Preliminary Study On Carbon Storage Mediated By Fenton-like Reactions In Long-term Fertilized Soil

In China,the cultivated land in red soil area accounts for 30%of the whole country.However,soil degradation is particularly serious,owing to long-term unsuitable utilization of red soil.Soil organic carbon(SOC)content in the agricultural ecosystem shows a trend of gradual decline due to long-term and excessive application of chemical fertilizers.According to the latest soil census data,approximately 68%of the farmland in the red soil area of China belongs to low and medium yield fields,and the cultivated land is generally short of organic matter.Furthermore,the decrease of SOC content further aggravates the decline of soil fertility.Therefore,the serious degradation of red soil fertility has become a great challenge to the whole world.Although Chinese red soil researchers have done a lot of effective work on improvement of SOC,there is still a lack of understanding in improving SOC and soil fertility of red soil.This study assessed the impacts of long-term fertilization treatments on Fenton reaction and soil C storage using a long-term fertilization experiment,locating at Qiyang city,Hunan Province.Several chemical selective extraction methods,high performance liquid/gas chromatograph,high-throughput sequencing,synchrotron radiation based technologies,X-ray photoelectron spectroscopy(XPS)analysis were combined to characterize reactive iron,soil hydrogen peroxide and hydroxyl radicals,microbial communities,iron coordination and species,respectively.Further experiments were conducted to conduct in-situ characterization of soil colloid through NanoSIMS,and visual evidence of Fenton-or Fenton-like reactions in soil and the interaction between iron and C.Finally,arbuscular mycorrhiza and plant roots were added in different soil texture types to explore the influences of different soil textures,microorganisms and roots on Fenton-or Fenton-like reactions in soils.The main research results are as follows:(1)The contents of H2O2,HO· and Fe(Ⅱ)in red soil with long-term application of chemical fertilizers were significantly higher than those with long-term application of organic fertilizers.Also,root exudates and microorganisms could regulate the contents of H2O2,HO·and Fe(Ⅱ),with a linear relationship between H2O2 and HO·.Red soil with long-term application of organic fertilizers had better H2O2 buffering performance than that with long-term application of chemical fertilizers,owing to soil degradation(i.e.,low soil organic matter content).Compared with red soil with long-term application of chemical fertilizers,the amount of CO2 released with long-term application of organic fertilizers was higher in soils with 1,000 m pore diameter and 20 μm pore diameter but no significant difference with 0.45μm pore diameter.(2)The richness and biodiversity indices in red soil with long-term application of organic fertilizer were significantly higher than those with long-term application of chemical fertilizer.The presence of plant root and microorganisms had a minor effect on the structure of bacterial communities in red soil with long-term application of organic fertilizers but a major effect on those with long-term application of chemical fertilizers.Bacterial communities with long-term application of organic fertilizers affected soil pH,DOC,soluble Fe and CO2 emmision.However,bacterial communities with long-term application of chemical fertilizers may trigger Fenton reaction in soil by producing both H2O2 and reduced Fe(Ⅱ).The distribution patterns of clay minerals(3 619 cm-1),aliphatic C(2 914 cm-1),carboxylic C(1725 cm-1)and hydroxyl C(1 135 cm-1)at the soil-root interface were significantly different.The presence of roots increased the percentage of Fe(Ⅱ)at the soil-root interface.(3)The spatial distribution of 12C-,12C14N-and 16O-,in soil treated with long-term organic fertilizer and soil treated with long-term chemical fertilizer,is heterogeneous.Among them,16O-had the highest intensity.Compared with soil treated with organic fertilizers,the intensity of 56Fe16O-in soil treated with chemical fertilizers was high.As for carbon-rich region of interests(ROIs),the relationship of treated by M and NPK,the median lines of 56Fe16O-/12C14N-and 56Fe16O-/12C-ratios were slightly higher in soils with long-term applications of organic fertilizers than that with long-term applications of chemical fertilizers.Compared with the chemical fertilizer treatment,the ratio of 56Fe16O-to 12C-in soil with long-term applications of organic fertilizers was higher than that with long-term applications of chemical fertilizers,suggesting that Fenton-like reactions increased the mobility of iron and C in soils with long-term applications of organic fertilizers and thus changed the ratio of 56Fe16O-to 11C-.In soil with long-term applications of organic fertilizers,the presence of microorganisms and roots could strengthen the correlation of iron and organic C,indicating the occurrence of microbial mediated Fenton-like reactions.In contrast,in soil with long-term applications of chemical fertilizers,the correlation of iron and C was strong in soil without the presence of microorganisms and roots,suggesting the occurrence of Fenton reaction rather than microbial mediated Fenton-like reactions in soil.(4)There was no significant difference for soil pH values among sandy soil,sandy-clay soil and clay soil.The presence of roots+AM Fungi could significantly improve soil pH values.As for soil total C,the C content of clay soil was much higher than that of the other two soils.The presence of roots+AM Fungi significantly increased soil total C content.Sandy soil had the lowest soil total nitrogen content.Among the sandy soil,compared with the control group(-root-AMF),soil treated with only AM Fungi significantly reduced the total soil nitrogen,while soil treated with AMF and root system significantly increased the total soil nitrogen.Clay type hydrogen peroxide(H2O2)content in soil>sand>sand clay,including the type of clay soil,AM Fungi+root treatment can improve the effect of the content of hydrogen peroxide soil but not significant,while in types of soil and sandy clay sandy soil type can significantly improve the hydrogen peroxide content in soil,soil type soil especially;The content of ferric iron[Fe(Ⅱ)]in soil of clay type was>.The content of ferric iron[Fe(Ⅱ)]in soil of sand type was>.Soil soluble iron content>sand soil type>clay type,relative to AMF treatment,AMF+root system treatment can significantly improve soil soluble iron content.Soil soluble organic carbon of clay type was significantly higher than that of sandy soil type and sandy soil type,while the difference between sandy soil type and sandy soil clay type soil was not significant AM Fungi+root system treatment could significantly improve soil soluble carbon content.In summary,a conceptual model was proposed to effectively increase the fertility of subtropical red soil.Specifically,the application of organic materials(such as livestock and poultry manures,straws,and organic fertilizers)can stimulate microorganisms and the availability of iron minerals in red soil,promote Fenton-like reactions,and then form both intramolecular covalent bonds and mineral-organic complexes,which increase the stabilization and accumulation of organic C in red soil.Long-term different fertilization treatments markedly affected Fenton-like reactions in soil.Organic fertilization could priming microorganisms and thus microbial mediated Fenton-like reactions.In addition,the presence of root and microorganisms could also promote Fenton-like reactions,which are benefit to soil C storage.This study provides novel insights for improving soil fertility.