Dynamics Of Aggregate Stability Of The Yellow Soil Under Acid Rain Background In The Three Gorges Reservoir Area

Acid rain and soil erosion are important environmental problems in the Three Gorges Reservoir area.Soil aggregate stability is an important factor in soil erosion processes.This paper amied at the soil degradation caused by acid rain in the Three Gorges Reservoir area,a systematic study on the dynamic mechanism of soil aggregate stability and its control factors was conducted.The objectives of this study were to investigated the dynamic mechanism of soil aggregate stability under drying-wetting cycles,the process of soil crusting and acid rain events.The interaction relationship between acid rain and typical land types and main factors of soil aggregate stability under acid rain background were also explored.Main results of this study were as follows:(1)The dynamic changes of soil aggregates and the redistribution of the particle size of soil aggregates under dry and wet alternate action were investigated.It was found that 1-2 drying and wetting alternation is beneficial to the aggregation of micro aggregates within the aggregate>0.5 rnm diameter,which can enhance the resistance of soil aggregates to the fast wetting treatmetn.However,the polymerization is unstable.When the 3 dry and wet alternate action was used,the polymerization disappeared,and the 0.5 mm particle size of the micro aggregate formed by the new polymerization showed a breaking trend.The resistance of soil aggregates to "dissipation" is also decreasing.The stability of soil aggregates under the slow wetting treatment and shaking treatmetn was always decreased with the increase of dry and wet alternate times.Dry wetting effect on the degree of soil aggregate stability under the effect of minimum dissipation,while the clay swelling and mechanical damage degree of aggregate stability under the action of the larger,and the effects of rewetting on large aggregates stability than small aggregates.(2)The dynamic changes of aggregation and stability of soil aggregates under the interaction of acid rain and litter are analyzed.Litter addition can promote soil respiration rate,greatly promote the aggregation of soil aggregates,promote the formation of>2 mm aggregates and enhance the stability of soil aggregates.Acid rain has a significant inhibitory effect on soil respiration rate,and greatly inhibits the aggregation of soil aggregates.The specific performance is to inhibit the 2 mm large aggregate in soil and reduce the stability of soil aggregates.The effect of acid rain leaching on the stability of soil aggregates was significantly different between different aggregate treatment methods.The effect of acid rain on soil aggregate stability was not obvious under rapid humid treatment.Under the slow humid and mechanical crushing treatment,acid rain leaching greatly reduced the stability of soil aggregates.With the decrease of pH value of acid rain,the more significant it has on the stability of aggregate.(3)The dynamic changes of soil aggregate stability during the development of the crust were revealed.Under the action of natural rainfall,the bulk density of the surface soil increased gradually,indicating that the crust gradually developed and completed,but its stability had a critical threshold.When encountering a critical threshold rainfall,the crust of the topsoil would break up and redevelop.The stability of the topsoil aggregates was significantly changed under the action of rainfall.With the increase of accumulated rainfall,MWDFW showed an obvious upward trend,while the increase of MWDsw and MWDws was small.Under the influence of natural rainfall,there are great differences in the stability of soil aggregates.The stability of the aggregates is generally higher than that of the lower soil.With the increase of rainfall,the difference between them increases.(4)The interaction between acid rain and typical land use types in the Three Gorges Reservoir Area(Chongqing section)is clarified.In the studied area,the pH value of acid rain is lower and acid rain frequency is higher in autumn and winter seasons,compared to spring and summer seasons,due to the higher frequent rainfall events in autumn and winter seasons.The influence of canopy layer on the composition of acid rain has significant difference in spring-summer and autumn-winter.The main result is that after spring and summer rainfall,pH rises and then falls in autumn and winter.The litter layer and soil layer have a significant influence on the acid rain composition,and the pH increases significantly after the rain passes through the litter layer and the soil layer.The buffer capacity of the same soil in different pH stages is very different.When pH>3,the main buffer effect is soil organic matter,CEC and exchangeable Al3+.When pH<3,the main buffer mechanism of soil is soil mineral weathering.According to the classification of acid damage capacity,all the four kinds of forest soil belong to the vulnerable and slightly vulnerable soil,and the acid settlement is very easy to cause damage to the plants.(5)The stability and main factors of soil aggregate in typical land soil under acid rain are clarified.The soil acidification in the study area under the long-term acid rain leaching is serious,and the pH value of soil is below 4.1.The content of ion and cation exchange(CEC)in soil salt base is low,but the content of exchangeable A13+,soil organic matter and soil clay grain is higher.The stability of soil aggregates under different crushing mechanisms is quite different in the study area.The stability of soil aggregates in all 5 soil types shows MWDST>MWDSW>MWDFW at all sections.On the whole,the stability of the 5 types of land aggregates showed as follows:shrub(2.61 mm)>broad-leaved forest(2.62 mm)>mixed forest(2.31 mm)>farmland(2.22 mm)>bamboo forest(1.61 mm).The main factors influencing the stability of soil aggregates under different crushing treatments were different in the study area.The stability of small aggregates in FW treatment mode was mainly affected by exchangeable Al3+and K+.The stability of soil aggregates under the treatment of SW and ST was mainly influenced by the exchangeable Ca2+,Mg2+,clay content and organic matter content.The soil pH value and exchangeable Na+were negatively correlated with the stability of soil aggregates under 3 treatments(P<0.05).