Characteristic Of Humus Substances And The Protection Mechanisim Of Organic Carbon In Aggregates In Purple Padyy Soil Of The Sichuan Basin, China

The close relationship betwween the stability,enhancement or release of soil organic carbon pool and the concentration of CO₂ in atmosphere has become one of hotspot in international science.Whether SOC pool continued to increase or stability subjected to the important theoretic baisis that terristral ecosystem was a sink for absortion and sequestration CO₂ in atomsphere,Which has been one of the key issue of the study on the soil and global change.Since the last 1980s',the increase of SOC storage in paddy soil of China indicated that paddy soil may be a sink for CO₂ in atomsphere.However,it was uncertain about the capacity of SOC sequestration and its stability mechanisms among different soil types.The dynamics of the stable fractions of soil organic matter(SOM),the humus substances,has been rarely studied in purple paddy soil of Sichuan basin.It is imperative to determine the characteristic of humus substances and the organic carbon protection mechanism in aggregates in paddy soil.In this paper,we evaluated extraction yields,elemental composition,isotopicδ¹³C abundance,Fourier transform infrared spectroscopy (FTIR)and solid-stae cross-polarization magic angle spinning technique(CPMAS)¹³C nuclear magnetic resonance(NMR)spectra of humic substances isolated from purple paddy soil in Sichuan basin in order to asses the influence of conservation tillage such as no-tillage,crop rotation and rice stalk mulch on the composition of soil humic acids(HA)and flumic acids(FA)and the physical protection mechanisms of SOM in aggregates under a long-term field experiment,including conventional tillage(rice(Oryza sativa L.)-fallow)(CT),no tillage and ridge culture(rice-fallow)(NT-R),no tillage and ridge culture (rice-rape)(NT-RR),no tillage and plain culture(NP-RR),no tillage and ridge culture(rice-rape/green manure)(NT-RR)conventional tillage and ridge culture(rice-rape)(CT-RR),convention tillage and plain culture(rice-rape)(CT-PR)and water-upland rotation(rice-rape)(PU-RR),which located in Southwest university experimental farm.1)The significant differences of organic carbon and humic acid carbon were observed between conservation tillage and conventional tillage.Conservation tillage made the extractable humus carbon increased in plow layer and plow pan.Extractable humus carbon accounted for 37%-40%of soil organic carbon in cultivated layer.Compared with the CT,it was reduced about 23.52%under PU-RR and increased about 39.29%and 8.70%under NR-RR and NP-RR,respectively.In soil of plow pan,the extractable humus carbon under NR-RR and NP-RR increased about 54.97%and 66.38%compared with those under CT,respectively.Humic acid carbon content accounted for the content of hurhic acid and organic carbon about 38%-65%and 12%-29%,respectively.The humic acid carbon content in plow layer of purple paddy soil increased under long-term conservation tillage.Compared with the original soil(in 1990 yr),it had increased 41.62%and 10.52%under NR-RR and NP-RR,respectively.In 20-40 cm soil layer,the humic acid carbon content only increased under NP-RR and about 81.91%and 70.82%of original soil(1990 yr)under NR-RR and PU-RR,respectively.It was found that the difference between NR-RR/NP-RR and CT was very significant.Soil FA carbon content was about 12%-22%and 11%-25%of soil organic carbon content in plow layer and plow pan of purple paddy soil,respectively.Compared the original soil(1990 yr),it increased 23.08% and 77.69%in surface layer under NR-RR and NP-RR,which was 25.49%and 20.45%higher than that in CT,respectively.There was no significant difference between CT and PU-RR.Compared with the original soil(in 1990 yr),the FA carbon content increased 61.00%and 10.91%in plow pan of under NR-RR and NP-RR,which was 43.79%and 50.69%higher than CT,respectively.It indicated that the conservation tillage contributed to increasing FA carbon content of purple paddy soil.The ratio of E₄/E₆ of HA was increased under long-term paddy planting with the value range from 4.65 to 5.89.The change of E4 of HA indicated that the molecular structure of HA tended to be more simplified whereas the aromatization degree of HA in plow pan was reinforced under conservation tillage.Thus the change of E₄ of HA maybe can better reflect the complicated degree of humic acid in purple paddy soil. The⊿log K of the NaOH+Na₄P₂O₇-extractable HA was at the range from 0.75 to 0.80 which was lower than that by NaOH(0.94-1.02)and the RF was at the range from 164 to 205 which was higher than that by NaOH(52-93).Compared with CT,⊿log K of