Changes Of Organic Carbon Fractions In Soil Aggregate And Their Response To Plant After Farmland Conversion In The Loess Hilly Region,China

Soil aggregate is the basic unit of soil structure and function,and it is also an important component of soil quality and ecological function regulation.Exploring the composition changes of soil organic carbon from the perspective of aggregates is the key to understanding the mechanism of soil organic carbon turnover.In this study,maize farmland widely planted in loess hilly region and artificial Robinia pseudoacacia forest for 16,22,32 and 47 years were selected as research objects.The contents of soil organic carbon components and their functional groups in 0-20 cm,20-40 cm and 40-60 cm soil layers were analyzed from the point of view of aggregates,and the relationship between soil aggregate organic carbon content and organic carbon chemical structure was clarified.Combined with the input of organic carbon and the physical and chemical properties of soil,the mechanism affecting the distribution of organic carbon components in soil aggregates was revealed.The results show that:(1)The stability of soil aggregates in the surface layer is more sensitive,while that in the deep soil is weaker.Compared with farmland,The water-stable aggregate content which >0.25 mm decreased 0.14,0.20,0.33,0.18 times in 0-20 cm soil layer of Robinia pseudoacacia for 16,22,32 and 47 years.The GMD of soil aggregates increased by 1.40,1.55,0.65 and2.05 times in 40-60 soil layer.(2)The content of SOC in each particle size of soil increased with the increase of years of returning farmland.The SOC content of large aggregates in the three soil layers ranged from 3.25 g/kg to 6.80 g/kg,which was the highest in Robinia pseudoacacia for 47 years,and decreased with the increase of soil depth.In addition,the increase of soil active organic carbon content after returning farmland determines the size of available organic carbon sources for microorganisms in different grain size soil units.(3)After returning farmland,the relative content of aromatic and aliphatic functional groups in soil increased obviously.Compared with farmland,the relative content of Comero functional groups of aromatic C in > 0.25 mm particle size aggregates increased,while the Cmurh deformation vibration functional groups of fat C were mainly concentrated in small and medium particle size aggregates.With the increase of conversion years,the relative content of Cmuro functional groups of polysaccharide decreased in large aggregates and increased in clay particles.(4)SOC and its components of aggregates are significantly affected by chemical structure,and different particle sizes of SOC are affected by different functional groups.There was a significant positive correlation between SOC content and MOC content in aggregates with >0.25 mm size,and there was a significant positive correlation between MOC content of macroaggregates and C=O functional groups of aromatic C in 0-20 cm soil layer and 40-60 cm soil layer,and between MOC content of macroaggregates in 20-40 cm soil layer and-OH vibration of carboxyl C.There was a significant positive correlation between the SOC content of aggregates and MBC content in clay particles,and the MBC content of aggregates in surface and deep soil was correlated with the C=O functional groups of aromatic C.(5)The effect of plant organic carbon input on organic carbon of soil aggregates gradually decreased with the increase of soil layer.The aggregate SOC in 0-20 cm and 20-40 cm soil was mainly affected by plant organic matter input,while the influence factor was transformed into nitrogen components in 40-60 cm soil layer.In conclusion,returning farmland to forest in the loess hilly region improved the stability of soil organic carbon,changed the distribution of organic carbon components in aggregates,affected the chemical structure of organic carbon in aggregates and soil physical and chemical properties,and increased the content of organic carbon components in aggregates.The soil structure and quality were effectively improved,while the surface layer of soil was mainly affected by plant factors and changed with the increase of soil depth.The results of this study are of reference value for revealing the response of soil aggregate carbon components to plant input in loess hilly region,and are of great significance for vegetation restoration to affect soil carbon cycle model.