the ratio of electron donor and receptor in the soil. Accordingly the flow of electron transfer is affected. Therefore, the question whether there exists competition of electron donors or interaction of productions has become to be the hotspot in the remediation of soil pollution. Fe(III) and SO₄^2- are the primary electron receptors in the rice soil. They play an important role in the electron transfer of anaerobic process. According to the principle of thermodynamics, the redox potential reduced by sulfate is equal to that reduced by Fe(III) in the waterlogging soil. Therefore, it's commonly considered that the sulfate-reduction and Fe(III)-reduction can react simultaneously in the sediment. Whereas, mostly iron exists as the form of ferric oxide, especially the infusible ferric hydroxide (FHO) in the sediment. Affected by specific surface area and crystallization, the competition ability of Fe(III), as the electron receptor, is less than SO₄^2-. So, it's possible that sulfate can be a competitive electron receptor to affect iron-reduction. The change of iron-reduction state in the soil can further affect the validity of nutrients and contaminations. Therefore, for the purpose of correctly understanding the quality of soil and environment, it is significant on theory and practice to systematically develop the research of relationship of sulfate-reduction between iron-reduction competitions.In this study, the typical paddy soil was chosen as the materials. Anaerobic slurry culture, anaerobic microorganism community culture and microorganism pure culture were used to be the research methods. By adding SO₄^2- into the culture system, the study discussed the influence of the SO₄^2- as a competitive electron receptor to Fe(III)-reduction of different iron-reduction systems. It provided us the essential academic evidence for understanding the competition relationship of different electron receptors and the change mechanism of oxide-reduction process result from the effect of microorganism in the anaerobic soil. The study gave us some conclusions:1. In the anaerobic slurry culture of different type of paddy soil, the reduction speed of Fe(III) decreased with the concentration increasing of SO₄^2-. But the final cumulative production of Fe(II) was significant increased compared with the control. The influence of Fe(II) reduction speed by adding SO₄^2- is as: calcareous paddy soil > acidic paddy soil. The final cumulative increasing ratio is as: acidic paddy soil > calcareous paddy soil.2. The test of adding different concentration of lactate indicated that the more carbon source (electron donor) can be used by soil microorganism, the more significant effect of electron competition the SO₄^2- to dissimilatory Fe(III) reduction process.3. Adding SO₄^2- brought strong inhibition to the Fe(III) reduction process of different paddy soil microorganism community. The result was that the reduction process of Fe(OH)3 lagged. The more concentration of SO₄^2- was added, the more significant lagged was. Adding SO₄^2- can enhance the ability of Fe(III)-reduction, can significantly increase the reaction speed at the late stage, and can sustain the final cumulative production of Fe(II) at the same level compared with the control. This result showed that adding SO₄^2- can induce an obvious competition relation of electron and lead to the distributary of electron. As the further result, the Fe(III)-reduction reaction was lagged. At the same time, some small molecular organic acid produced by sulfate-reduction-microorganism in the metabolism process, can accelerate the reaction of Fe(III)-reduction. Furthermore, partial sulfate-reduction-bacteria can use Fe(III) as their electron donor to accelerate the Fe(III)-reduction.4. In the pure culture test of iron-reduction-bacteria, adding SO₄^2- bring no inhibition to the Fe(III)-reduction process of the four strains of iron-reduction-bacteria for test. The result indicated that the iron-reduction-bacteria them self were not influenced by SO₄^2-. The inhibition effect of Fe(III) from SO₄^2- was not directly reacted to the iron-reduction-bacteria, but it affected the process of ferric oxide reduction by the competition of electron flow of sulfate-reduction-bacteria.5. In the light culture test of soil slurry, the cumulative production of Fe(II) at the initial stage presented increasing trend. With the increase of cyanobacteria produced by illumination, the cumulative production of Fe(II) became to decrease. When the condition shifted to dark culture, the cumulative production of Fe(II) rapidly increased. It indicated that illumination can result in the change of iron-redox-balance stage in the soil.6. By examination and chlorophyll chromatogram scanning of green bacteria produced under the condition of light culture, it was found that both of the two paddy soil can produce photosynthetic oxygen-evolving cyanobacteria under this condition. The absorb wave length of feature-optical-light is 664 nm. The conclusion approved that the reason why the cumulative production of Fe(II) of paddy soil in the light system was low was resulted from the chemical oxidation of photosynthetic oxygen evolving reaction of cyanobacteria in photosynthetic system II.7. Adding SO₄^2- can significantly change the metabolism of photosynthetic bacteria, delay the process of Fe(II)-oxidation and the time delayed was increased by the increasing of SO₄^2- concentration. When the SO₄^2- was added, the decomposition of H2O of metabolism of photosynthetic bacteria was transferred to H2S. The release of O2 was decreased consequently. The result finally led the process of Fe(II)-oxidation to be weaken in the soil.