HA was decreased and RF was increased under NR-RR.The contents of Carboxyl content,carbonyl content and phenol-hydroxy content of HA in cultivated horizon were increased whereas the contents of methoy and alcoholic hydroxyl reduced.The oxidation degree and aromatic components reinforced.The RF of soil FA(extracted by 0.1 mol L^-1NaOH+Na₄P₂O₇)was higher than that by 0.1 mol L^-1 NaOH in plow layer and plowpan(in the range of 40-43 and 39-59,respectively).The increasing order sorting of The RF,oxidation degree and aromatization degree of FA in cultivated horizon were NR-RR, NP-RR and PU-RR.In cultivated layer under NR-RR and NP-RR,the⊿log K of FA was decreased and the RF of FA was significantly higher than the control treatment(CT).Therefore with the increase of soil depths, the content of carboxyl,carbonyl and phenol-hydroxy of FA increased,and the contents of methys and alcoholic hydroxylof FA reduced.The oxidation degree and aromaticity of FA were more significant.The distribution of SOM under long-term conservation tillage was studied by isotopesδ¹³C abundance.The results showed that soil organic carbon content increased after long-term paddy planting, and organic carbon content,nitrogen content,ratio of C/N andδ¹³C values were all significantly affected by tillage systems in plow layer and plow pan.The decreasing order soring of organic carbon contents in different tillage systems was NP-RR＞NR-RR＞PU-RR and the reduced range was not the same for different soil layer.Theδ¹³C values of whole soil organic matter were between - 27.85‰and - 25.56‰, and theδ¹³C values under NP-RR and NR-RR were about - 27‰.The increase of stable carbon isotope values with depths was was about 1.92‰under PU-RR and the dispersion between 20-40 cm and 0-5 cm soil layers.Theδ¹³C values of soil under different tillage systems were higher than those of stalks and roots of rape and paddy.Theδ¹³C values of HA were at range from -28‰to -30‰,which were lower than those of SOM(about 1‰-2‰)but closer to these of stalks and roots of rape and paddy.It indicated that more fatty matters could be stored in HA.Except that under CT,theδ¹³C values of soil HA were a bit increased with the increasing of soil depth under the other treatments.The decreasing order sorting of theδ¹³C values was FA＞SOM＞HA.The margin value of theδ¹³C values was about 2‰between soil HAs and SOM,and about 4‰difference between soil HAs and humin.It indicated that there were more carbohydrate components of soil HAs.With the increase of soil depths,theδ¹³C values of soil HAs were decreasing under CT and NP-RR and about 1‰increased under PU-RR and NR-RR.Theδ¹³C values of soil humin in plowlayer and plowpan were at the range of - 23.68‰～24.85‰and - 22.59‰～24.21‰,respectively.With the increase of soil depths,theδ¹³C values of soil humic acid increasing 1.86‰,0.14‰,0.64‰and 1.51‰under CT,NR-RR,NP-RR and PU-RR,respectively.The decreasing order of theδ¹³C values in each fraction was humin＞FAs＞SOM＞rice(rape)residues＞HAs.The main factors that affected theδ¹³C values of SOM and humus substance were vegetable types,soil depths,patrticulate composition,and conservation tillage.2)The carbon was the main contents(50%-53%),followed by oxygen(30%-36%),hydrogen(5%-6%), sulfur(4%-7%)and nitrogen(4%-6%)of the soil HAs in purple paddy soil.Compared with the original soil(1990yr),the total acidity,carboxyl and phenolic hydroxyl content of the soil HAs decreased and the carbonyl content increased about 1 times under NR-RR.Compared with the HA in plow layer under CT, the total acidity,carbonyl and phenol-hydroxyl contents increased and the carboxyl groups reduced under NP-RR,and the total acidity,carboxyl and phenolic-hydroxyl content of the soil HAs decreased and the carbonyls content increased under PU-RR.The total acidity and carboxyl groups content of the soil HAs were nearly same between NP-RR and CT.The carbonyls content was significantly increased under NP-RR. By means of infrared spectrum analysis,it was found that the major oxygen-containing functional groups of the soil HAs in purple paddy soil were carboxyl groups,hydroxyls and carbonyls,and their contents were different under each tillage system.The carbon of aliphatic components(including alkyl-C and carbohydrate-C)accounted for the largest part(50%-86%)of humic acids in purple paddy soil,followed by carbonyl C(18%-31%)and aroma carbon (9%-24%).Carbohydrate components were the major part in aliphatic components,and then was the alkyl-C.In plow layer the decreasing order sorting of aliphatic components of HA was CT(70.91%)＞NP-RR(60.21%)＞PU-RR(52.80%)and in plough pan,the increasing order sorting of the aliphatic characteristics of HA