Mechanisms Of Mineral Transformation And Soil Organic Matter Sequestration In The Upland Red Earth Under Long-Term Different Fertilization Regimes

Soil organic carbon(SOC)is the largest pool of biologically-active carbon on earth,and thus its stability is a central interest of global carbon biogeochemistry.Meanwhile,SOC plays a key role in maintaining soil fertility to ensure agricultural production.The consensus view of mechanisms of SOC sequestration has changed greatly over the past decade.Informed by new techniques of in situ soil microscopy and mass spectrometry,a growing body of observations downplay the role that molecular structure plays in SOC stability,while support the paradigm explaining SOC persistence as one that is driven by ecosystem properties,especially physicochemical interactions,such as the co-precipitation of SOC with Fe/Al(hydr)oxides at nano-scale.Moreover,the chemical nature of the soil minerals has a significant impact in controlling SOC content.Poorly crystalline Fe is more effective than crystalline Fe in stabilizing SOC.More than 38%of the earth's land surface is consisted of agricultural land,and human activities strongly affect its SOC stock.For example,fertilization regime dramatically alters the physical,chemical and biological properties of the soil and has an effect of SOC composition and the molecular crystal structure of Fe/Al(hydr)oxides.Microbe-mediated Fe redox cycling triggers precipitation and dissolution of Fe minerals and affects the Fe mineralogy significantly.However,we still have a limited understanding of complex response of the Fe/Al(hydr)oxides and OC associations to different fertilization regimes,as wells as the role of the microbe-mediated Fe redox cycling played in this regulatory mechanism.In this paper,soils under long-term(23 years)treatment with chemical and/or organic fertilizers were collected to evaluate the effects of long-term fertilization regimes on the association of Fe/Al(hydr)oxides and organic matter in soils.Besides,we conducted an incubation experiment to examine the differences in the Fe redox bacterial community in contrasting fertilized soils and their influence on the transformation of ferrihydrite to assess the role of the microbe-mediated Fe redox cycling in different fertilized soils.Furthermore,an incubation experiment was designed to study the adsorption or coprecipitation of Fe-OC complexs under different fertilization treatments.The main results were listed as follows:(1)After 23 years of fertilization,the SOC ranged from 7.23 ± 0.11 g kg-1 to 15.01 ±0.47 g kg-1 in the different fertilization treatments,with the highest SOC in the M treatment,followed by NPKM,NPK,NPKCa,NCa,and N.The chemical fertilization treatments(i.e.,N and NPK)greatly decreased the soil pH,i.e.,approximately 1.85-2.59 and 1.43 units,when compared with organic manure fertilization(i.e.,M and NPKM)and no fertilization(i.e.,Control)treatments.The addition of lime(i.e.,NCa and NPKCa)increased the soil pH to the level of the organic fertilization treatments.The application of organic manures or lime could increase soil pH and effectively alleviate soil acidification.(2)Long-term(23 years)fertilization significantly affected the Al fractions in soils.Organic fertilization increased poorly crystalline Al but decreased exchangeable Al compared with chemical fertilization.The addition of lime significantly increased weakly organically bound A1 but decreased exchangeable Al.A higher poorly crystalline A1 level could be beneficial to the sequestration of soil C.In contrast,a higher exchangeable Al level could induce soil acidification.Poorly crystalline Al was present as allophane and imogolite in the soils under no-fertilization and organic fertilization but not under chemical fertilization.(3)The Fe fractions in soils were significantly affected by long-term fertilization regimes.Organic fertilizations(NPKM and M)increased the concentrations of Fe in soil colloids(Feco),ammonium oxalate extracted Fe(Feo)and poorly crystalline Fe(Feam),whereas chemical fertilization increased total Fe in soil(Fet),organically complexed Fe(Fep),CDB extracted Fe(Fed)and crystalline index(CI)(P<0.05).