Effects Of Straw Returning And No-tillage On Soil Organic Carbon Characteristics And Soil Water Stable Aggregates

Understanding the different C pools,molecular characteristics of soil organic carbon(SOC)and contribution of water stable aggregates in cropping system is imperative for sustaining the soil quality of agroecosystems.The conservation tillage practices,such as straw returning,fertilization,and no-tillage,could distinctly change the soil organic carbon(SOC)stock,labile organic carbon(C)pools and soil water stable aggregates,the combined effects of such management practices on the quantity and quality of soil SOC in rice-rapeseed rotation are poorly understood.Field experiments were conducted on the long-term experimental stations of rice-rapeseed rotation in Wuxue,Jingzhou,and Wuhan of Hubei province of central China.All the experiments for present work were conducted to investigate the effects of straw retuning on the soil organic carbon(SOC),particulate organic carbon(POC),microbial biomass carbon(MBC),easily oxidizable carbon(EOC),dissolved organic carbon(DOC),soil carbon management index(SCMI),the water-stable aggregation(>5,5-2,2-1,1-0.5,0.25-0.5,and<0.25 mm),SOC contents in soil aggregates,stability of water stable aggregates(WSA)and the chemical nature of soil organic matter were investigated by solid-state CPMAS 13C nuclear magnetic resonance(NMR)spectroscopy and Fourier transform-infrared(FTIR)spectroscopy.Compared to Conventional tillage(CT)treatment,straw returning treatments significantly increased SOC stocks,SCMI and C fractions of 0-20 cm depth by 6-50%at Wuxue,7-35%at Jingzhou and 4-25%at Wuhan.Compared to CT treatment,combine practices of straw returning significantly increased SOC(by 10-25%),MBC(by 12-24%),POC(by 20-94%),EOC(by 9-22%),DOC(by 5-13%),SCMI(by 10-25%)and SOC stocks(by 10-25%)at 0-20 cm depth.However at 20-40 cm depth CT treatment increased the SOC stocks(by 5-19%)and DOC(by 3-10%)as compared to other treatments.Significant and positive correlations were observed between SOC and numerous active SOC fractions,including DOC,EOC,MBC and POC.Also,total SOC and SCMI were observed to correlate significantly and positively each other.Of the four labile organic C fractions,POC and MBC were the most sensitive indicators of changes in TOC induced by straw management practices.CPMAS 13C NMR results demonstrate that Wuxue site was dominated by aromatic-C(42.65-45.54%)followed by O-alkyl C(22.92-34.33%)under CT and straw returning treatment(CTS),respectively.However,Jingzhou site was dominated by aromatic C(33.01-43.58%)followed by the O-alkyl C(17.50-36.75%)and Carbonyl C(42.52%-20.44%)in CT and CTS treatment,respectively.FTIR spectroscopy depicted a relative improvement of aromatic,carboxylic,aliphatic and NH(amide II)groups in CTS treatment after rice-rapeseed rotation system.Alkyl C and O-alkyl C ratio increased under CTS practices in both sites.While,aromatic C and aromaticity index changed with respective to different sites,in Wuxue aromaticity index increased and in Jingzhou site decreased due to increase in the Carbonyl C under CT.CPMAS 13C NMR data delivered a noticeable evidences about the chemical stabilization of SOM than FTIR spectra under rice-rapeseed rotation system in central China.Compared to CT treatment,straw returning treatments significantly increased water stable aggregates contents of>5 mm aggregate(by 40-78%),promote the aggregate contents of<0.25 mm and 0.25-0.5 mm(by 25-60%),and promote the contents of soil aggregates 2-5 mm,1-2 mm and 0.5-1.0 mm(by 10-15%)at 0-20 cm.In 0-20 cm depth,straw returning and no-tillage not only promoted the formation of macroaggreagtes but also increased aggregate stability as expressed with MWD and GMD index and decreased Fractal dimensions(FD)of soil aggregates.Among all defined aggregates,the SOC contents of>5 mm was highest and followed by SOC contents in 5-2 mm and<0.25 mm aggregates.The carbon content of soil aggregates under CT treatment ranged from 8.52 to 18.30 g kg-1 at Wuxue and 10.0 to 15.20 g kg-1 at Jingzhou.The average carbon content in straw returning(CTS)was 9.35 to 19.50 g kg-1 at Wuxue and 11.25 to 16.60 g kg-1 at Jingzhou site in 0-20 cm depth.The content of SOC was higher in macroaggregates as compared to soil microaggegates.There were positive and significant linear correlations of SOC with C fractions,SCMI and C fraction with each other.Soil organic carbon has significant relationship with Alkyl C,aromatic C,Alkyl-O-alkyl ratio and aromaticity index.O-alkyl C was positively correlated with C fractions,aromatic C and carbonyl C,which implied that O-alkyl C might be the main C component of SOC,MBC,DOC and POC.The SOC had positive linear relationship with>5mm,2-5 mm,1-2 mm and negative correlation with 1-0.5,0-5-0.25 and<0.25 aggregates.The correlation analysis revealed that EDX-C contents was positively or negatively(p<0.05)correlated with>5,2-5,1-2,0.5-0.25,<0.25mm.SOC contents of macroaggegates>5 mm and 2-1 mm had higher negative and positive correlation with SEM-EDX C content and lowest was found with SOC contents of 0.25-0.5 mm soil aggregates.The strongest positive correlation coefficients were noted between O-alkyl C with 2-1 and>5 mm water stable aggregates.Similarly,SOC content of 2-1 and>5 mm water stable aggregates had highest correlation with O-alkyl C.Our findings concluded that POC was an earlier indicator of SOC changes in agricultural management practices and O-alkyl C was a better molecular indicator for observing the effect of tillage and straw management practices on the quality of SOC in rice-rapeseed rotation system.Straw returning significantly enhanced the formation of stable macroaggregates,stability of all aggregate fractions,and increased SOC concentration in the macro-aggregate fraction,representing a potential method of enhancing SOC sequestration through physical protection.This study proves that chemical quality of SOC plays an important role in soil organic carbon management,it also provides a novel contribution and clarifies the chemical structures and molecular forms of C and explains the relationships between chemistry of SOC and labile fractions of SOC after straw returning management practices in rice-rapeseed rotation system.These findings convey a new evidence to support the positive effects of straw returning on soil SOC sequestration in central China,in view of the improvement of SOM structure and stability.