Study On The Distribution And Structural Characteristics Of Soil Organic Matter Fractions Within Soil Aggregates Of Tea Plantations With Different Ages

Soil aggregates and organic matter are both the basis for maintaining soil quality,and the two are closely related.Soil aggregate is the place where organic matter is maintained,and organic matter is the main cementing material to form stable aggregates.Study on the distribution of soil organic matter(SOM)components and their functional groups in soil aggregate can be used to explain the relationship between soil organic matter and aggregates.Therefore,this study regarded four tea plantations with different ages in hilly region in the Western Sichuan as the research objects,with the control of nearby abandoned land,to explore the distribution characteristics of SOM components and the structural characteristics of them,in order to provide a theoretical basis for the sustainable use of tea pantations and to reveal the physico-chemical protection mechanism of SOM in soil aggregates of tea plantation.The main results are as follows:(1)Contents of soil organic carbon(SOC),fulvic acid(FA),humic acid(HA)and humin(HM)increased first and then decreased with the increasing tea plantation age.All of them reached the maximum in 25-year tea plantation.The content of SOC fluctuated with the decrease of soil aggregate size.SOC content in >2mm aggregates was significantly higher than that of others,and the content of FA gradually decreased.>5 mm aggregate showed a higher HA content than other sizes.With the increasing of tea plantation age,the proportion of HA and HM in SOC increased gradually,which were higher in 25-year tea plantation,and the proportion of FA showed the opposite trend.The soil HA/FA ratio was between 0.30~0.88.The humification degree of >5 mm and <0.25 mm aggregates were higher than others.The increase in the number of macroaggregates in 25-year tea plantation was beneficial to the improvement of the quantity and quality of SOM.(2)Contents of concentrate-acid extracted carbohydrates,EE-GRSP and T-GRSP increased significantly after the tea plantation,which were higher in 25-year tea plantation.EE-GRSP decreased gradually with the increasing tea plantation age.Tea plantation was beneficial to the accumulation of concentrate-acid extracted carbohydrates and GRSP content.Long term tea planting(33 years)was not conducive to the preservation of soil carbohydrates and GRSP.The dilute-acid extracted carbohydrate was mainly distributed in >0.25 mm aggregates.The concentrate-acid extracted carbohydrate was mainly distributed in >5 mm and 2~5 mm aggregates,and the GRSP was mainly distributed in >5,2~5,1~2 and <0.25 mm aggregates.(3)SOM,HM and carbohydrates were significantly correlated with the MWD of water stable aggregate,and the correlation coefficients were 0.87(SOC),0.85(HM),0.81(hotwater extracted carbohydrates),0.83(dilute-acid extracted carbohydrates)and 0.75(concentrate-acid extracted carbohydrates),respectively.FA and T-GRSP showed significant positive correlation with MWD of water stable aggregates,and the correlation coefficients were 0.70 and 0.63,respectively.As an important cementation of soil aggregates,carbohydrates had great contribution to the stability of soil aggregates in tea plantations and can be used as indicators of soil quality changes during the process of tea plantation.(4)The H/C and O/C of HA,FA and HM in soil and aggregates were reduced after tea plantation.The aliphatic chain hydrocarbons in HA and HM molecules were relatively reduced,and the aromatic structure was relatively increased.In 0~20 cm soil layer,H/C of HA in large aggregates(>5 mm,2~5 mm and 1~2 mm)showed an increasing trend with tea plantation age.In 20~40 cm soil layer,H/C value of HA in aggregates was lower in 25-year tea plantation.Both H/C and O/C of FA and HM were lower in 25-year tea plantation.Tea plantation was beneficial to enhance the degree of condensation and aromatization of humus components in soil,which was conducive to the complexity of their structure and the stability of organic matter,which was mainly manifested in <1 mm aggregates.The soil humus components in 25-year tea plantation had low oxidation degree,high condensation degree,high aromatization degree and strong stability,which contributed to the fixation of carbon to soil.