| The saline-sodic soils in Songnen Plain has the characteristics of high exchangeable sodium content,soil compaction,poor permeability,and bad physical and chemical properties.The higher the salinization degree of soil,the worse the stability of aggregates,clay disperses and precipitates down to the alkalization layer,hindering plant growth.Soil organic matter is an important component of soil and is composed of different bioavailable components.The response of soil organic matter to soil quality changes will be different due to different organic matter components.This study,salinesodic soils as the research object,discussed the relationship between active organic matter components and aggregate stability in saline-sodic soils at different saline-alkali levels(mild,moderate,and severe),the relationship between humus proportion,structure and aggregate stability of saline-sodic soils under different vegetation cover(Suaeda glauca,Puccinellia distans,Phragmites australis,Leymus chinensis and Alkali spot bare),and the effects of adding different modified substances(control treatment,farmyard manure,desulphurization gypsum,sandy soil,and Dealkalization.1)on aggregate stability,quantity and structure of humic acid(HA)in saline-sodic soils by quantitative analysis and structural characterization,organic matter accumulation and short-term artificial application.The purpose of this study was to identify the key factors of organic matter composition affecting the stability of sodic soil aggregates,propose a unified evaluation system for the stability of sodic soil aggregates,and reveal the contribution rate of salinization degree of sodic soil to the stability of sodic soil aggregates.This study would provide basic data for the improvement and utilization of soda-saline soil,and has important guiding significance for vegetation restoration of Songnen Plain.The main results were as follows:1.The order of the contents of water-stable aggregates with particle size greater than 0.25 mm and the MWD and GWD of water-stable aggregates were LS>MS>HS.These results showed that the lower the degree of salinity,the more conducive to the formation of large aggregates with a particle size greater than 0.25 mm,and the higher the stability of soil aggregates.The content of water-stable aggregates with particle size greater than 25 mm and the MWD and GWD of water-stable aggregates at 0-20 cm soil layer were higher than those at 20-40 cm soil layer.The content of organic carbon and soluble organic matter in soil aggregates with particle size less than 0.053 mm was higher than other particle sizes under different saline-alkali levels.The content of easily oxidized organic carbon in soil aggregates with 0.25-0.053 mm particle size was lower than that in other particle sizes,and the content of microbial biomass carbon with 2-0.25 mm particle size was higher than that other particle sizes.The lower the salinity degree of saline-sodic soils,the higher the content of easily oxidized organic carbon,soluble organic matter,and microbial biomass carbon in each particle size of agglomerates.2.The content of water-stable aggregates with particle size greater than 0.25 mm and MWD and GWD of water-stable aggregates in soil under different vegetation coverage were higher than those in alkali spot bare land,indicating that vegetation coverage improved the stability of soil aggregates.The content of water-stable aggregates with particle size greater than 25 mm and the MWD and GWD of waterstable aggregates at 0-20 cm soil layer were larger than those at 20-40 cm soil layer.The organic carbon content of soil aggregates under different vegetation coverage was higher than that in alkali spot bare land,FA content was lower than that in alkali spot bare land.HA content,Hu content and PQ value of soil aggregates under different vegetation were higher than that in alkali spot bare land,indicating that the humification degree of soil aggregates under vegetation coverage was higher than that in alkali spot bare land.Compared with the alkali spot bare land,HA was significantly affected by different vegetation coverage.The vibration of HA absorption peak was weakened at 1720 cm-1 in the infrared spectrum,but it increased to different extents at the(2920+2850)/1720,(2920+2850)/1620,and 2920/2850 value.The contents of C,H,and H/C in HA were higher than those in alkali-spotted bare land,which indicates that the aliphatic hydrocarbon carbon of HA increased,while the carboxyl carbon content decreased,and the degree of polymethylene methylation enhanced,further leading to the improvement of the aliphatic property of HA.The fluorescence intensity of HA is higher than that of alkali spot bare land.With the increase of the fluorescence peak intensity of HA in soil,the number of electron-donating groups and the phenolic structure in lignin structure are increased.The high/medium ratio of heat and high/medium ratio of weight loss of HA are lower than those in alkali-spotted bare land.The lower the degree of condensation and oxidation of HA structure,the lower the thermal stability.The ratio of aliphatic C to aromatic C of HA is greater than that of alkali-spotted bare land.Because of the HA aromatic chain hydrocarbon reduction,the molecular structure tends to simplify.3.The content of water-stable aggregates with particle size greater than 0.25 mm and MWD and GWD of water-stable aggregates after adding soil amendments was greater than control treatment,indicating that vegetation coverage improved the stability of soil aggregates.The content of water-stable aggregates with particle size greater than 0.25 mm and MWD and GWD of water-stable aggregates at 0-20 cm soil layer were larger than those at 20-40 cm soil layer.The content of organic carbon at 0-20 cm soil aggregates increased after adding different modified substances,and FM had the most significant effect.The content of FA was lower than that of CK,and the content of HA,Hu,and PQ was higher than that of CK,indicating that adding different modified substances could promote the humification degree of sodic saline soil.Compared with the control treatment,HA was significantly affected after adding soil amendments.HA infrared spectrum,(2920+2850)/1720,and(2920+2850)/1620 were increased to different extents.The intensity of the three-dimensional fluorescence D peak was enhanced,and the content of C,H and C/H in HA all increased,while O and O/C decreased.HA calorie high/medium ratio and weight loss high/medium ratio decreased,while HA aliphatic carbon/aromatic carbon ratio increased to varying degrees.These results indicated that the aliphatic hydrocarbon carbon of HA increased,while the carboxyl carbon content decreased,and the degree of polymethylene methylation enhanced,further leading to the improvement of the aliphatic property of HA.The fluorescence intensity of HA is higher than that of alkali spot bare land.With the increase of the fluorescence peak intensity of HA in soil,the number of electrondonating groups and the phenolic structure in lignin structure are increased.The high/medium ratio of heat and high/medium ratio of weight loss of HA are lower than those in alkali-spotted bare land.The lower the degree of condensation and oxidation of HA structure,the lower the thermal stability.The ratio of aliphatic C to aromatic C of HA is greater than that of alkali-spotted bare land.Because of the HA aromatic chain hydrocarbon reduction,the molecular structure tends to simplify. |
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