Effects Of Iron-manganese Oxides Photocatalysis On Greenhouse Gas Emissions From Paddy Soils Mediated By Microorganisms

Rice cultivation is an important source of agricultural greenhouse gas emissions,it is accounting for about 8%of anthropogenic methane emissions.Greenhouse gas emission mitigation in paddy soils is the core of carbon sequestration and emission reduction in agricultural and rural areas.It has been shown that the iron and manganese oxide semiconductor minerals rich in paddy soils,which can produce electron-hole pairs with redox effect under photocatalysis.we infer that these electron-hole pairs must participate in a series of redox that affect greenhouse gas emissions from rice paddies.However,there are few work about the effect of iron-manganese oxide semiconductors on greenhouse gas emissions from paddy soils.In order to reveal the mechanism of semiconductor minerals in paddy soil on greenhouse gas emission.The effects of iron-manganese oxide semiconductor minerals on greenhouse gas emissions from paddy soils were studied.The main content of this paper are including:the soil samples collected from five major rice-growing areas in China,and analyzed the relationship between iron-manganese oxide semiconductor minerals and greenhouse gas emissions;slags rich in iron-manganese oxide semiconductor minerals were addition to paddy soil and measured their greenhouse gas emission;selected key semiconductor minerals which can mitigation greenhouse gases emission from paddy soil;evaluated the effects of slag application on soil physical and chemical properties and soil health.The main results were as follows:（1）Relationship between iron-manganese oxide and carbon emissions from paddy soils.Five rice cropping areas in China were chosen for soil sampling.Semiconductor minerals were measured,and three main semiconductor minerals including hematile,rutile,and manganosite were identified in the paddy soils.The identified semiconductor minerals consisted of iron,manganese and titanium oxides.Content of Fe₂O₃,Ti O₂ and Mn O in the sampled soil was between 4.21-14%,0.91-2.72%and 0.02-0.22%,respectively.Single-cropped rice cropping area in northeastern China of Jilin province（DBDJ）have the highest content of Fe₂O₃ of 14%.Soils from the five main rice cropping areas were also identified as having strong photoelectric response characteristics.The highest photoelectric response was found in the DBDJ rice cropping area in Jilin province with a maximum photocurrent density of 0.48μA/cm².Soil respiration was monitored under both dark and light conditions.Soil respiration rates in the five regions were（from highest to lowest）:DBDJ>XNDJ>XBDJ>HZSJ>HNSJ.Soil respiration was positively correlated with semiconductor mineral content,and soil respiration was higher under the light treatment than the dark treatment in every rice cropping area.This result suggested that soil respiration was stimulated by semiconductor mineral photocatalysis.This analysis provided indirect evidence of the effect semiconductor mineral photocatalysis has on the carbon cycle within paddy soils,while exploring carbon conversion mechanisms that could provide a new perspective on the soil carbon cycle.（3）Effect of iron-manganese oxide-rich slag on greenhouse gas emissions from paddy fields.Three kinds of slag（steel slag,manganese slag and titanium slag）were applied to paddy soil to monitor the greenhouse gas emission fluxes and cumulative greenhouse gas emission fluxes during the whole growth period of rice,the global warming potential of greenhouse gas emissions from rice fields was also calculated.The results showed that all the three kinds of slag addition can reduce methane emission flux and the cumulative methane emission.Compared with the control treatment,the application of manganese slag can reduce methane emission by 55%.There was no significant effect on carbon dioxide emission of different slag treatments,but the cumulative carbon dioxide emission of steel slag treatment was significantly lower than that of control treatment.The global warming potential of manganese slag treatment was 63%lower than that of control treatment.Manganese oxide as an electron acceptor and photocatalysis of manganese oxide may be the main reasons that lead to the reduction of greenhouse gas emissions.（4）Select key iron-manganese oxide semiconductor mineral for greenhouse gas reduction and analyzing their potential microbial mechanism.Effects of iron-manganese oxide on greenhouse gas emissions from paddy soils mediated by microorganisms.The effects of three kinds of iron oxides and three kinds of manganese oxides on the phase change,microbial community and structure,and methane emission in paddy soil were studied by field and experimental incubation.The results showed that slag addition increased the content of goethite in topsoil and decreased the abundance of acidobacteria.Goethite and lepidolite are the most abundant iron-manganese oxide semiconductor minerals in the soil.The incubation experiment shows that the addition of goethite and lepidolite can reduce methane emission under the condition of illumination.Compared with the control,the application of goethite under the light condition reduced the methane emission by 48%,and the addition of lepidolite and vernadite under the light condition also significantly reduced the methane emission of rice fields,with the reduction amounts of 36%and 38%.（5）Effects of iron-manganese oxide-rich slag on soil physical and chemical properties and rice yield.The three kinds of slags are not only rich in Si,Ca,Fe,Mg,p,N,but also rich in heavy metals.1.66 mg/kg of cadmium and 14.16 mg/kg of lead in manganese slag significantly higher than steel slag and titanium slag.The contents of Cr and Zn in steel slag were 2717 mg/kg and 3462 mg/kg respectively,which were significantly higher than those in titanium slag and manganese slag.The effective manganese content in manganese slag is up to 15084 mg/kg,and the effective zinc content in steel slag is up to 1985 mg/kg.Slag addition changed the physical and chemical properties of paddy soil.Compared with the control,the soil p H were slightly increased by three different slag treatments.The contents of total nitrogen,total phosphorus and organic matter increased by 12%,10%and 9%,respectively.The content of organic matter in steel slag treatment was the highest and significantly higher than that in manganese slag treatment.After two years of slag application,it was found that slag application did not pose an increased risk to soil cadmium and lead,and even resulted in a significant reduction in total cadmium and lead levels after manganese slag application.The addition of three kinds of slag significantly reduced the total chromium content,and the steel slag addition reduced the total chromium content by 25%.The effective content of heavy metals increased significantly with the application years.But it is not higher than the risk assessment standard of agricultural land soil pollution.The results showed that different slags addition did not significantly increase heavy metals in paddy soil.In addition,the slag is rich in silicon,phosphorus and other nutrients,these elements improve the soil nutritional status,and finally increased rice yield.To sum up,this study found that there are a large number of semiconductor minerals in paddy soil,which not only stimulate soil respiration,but also have methane mitigation effects when amendment manganese containing semiconductor minerals.In addition,adding slag rich in iron and manganese oxides did not cause heavy metal pollution in the paddy soil while reducing methane emissions,and had a certain improvement effect on rice yield.