Effect of straw returning on the characteristics of Fourier Infrared Spectroscopy organic carbon within aggregates in a Mollisols

The effects of short-term different straw returning managements on the content and infrared spectrum characteristics of organic carbon within aggregates were investigated in an arable Mollisols.The results were expected to provide a theoretical basis for optimizing the straw returning practices and enhancing the potential of organic carbon sequestration.Six field treatments with different patterns of crop returning were established in the Hailun Station as follows:(1) S0,plowing 20 cm,crop straw removal;(2) S1,plowing 20 cm,crop straw returning at 5 t·hm^-2;(3) S2,plowing 20 cm,crop straw returning at 10 t·hm^-2;(4) S3,deep plowing 35 cm,crop straw returning at 10 t·hm^-2 at 20-35 cm layer;(5) S4,no-tillage,10 t·hm^-2crop straw mulching at the surface;(6) S5,plowing 20 cm,10 t·hm^-2 crop straw after burning.The water-stable aggregate fraction and Fourier Infrared Spectroscopy(FTIR) methods were utilized to analyze the characteristics of infrared spectrum of organic carbon in both bulk soil and aggregate-sized fractions under different straw returning managements.Our results showed that compared to S0 treatment,different straw returning managements increased the surface soil organic carbon contents in an order of S2>S1>S4>S5>S3>S0.The mass percentages of >2 mm and 2-0.25 mm aggregates increased by 16.4%-112.9% and 19.0%-29.4%,respectively under different straw returning managements.The content of organic carbon within >2 mm and 2~0.25 mm aggregate increased by 14.4%-55.5% and 32.7%-54.7%,respectively.Compared to the S0 treatment,the infrared absorption peaks of aliphatic-CH increased,and aromatic C=C decreased after short-term straw returning.Correlation analysis and principal component analysis showed that the content of organic carbon was positively correlated with the relative peak area of aliphatic,but negatively correlated with the relative peak area of aromatic(P<0.05) within >2 mm and 2-0.25 mm aggregates.On contrast,the organic carbon contents were negatively correlated with the relative peak area of aliphatic group and positively correlated with the relative peak area of aromatic group within <0.053 mm aggregate.These results indicates that the organic carbon within macroaggregates is mainly composed of simply-structured and easily-decomposed organic matter,while the organic carbon within microaggregates is mainly composed of aromatic carbon with higher stability.Long-term different straw returning managements are suggested to be beneficial in increasing SOC contents,promoting the formation of macroaggregates,enhancing the stability of aggregates and carbon sequestration.The crop straw returning at 10 t·hm^-1 combined with 20 cm soil tillage management is proposed to be the most effective measure to increase surface SOC and its stabilization in the Mollisol region.