(4)The Scanning Transmission X-ray microscopy(STXM)images and PCA-CA analysis showed that the distribution of Fe varied in the NPK-and NPKM-treated soils.The Fe L-edge near edge X-ray absorption fine structure(NEXAFS)spectra from all of the samples showed multiple peaks at the Fe L3 edge,which are sensitive to the oxidation state and the coordination environment of Fe.Based on the energy differences(AeVs)and intensity ratio values in the Fe L3 edge,octahedral Fe phases were predominant in the NPKM treatment,while the mixed octahedral Fe and tetrahedral Fe(?)phases presented in the NPK treatment.Also,there were more reduced Fe phases observed inside the soil colloid particles than in the peripheral particles.Besides,the LCF analysis of the Fe K-edge XANES spectra goethite indicated that Goethite was dominant under all fertilization treatments,the remaining Fe phases were composed of poorly crystalline ferrihydrite species,the percentage of which was significantly higher in soils under NPKM fertilization than under NPK treatment.(5)Ferrihydrite could transform to well crystallized Fe minerals after boiling in 1 mol L-1 KOH for 4 h.The adding of DOM from the different fertilized soils had an inhibitory effect on the crystallization of ferrihydrite to varied degrees.The composition and content of organic C played important roles in this process.The DOM extracts from the NPKM treatment exhibited the strongest inhibitory effect,whereas the DOM extracts from the NPK treatment had the weakest effect.This was mainly due to that the DOM from the NPKM treatment contained larger concentration of aromatic C groups which was found to be preferentially absorbed onto reactive poorly crystalline Fe oxides.(6)Compared with no fertilization(Control),Fe redox cycling bacteria were present at a higher abundance and diversity in organically fertilized soils but at a lower abundance and diversity in chemically fertilized soil.During Fe reduction stage,Geobacter was the important active Fe-reducer,with a higher relative abundance in both organically and chemically fertilized soils than in Control.Greater consumption of ferrihydrite was observed in chemically fertilized soil than in organically fertilized soil,due to the higher relative abundance of Geobacter.However,during Fe(?)oxidation stage,Pseudomonas and Anaerolinea were more abundant,and produced higher levels of poorly crystalline Fe oxides under organic fertilization.(7)The C/Fe molar ratio and the Fe-OM associations caused by OM adsorption versus co-precipitation determines the properties of the Fe-OM complex.The maximum C loading was significantly higher in the co-precipitation than adsorption experiment.However,in the desorption experiment,the adsorption complex released more C to the solution.That is to say,the chemical stability of the co-precipitated OM against desorption is greater in comparison to the adsorbed OM.The DOM from the organic fertilized soils were more likely to co-precipitate with Fe minerals.While the Fe-OM associations were formed by adsorption in NPK-treated soils.In this way,the NPKM soils has greater potential for the SOC sequestration.Taken together,this paper linked fertilization regimes,Fe/Al(hydr)oxides-SOC complexes and the microbe-mediated Fe redox reactions in red earth.Increased concentrations of poorly crystalline Fe/Al(hydr)oxides were present in soils treated with organic fertilizers,whereas increased concentrations of crystalline Fe/Al(hydr)oxides were present in soil treated with NPK fertilizer,which,to a large extent,is a result of the shifts in the Fe redox bacterial community.Long-term chemical and organic fertilization regimes have distinct influences on the Fe redox microbial community and the corresponding Fe mineralogy in soils,which is closely related to soil physiochemical characteristics.Long-term NPK fertilization enhanced the Fe reducing bacteria and promoted Fe reduction in the incubation experiment,resulting in increased consumption of ferrihydrite.Meanwhile,soils under long-term organic fertilization contain more Fe(II)oxidisers,which produce relatively higher concentrations of poorly crystalline Fe oxides.Larger amount of poorly crystalline Fe/Al(hydr)oxides control the SOC stability,and in turn,SOC protect the poorly crystalline from transforming to the crystalline counterparts.Besides,the DOM from the organic fertilized soils were more likely to co-precipitate with Fe minerals,the chemical stability of which against desorption is greater in comparison to the adsorbed OM.In this way,soils under long term organic fertilization have greater potential for the SOC sequestration.