was(50.80%)＞CT(59.13%)＞PU-RR(85.75%).The aliphatic characteristics of the soil HAs decreased and the aromaticity of HA increased under CT and NP-RR.The aliphatic characteristics of HA increased and the aromaticity of HA decreased rapidly under PU-RR.The differences of soil HAs carbon types among treatments mainly resulted from the changes of oxygen-containing functional groups with soil depths.It indicated that the chemical compositions of soil HA were affected by agriculture management practices such as no-tillage,crop rotation and stalk mulch.The oxygen was the largest composition(71%-81%)in FA constitution of purple paddy soil,followed by sulfur(6%-10%),carbon(7%-18%),hydrogen(about 2%)and nitrogen(about 1%).Compared with the infrared spectra characteristics of HA,the absorption peaks in infrared spectra characteristics of soil FAs were not so many.The major oxygen-containing functional groups included carboxyl,carbonyl and hydroxyl groups.Differences among these different functional groups were found.The aliphatics carbon was the largest composition of FA(50%-68%),followed by carboxyl carbon(19%-50%),and the aroma carbon was the lowest part.The aliphatics carbon was composed of carbohydrate-C(25%-43%),alkyl-C (24%-31%)and some others.In cultivated layer,the aliphatic characteristics of FA could be ranked as CT (49.34%)＜NP-RR(52.57%)＜NR-RR(59.37%)＜PU-RR(67.27%)and among tillage systems the decreasing order was CT(67.52%)＞NR-RR(66.31%)＞NP-RR(61.22%)＞PU-RR(52.45%).So,the carbohydrate and saccharide were more sensitive to the soil depths change and the aromaativity was decreased with the increasing soil of depths(except under PU-RR).3)Reclaiming of the straw mulch has been taken as an important method to improve soil in many areas of China.In order to evaulate the mechanism of straw mulch improving soil,the effects of straw mulch on the soil humus substances(humic acids and fluvic acids)were studied by elemental composition,FTIR and solid-stae CP/MAS ¹³C NMR spectra in purple paddy soil with incubation experiment.SOC contents were significantly increased in the application of rice stalk.Compared weith those of CK,the SOC contents of the application of rice stalk were about 1.64,1.56 and 1.31 times in the incubation time of 45d,90d and 135d,respectively.The HS under the application of rice stalk extracted by 0.1 mol L^-1NaOH+Na₄P₂O₇ at 70℃and 100℃was about 24%-32%and 47%-53%of soil carbon content and the PQ was about 57%-67%and 46%-52%,while about 22%-26%and 40%of soil carbon content and,the PQ was about 22-26%and 50%-54%by 0.1 mol L^-1NaOH at 70℃and 100℃,respectively,Which would significantly affected⊿log K and RF of HA whatever adding rice stalks or not.The Rp type was the main sort of the extracted HA.For both the application of rice stalks and CK,the carbon was the largest composition(about 52%)in HA constitution,followed by oxygen(27%-31%),hydrogen and sulfur,and nitrogen was the smallest part (about 4%-6%).No obvious differences were found between the application of rice stalks and CK.With incubation time increase,the total acid and phenolhydroxyl content of soil HA increased and the carboxyl and carbonyl content of soil HA maitained stable under CK.The total acid,phenolhydroxyl and carbonyl content increased during the incubation time and the peak of the carboxyl content changes appeared at the 90th day under application of rice stalks.The results of FTIR Showed that the aliphatic characteristics of the soil HAs increased and the mainly oxygen-containing functional groups were carboxyl,hydroxyl and carbonyl in the application of rice stalks.The alphiatic components,about 70%of which was carbohydrate or saccharide,was the main part of the soil HAs(57%-60%),and then the aromatic components (21%-24%),the carbonyl carbon content was the lowest(18%-19%).Except the O and S content of soil FA among treatments were much higher than those of main soil types of China,other elements are all lower than those of China.⊿log K of FA extracted by 0.1 mol L^-1NaOH at 70℃decreased with the incubation time,and the trend of RF was the on the opposite.At the same incubation time,the⊿log K of FA extracted at 100℃was lower than that extracted at 70℃,and one turning point was observed at 90 d.the⊿log K of FA under the application of rice stalks extracted by 0.1 mol L^-1NaOH+Na₄P₂O₇ at 100℃was increased and the RF decreased with the incubation time,and their values were higher than those extracted by 0.1 mol L^-1NaOH.The change degree of FA was not as severe as that of HA at the same incubation time.The results of FTIR showed that soil FA contained these functional groups such as hydroxyl,alkyl and carbonyl and benzene.With the