Influencing Factors Of Soil Aggregate Stability Under Different Stands In Acid Rain Region

The problem of soil erosion in the Three Gorges reservoir area cannot be ignored,and acid rain still exists in the Chongqing section of the reservoir area.As an important land resource in Chongqing,the stability of yellow soil aggregates directly affects the water and soil loss in the Three Gorges reservoir area.This study focused on the influencing factors of aggregate stability of yellow soil under different forest stands and the changes of aggregate stability under acid rain,aiming to provide theoretical support and scientific basis for soil and water conservation in the Three Gorges Reservoir area.In this study,we selected five typical forests in Jinyun Mountain as research objects,and systematically analyzed the main influencing factors of yellow soil aggregates in different stands by measuring soil properties,root traits,arbuscular mycorrhizal fungi content and aggregate stability.In addition,four initial aggregate size classes(5-7,3-5,2-3 and 1-2 mm)were slowly capillary-wetted with three levels of simulated acid rain(pH1,pH3,pH5)to study the dynamic changes of aggregate stability under acid rain wetting.Soil aggregate stability was determined according to Le Bissonnais,the mean weight diameter of aggregates(MWD_FW,MWD_WS,MWD_SW)were measured by fast wetting,shaking after pre-wetting and slow wetting respectively.The main conclusions of this study are as follows:(1)The MWD_FWof each stand was the lowest,but there were significant differences in aggregate stability under different stands,and the size relationship of aggregate stability was different among the three treatments.The MWD_FW(2.03±0.26 mm)and MWD_SW(2.75±0.01 mm)of the Gorodnia axillaris and other Broad-leaved trees mixed forest were the highest,while the MWD_WS(2.78±0.16mm)of the Cunninghamina lanceolata and Lindera kwangtungensis mixed forest were the highest.In addition,MWD_FWwas more sensitive to the change of tree species.(2)The soil organic carbon content of each forest land is high,the soil acidity is strong,and the soil texture is mainly sand.The soil physical and chemical properties of different stands were significantly different,the organic carbon content and Fe and Al oxide content of the Cunninghamina lanceolata and Lindera kwangtungensis mixed forest were the highest,and the cation exchange capacity of the Gorodnia axillaris and other Broad-leaved trees mixed forest and Cunninghamina lanceolata and Lindera kwangtungensis mixed forest was significantly higher than that of other stands,and the soil texture difference between the stands was significant.The difference of vegetation root system among stands is that Phyllostachys pubescens Pure forest have higher root mass density and root length density,and root mass density of Pinus massoniana and Symplocos setchuanensis is significantly lower than other stands,but there is no difference in root morphological characteristics between the two stands.We did not observe the difference of arbuscular mycorrhizal fungi between the stands because the standard deviation between plots in the same stand is larger.(3)MWD_FWwas positively correlated with cation exchange capacity,organic carbon,iron and aluminum oxides,sand content and fine root proportion,and negatively correlated with silt content,specific root length and very fine root proportion.We did not observe the effect of root density on MWD_FW,but root morphological characteristics significantly affected MWD_FW.Cation exchange capacity,root morphology and soil texture were the most important factors.MWD_WSwas positively correlated with clay content,organic matter,cation exchange capacity and amorphous alumina,and negatively correlated with pH.The effect of stand on MWD_SWwas small.When afforestation is carried out in areas with serious soil erosion,we should pay more attention to the morphological characteristics of plant roots and the effects of vegetation on soil texture and cation exchange capacity.(4)Under the same experimental conditions,the stability of soil aggregates with different initial particle sizes was different.In general,there was no significant difference in the aggregate stability when they were wetted with pH3 and pH5 acid solutions,but the aggregate stability changed more sharply when they were treated with pH1 acid solution.Acid rain with pH value of 3 and 5 can improve the ability of aggregates to resist slaking and mechanical breakdown,and reduce the ability of aggregates to resist differential swelling.However,the improvement of aggregate stability by acid rain is at the cost of soil acidification.