Mechanisms Of The Interaction Between Poorly Crystalline Iron Oxides And Soil Organic Carbon In A Rice-wheat Cropping System

The total arable soil of the world is approximately 1.369 billion hectare,whereas there is only 0.1217 billion hectare in China.To feed more than 21%of the world's population with less than 9%world's arable soil,it is urgent to take effective agricultural management practices to enhance the storage and stability of soil organic carbon(SOC)and to improve the soil fertility and agro-ecosystem productivity.The rice-wheat cropping system in the middle and lower valley of the Yangtze River region is one of the main cropping systems for cereal food production in China.However,the mechanism of SOC sequestration in the rice-wheat rotation paddy soil is still unclear,which severely inhibits the improvement of soil fertility and agricultural productivity.Newly emerging evidence suggests that the stability of SOC depends mainly on its environmental and biological factors,rather than on its chemical recalcitrance alone.Therefore,occlusion within soil aggregates,attachment to soil reactive minerals and translocation to deep soil profile are considered important mechanisms of SOC sequestration.Many investigations have shown that reactive iron oxides bind and preserve more than 20%of SOC in soils and sediments.However,knowledge regarding the effect of seasonal redox cycles caused by repeated drying and wetting conditions in the rice-wheat cropping system on the redox transformation and subsequent accumulation of iron oxides and their implications for the storage and stability of SOC is scarce,and thus the mechanisms that drive the above processes remain poorly understood.In the present study,soils from a typical rice-wheat cropping system in the middle and lower valley of the Yangtze River were used to explore the processes of the SOC sequestration,the transformation and accumulation of reactive iron oxides,and the formation and turnover of soil aggregates by studying the contents of SOC and poorly crystalline iron oxides(Feo)in the bulk soil and the soil aggregates and the dynamics of aggregate mean weight diameter(MWD).A short-term laboratory incubation experiment was conducted to study the potentially mineralizable SOC in the bulk soil and the soil aggregates and to evaluate the stability of the associated S.C.Furthermore,solid-state 13C nuclear magnetic resonance(NMR)spectroscopy was used to explore the SOC chemical structures in the bulk soil and the soil aggregates.Moreover,an elemental analyzer coupled to an isotope ratio mass spectrometer was used to analyze the stable isotope composition of SOC in the bulk soil and soil aggregates.The main results were listed as follows.1)The application of organic fertilizers in combination with chemical fertilizer significantly(P<0.05)increased the contents of SOC and Feo compared to chemical fertilization alone.The concentrations of SOC and Feo were significantly(P<0.05)higher after rice harvest than after wheat harvest and continued to increase significantly(P<0.05)over time.Seasonal drying and wetting conditions in the rice-wheat cropping system contributed to the redox transformation of the reactive iron oxides and the accumulation of Feo.The application of organic fertilizers significantly increased the soil microorganism biomass carbon(MBC)and the carbon mineralization rate(CMR,rate per unit soil).The increased mineralization due to the application of organic fertilizers during the paddy rice inundation may need more iron as alternative electron acceptors,which contributed to the reductive transformation of the reactive iron oxides.In the subsequent wheat season,the strong binding interaction between SOC and Feo may inhibit the re-crystallization of the organo-Fe complexes,which contributed to the accumulation of Feo in the rice-wheat cropping system.A significant positive correlation(P<0.001)between SOC and Feo was observed in the bulk soil and the soil aggregates,which indicated that Feo may play an important role in the SOC preservation.Variation partitioning analysis showed that the Feo alone could explain 52.57%,and the fertilization treatments and crop season could also significantly explain 1.43%and 1.5%of the variation in the SOC content in the rice-wheat cropping system.2)The application of organic fertilizers in combination with chemical fertilizer significantly(P<0.05)increased MBC and aggregate MWD compared to chemical fertilization alone.The MBC and aggregate MWD were significantly(P<0.05)higher after wheat harvest than after rice harvest.A significant positive correlation(P<0.001)between MBC and aggregate MWD was observed in the rice-wheat cropping system,which indicated that soil microorganism may play an important role in soil aggregation.Variation partitioning analysis showed that fertilization treatments and crop season could explain 11.32%and 7.05%,respectively,whereas MBC could hardly significantly explain the variation in the aggregate MWD.The specific carbon mineralization rate(SCMR,rate per unit SOC)continued to decrease