| Soil is the largest pool for bioactive carbon(C)storage on Earth,playing an extremely important role in global climate change.Thus,investigations of the responses of soil organic C(SOC)to climate change and high-intensity human activities are of great importance.Organic amendment is an important fertilization regime to increase SOC content and soil fertility.Organic decomposition regulates the proportion of derived C that is either retained in the system as SOC by spatial isolation of soil aggregates or surface bonding of minerals,or lost as CO2 after the degradation and utilization of microorganisms.Soils amended with organics create extremely high temporal and spatial variability of greenhouse gas(GHG)fluxes,making it difficult to accurately predict decomposition of SOC and GHG emissions.In general,this temporal and spatial variability of GHG fluxes occurs at very small volumes(<1 cm3),so-called "hotspots",and persists for very short periods(<2 weeks).Therefore,a better comprehending of soil biogeochemical processes within soil hotspots is crucial for understanding the mechanism of soil C sequestration and response to global climate change.In this paper,to explore the effects of microorganism-mineral interaction mechanisms on GHG emissions and the formation of organo-mineral complexes in soil hotspots,the Fe redox cycle and reactive oxygen species(ROS)as well as their effects on soil C emission were studied through a series of well-controlled soil microcosm experiments,combining with the combination of synchrotron radiation based fourier transform infrared(SR-FTIR)spectroscopy,?-X-ray fluorescence(?-XRF),near-edge X-ray adsorption fine structure spectra(NEXAFS),X-ray photoelectron spectroscopy(XPS),gas chromatography(GC),high performance liquid chromatography(HPLC),ferrous oxidation in xylenol orange(FOX)analysis and other multi-discipline analysis methods.Meanwile,to further verify the interaction mechanisms of microorganism-mineral,the interface processes between soil minerals and microorganisms were studied through the incubation experiments of model bacteria(Pseudomonas brassicacearum J12)and different minerals(kaolinite,montmorillonite,hematite,goethite and ferrihydrite),the ability of the production of ROS by mineral catalysis and its effects on bacteria were investigated.The main results are as follows:(1)CO2 emission from the microcosm experiments were enhanced by straw amendment and high moisture content,following the order:WHC(water holding capacity)-90%+S>WHC-45%+S>WHC-90%>WHC-45%.Compared to soils without straw,straw amendment increased CO2 emission during the first 14 days by 4.9-16.6 times and 4.5-13.4 times for 45%and 90%moisture levels,respectively.The highest CO2 efflux was over 22 mg C kg-1 soil d-1 during 10-14 days,which occurred at 90%moisture level with straw addition(WHC-90%+S).Afterwards,CO2 efflux under straw addition sharply declined and almost reached zero at 42 days.(2)DOC,Fe2+,H2O2 and HO' showed consistent patterns of being highest at the straw-soil interface(distance from straw 0-3 mm).A significantly positive correlation existed between DOC and Fe2+(r2=0.98,p<0.001),H2O2(r2=0.96,p<0.001)and HO·(r2=0.82,p<0.01),suggesting that all of Fe2+,H2O2,and HO· contribute to the bioavailability of OC in soils.XPS fitting results from the Fe 2p3/2 peak demonstrated that Fe2+ was generated on the surface of soil particles by straw amendment,and that HO· was subsequently generated by Fe2+ reacting with H2O2 through a Fenton reaction(Fe2+ +H2O2+H+?Fe3++H2O+HO·).The concentrations of Fe2+,H2O2,and HO· at the layer of 0-3 mm(distance from straw)in WHC-90%+S treatment were 73.6%,25.8%and 121%higher than in WHC-45%+S treatment.CO2 emission was significantly(p<0.05)increased when Fe2+and H2O2 were added together or H2O2 was added alone in the incubation experiments,whereas Fe2+ amendment alone did not increase CO2 emission through the experiment of the effects of Fenton reagents on CO2 emission.Moreover,a substantial increase in CO2 emission was only observed in the Bulk soil+S treatment,relative to the other treatments,when H2O2 concentrations were low;a marked increase of CO2 emission was found when the added H2O2 concentration was high.(3)SR-FTIR mapping showed a heterogeneous distribution between mineral-OH(3625 cm-1)and organic functional groups at 90%and 45%moisture levels.Chemical imaging of straw/soil interfacial regions showed a shift in functional groups from mineral-OH(3698 and 3625 cm-1)in the bulk soil to organic-OH(3418 cm-1)in the straw.Meanwhile,both mineral-OH and organic-OH co-existed at the straw/soil interface.Notably,the straw/soil interface was approximately 3 times wider at high moisture(?50 pm)than at low moisture(?15 ?m)levels.The results from ?