incubation time increasing, the aliphatic compounds was the main part of soil FA chemical composition in both treatments with and without rice stalks addition,which was about 42%-53%and 58%-72%,respectively.The aromaticity was about 8%-17%and 15%-21%in CK and the application of rice stalks,respectively.The main component of FA was alkyl-C in CK and carboxyl-C was the major part in the application of rice stalks.4)A 13 years long-term experiment in purple paddy soil of Sichuan Basin was used to evaluate effects of conservational tillage systems on aggregates composition and carbon storages.The results showed that aggregates composition and soil carbon storages were significantly controlled by conservational tillage systems.In the 0-10 cm layer,the amount of macroaggregates under conservational tillage systems: no-tillage and ridge culture(rice-rape)(NT-RR),no-tillage and plain culture(rice-rape)(NP-RR),no-tillage and ridge culture(rice-green manure)(NT-RGM),tillage and ridge culture(rice-rape)(TR-RR),tillage and plain culture(rice-rape)(TP-RR)were 23%,69%,9%,36%and 28%higher compared with conventional tillage(CT)(12%),respectively.In the 10-20 cm layer,the macroaggregates proportion was 9%-38%lower than that of CT.The organic carbon concentration in macroaggregates at 0-10 cm depths under no-tillage and ridge culture(rice-fallow)(NR-RF),NR-RR and NR-RGM were 13%,31%and 32%higher than that of CT,while 28%-54%lower at 10-20 cm depths,respectively.The differences of organic carbon concentration between macroaggregates and microaggregates in the 10-20 cm layer were lower than those in the 0-10 cm layer.The organic carbon storages among conservational tillage systems in the 0-10 cm layer were 8%-28%higher than that of CT,while lower 4%-22%in the 0-10 cm layer.The ratio of mean organic carbon sequestration on the conversion from CT to conservational tillage at 0-10 cm and 10-20 cm depths since 1990,were 53 g m^-2y^-1and 25g m^-2,y^-1,while under CT were 26 g m^-2y^-1and 33 g m^-2y^-1, respectively.The conservational tillage systems favorably led to the formation of macroaggregates and the enhancement of soil organic carbon storages.Long-term conservation tillage is an effective agricultural management to improve soil structure in severely destroyed paddy soil by increase of soil organic carbon (SOC),more work is still needed to understand the impacts of conservation tillage on the mechanisms of aggregates stability.Soil samples in 0-20 cm layer were adopted from the long-term experiment:(1), paddy-upland rotation and plain culture(rice-rape)(PU-RR),(2)no-tillage and ridge culture (rice-fallow)(NT-RF),(3)no-tillage and ridge culture(rice-rape)(NT-RR)and(4)no-tillage and plain culture (rice-rape)(NT-PR).Different pretreatment,such as slaking in fast wetting,wetting and subsequent slaking, were applied to simulate the breakdown mechanisms of aggregate in paddy soil.The results showed that there was no significant difference of aggregate stability between slaking in fast wetting and wetting,but significant difference of aggregate stability exited between conservation tillage and conventional tillage. The aggregate stability in slaking pretreatment ranked in the order of NT-RR＞NT-PR＞NT-RF＞PU-RR. The aggregate stability order under wetting was NT-PR＞NT-RR＞NT-RF＞PU-RR.The organic carbon content in aggregates was significantly positive related to the aggregate stability under wetting(r=0.626^* *),while low correlation under slaking treatment(r=0.432).The results suggest that long-term conservation tillage favorably led to the increase of SOC in aggregates,and the enhancement of aggregates.As above all,the main SOC was observed in soil humin,followed by soil humic acids and soil fluvic acids in purple paddy soil of the Sichuan basin.The humic acids and fluvic acids mainly comprise of alphiatic components,and alkyl-c maitained stability,land use practices have greatly effects on the multitude of carbohydrate-C,aromatic C and carboxyl-C.The evnidences surggested that conservation tillage favorably led to the increase of OC content in mcroaggregate,enhancement of mcroaggregates stability,increase of the mcroaggregates propertion and SOC storage.In this paper,characteristics of humus substances and organic carbon physical protection mechanisms in aggregates were investgated,and achieved basis information about HS composition,which could surpported the firstly reference for the study on the recognization of substances cycle and energy transformation in paddy field system.In the future,more ephersis should be played on the important role of humus substances in the processes of SOC pool stability,acumulation,and its transformation.