significantly(P<0.05)over time and was significantly(P<0.05)higher after wheat harvest than after rice harvest.There was a significant positive(P<0.001)correlation between MBC/SOC and SCMR in the bulk soil,which indicated that MBC/SOC may be an effective index to evaluate the stability of SOC.The SCMR in the aggregates decreased in the order of silt and clay fraction>large macroaggregates>small macroaggregates>microaggregates.There was a significant(P<0.001)negative correlation between Feo and SCMR in the bulk soil and the soil aggregates,which indicated that Feo may play an important role in the stabilization of SOC.3)Solid-state 13C MNR spectra analysis showed that the alkyl C-to-O-alkyl C ratio was higher after wheat harvest than after rice harvest and increased with decreasing aggregate size.However,the relative intensity of aromatic C was lower after wheat harvest than after rice harvest and those of the small macroaggregates and microaggregates were higher than that of the silt and clay fraction.There was a significant(P<0.001)negative correlation between the intensity of aromatic C and the ?13C value,indicating that the aromatic organic compounds are 13C depleted.There was a significant(P<0.001)positive correlation between Feo and the intensity of aromatic C,but a significant(P<0.001)negative correlation between Feo and the?13C value,indicating that Feo may selectively preserve 13C-depleted aromatic organic compounds.4)There were significant(P<0.001)positive correlations between Feo and SOC in different soil depth intervals,which indicated that Feo may play a role in SOC preservation.The SOC,MBC and CMR significantly(P<0.05)decreased with increasing soil depth,whereas SCMR increased with increasing soil depth.There was a significant(P<0.001)positive correlation between MBC and CMR,and between MBC/SOC and SCMR in the rice-wheat cropping system,which indicated that microorganism may affect the SOC stability.The intensity of aromatic C decreased with increasing soil depth,whereas the alkyl C-to-O-alkyl C ratio increased with increasing soil depth.The ?13C value tended to increase with increasing soil depth.The Feo was significantly(P<0.001)positively correlated with the aromaticity of SOC,Uut negatively correlated with the ?13C value,indicating that Feo may selectively preserve-UCIdepleted aromatic organic compounds.5)The application of organic fertilizers significantly(P<0.05)increased DOC and dissolved Fe,A1 and Si concentrations,whereas decreased that of the dissolved Ca concentration.The DOC concentration was significantly(P<0.001)positively correlated with the dissolved Fe,A1 and Si,but negatively correlated with the dissolved Ca,suggesting that Fe,A1 and Si may play important roles in soil colloids in preserving the DOC.Fertilization significantly influenced the SEM images of the soil colloids.The EDS analysis showed that chemical fertilization alone increased the interaction between DOC and Ca,but organic fertilization increased interaction between DOC and Fe or Al.TEM analysis indicated that the application of organic fertilizers increased the proportion of the gray region,but decreased that of the dark region,whereas chemical fertilization alone showed a contrary trend.The EDS analysis showed that the gray region was comprised of non-crystalline Al and Si,whereas the dark region was comprised of crystalline Fe.Therefore,we argued that the application of organic fertilizers increased the non-crystalline nano-minerals and thus the SOC sequestration potential in paddy soils.Above all,we argue that the seasonal redox cycles caused by repeated drying and wetting conditions in the rice-wheat cropping system contributed to the redox transformation of reactive iron oxides and the accumulation of Feo.The application of organic fertilizers further enhanced these effects.The rice-wheat rotation paddy soil underwent a seasonal dynamic processes of aggregation,disaggregation and re-aggregation.Microaggregate-associated SOC was relatively stable,whereas that in the silt and clay fraction was the most labile.Feo played an important role in the accumulation and stabilization of SOC in the bulk soil and the soil aggregates in the rice-wheat cropping system,which endowed paddy soil with a physico-chemical protection of SOC while application of organic fertilizers enhanced this protection.Therefore,the long-term rice-wheat rotation combined with organic fertilizers application contributed to the SOC sequestration and stabilization.The SOC degradation status was higher after wheat harvest than after rice harvest.In the rice-wheat cropping system,Feo preferentially preserved 13C-depIeted aromatic organic compounds at the redox interfaces,leaving other organic compounds migration into the deep soil profile.Soil DOC was closely associated with the dissolved Fe,Al and Si.Organic fertilization decreased the crystalline Fe nano-minerals and increased that of the non-crystalline Al and Si nano-minerals,while chemical fertilization alone increased that of the DOC associated with Ca and the crystalline Fe nano-minerals.