-FTIR spectra showed that the intensity of some peaks,such as organic-OH(3418 cm-1),aliphatic-C(2922 cm-1),carboxylic-C(1721 cm-1)and aromatic-C(1645 cm-1)at the interface soil markedly increased.The above results reveal that this distribution pattern of functional groups at the straw/soil interface is distinct from that of the bulk soil.By semi-quantitative analysis of organic functional groups in soil hotspots,the results showed that organic-OH(3418 cm-1),aliphatic-C(2922 cm-1)and aromatic-C(1645 cm-1)in rhizosphere were 2.22-3.86,5.33-7.8 and 5.24-6.41 times higher than in bulk soil,and in detritusphere were 4.34-5.88,4.75-7.25 and 1.58-2.06 times higher than in bulk soil,respectively.(4)The results of C 1s NEXAFS analysis showed that straw amendments at 90%moisture level significantly(p<0.05)increased the percentage of aromatic C and decreased the oxidation state of OC in soil pore water.Aromatic compounds are often associated with SRO minerlas,indicating that a considerable proportion of straw-derived bioavailable C may be retained by reactive Fe-containing mineral particles.The results of incubation experiment of model bacteria and minerals showed that,compared to no mieral(Control)treatment,the presence of montmorillonite significantly increased OD600(p<0.05).Presence of all other investigated minerals decreased OD600 in the following order:ferrihydrite>goethite>hematite>kaolinite at 5 and 25 mg mL-1,respectively,and ferrihydrite>goethite>kaolinite>hematite at 10 mg mL-1.An increase in mineral concentration resulted in a significant decrease in OD600(p<0.05).This phenomenon showed that minerals(especially active minerals)can inhibit the activity of heterotrophic bacteria that protecting OC from decomposition and utilization by microorganisms,thereby increasing the potential of carbon storage.(5)Through the detection of HO· in the bacteria-mineral culture system,the results showed that the generation of HO· radicals in the cases of montmorillonite,kaolinite,and hematite was similar to the control(p>0.05).However,goethite and ferrihydrite significantly increased the production of HO' radicals,which increased with concentrations.The order of Fe(II)and soluble Fe which were released from Fe(?)-containing minerals was as follows:ferrihydrite>>goethite>hematite.A positive correlation existed between HO' with soluble Fe and Fe(?)content(p<0.01).However,a significant but negative correlation between OD600 with soluble Fe and Fe(?)was found(p<0.05).A high concentration of soluble Al was released in the montmorillonite and kaolinite solutions.However,almost no correlation was found between soluble Al with HO· and OD600.(6)The ?-XRF spectromicroscopy showed a distinct density of Fe distributed on iron particles.The linear combination fitting(LCF)results from Fe K-edge ?-XANES spectra indicated that ferrihydrite was dominant(?82%),with a lesser percentage(?17%)of FeC2O4 among the mineral particles.However,considerable percentages of hematite(?13%),goethite(?19%)and FeC2O4(?25.9%)were present on the edge of these mineral particles.The results of XPS analysis showed that the peak of Fe(?)was bigger in the F+bacteria treatment than that in F-bacteria treatment,suggesting that surface Fe(?)was produced from the reduction of Fe(?)on the surface of Fe(?)-containing minerals,promoting the production of extracellular HO' through the Fenton or Fenton-like reactions.For SR-FTIR spectromicroscopy,correlation analysis confirmed obvious linear correlations(p<0.01)between ferrihydrite(Fe-OH,3344 cm-1)and lipid(C-H,2921 cm-1),amide I(C=0,1632 cm-1),amide II(C-N,1513 cm-1)and polysaccharides(C-OH,1030 cm-1).In summary,a novel mechanism driving CO2 emission and the production of organo-mineral complexes with straw amendment is identified as microbial-initiated Fenton chemistry,which mainly occurs within hotspots(i.e.,at the layers of 0-3 mm distance from straw).Thus,the interation mechanisms of microorganism-mineral produce a burst of hydroxyl radicals that perfect explains extremely high variability of GHG fluxes in soil hotspots.Together,these findings indicate that the free-radical mechanisms that may serve as an ubiquitous mechanism between iron minerals and all of heterotrophic bacteria,which may play an important role in soil C storage and global climate change. |
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