Properties Of Soil Water Dispersible Colloids And Aggregate Stability Evaluation Of Ziwuling Forest Soils

The ecological environment in the Loess Plateau is fragile,the soil structure loose,and the soil erosion serious.Vegetation restoration and reconstruction is an effective measure to improve the degraded ecosystem in this area.Soil aggregate is the basic unit of soil structure,and its size distribution and stability are important factors affecting soil fertility and soil quality.A lot of work has been carried out on soil physical and chemical properties,microbial properties and ecological environment impact during vegetation restoration.However,the changes of soil electrochemical properties,colloidal properties and aggregate stability after vegetation succession to forest communities are still unclear.In this paper,the surface soil（Populus davidiana,Betula platyphylla and Quercus liaotungensis）of typical forest in Ziwuling of the Loess Plateau was taken as the research object,and the changes of its basic physical and chemical properties,nutrient content and electrochemical properties with vegetation restoration were analyzed.The water dispersible colloid with soil diameter<2μm was extracted to study the changes of its physical and chemical properties,electrochemical properties,morphology,functional groups and colloid stability.On this basis,the particle size distribution and stability characteristics of soil aggregates were studied by dry sieving method and alcohol and pure water rapid wetting wet sieving method,combined with the calculation of interaction force between soil particles,and then the internal reasons for the stability change of aggregates in the process of forest vegetation restoration were explored,which provided some theoretical guidance and technical support for forest management in the Loess Plateau.The main results are as follows:（1）With the increase of vegetation restoration time,soil bulk density decreased,water content increased,organic carbon content（SOC）and cation exchange capacity（CEC）gradually increased,soil calcium carbonate（Ca CO₃）content gradually decreased,and soil particles tended to be sticky.The contents of total nitrogen（TN）and total phosphorus（TP）in forest soil were lower than those in farmland soil.The content of nitrate nitrogen（NO₃^--N）in forest soil increased gradually with the time of vegetation restoration,while the content of ammonium nitrogen（NH₄⁺-N）was opposite.The total potassium（TK）content of forest soil did not change much,while the available potassium content was greater than that of farmland soil.Except for the soil of Quercus liaotungensis-160 years plot,the C/N and C/P of other forest soils showed a trend of increasing gradually with vegetation restoration,and were higher than those of farmland soil.There is P limitation in forest soil.With the progress of vegetation restoration,the surface charge number,surface charge density and surface electric field intensity of forest soil increased,while the specific surface area decreased.Correlation analysis showed that the amount of organic carbon,cation exchange capacity and conductivity were the most important factors affecting the changes of soil surface electrochemical properties in Ziwuling forest.（2）The Ziwuling forest soil had the highest proportion of 0.053–0.002 mm components,and the mass fraction of soil water-dispersible colloids was 12.46%–16.34%.The soil colloid content was the highest in the farmland,followed by the Quercus liaotungensis-130 years plot,indicating that long-term ripening and tree growth can promote the formation of secondary minerals.The soil water dispersible colloidal particles are mostly irregular flakes,and the main clay minerals are hydromica,chlorite and kaolinite.Compared with farmland soil colloids,forest soil water-dispersible colloids have more aromatic groups and less hydrophilic and polar groups.The contents of SOC,CEC,TN and TK in soil water-dispersible colloidal particles gradually increased with vegetation restoration time.With the increase of vegetation restoration time,the CCC value of soil water dispersible colloids gradually increased,and the suspension stability was significantly enhanced,which was related to the increase of CEC caused by the increase of SOC and Ca CO₃ content.The characteristics of soil colloids are the concentrated expression of soil properties.The contribution rates of soil water dispersible colloids to surface charge,available phosphorus and organic carbon were 80.22%,67.93%and25.07%,respectively.Compared with farmland,vegetation restoration reduced the release of fine particles in forest soil.Vegetation restoration also significantly reduced the amount of particulate phosphorus in forest soil,thereby reducing the environmental risk of vegetation restoration forest community soil deterioration downstream water quality.（3）Under the guidance of soil external force,the content of soil aggregates measured by dry sieve and alcohol rapid wetting method was mainly 2–0.25 mm and 0.25–0.053 mm,followed by 5–2 mm.Under the combined action of soil internal force and external force,the soil particle size distribution measured by pure water rapid wetting method showed an inverted V-type,and the content of 2–0.053 mm increased significantly.The proportion of large aggregates(R_0.25),mean weight diameter（MWD）and geometric mean diameter（GMD）of soil aggregates measured by the three methods showed that the dry sieve method was slightly larger than the alcohol rapid wetting method,and the fractal dimension（D）was opposite.The relative mean weight diameter index(R_MWD)value and aggregate destruction rate（PAD）value of forest soil in Ziwuling were farmland>forest soil,indicating that with vegetation restoration,soil internal force decreased,so that the degree of aggregate fragmentation decreased and the stability of soil aggregates increased.The net resultant force between forest soil particles was much smaller than that of farmland soil,indicating that vegetation restoration enhanced the stability of soil aggregates.With the progress of vegetation restoration,the destructive effect of soil internal force on aggregates was weakened,and the stability of aggregates was enhanced.