| In Hilly red soil region in the southeast China, soil degradation caused by soil erosion have greatly endangered agricultural development of this region. It is urgent to study and find a effective way to control soil erosion. Soil erosion is a complex process, rainfall and runoff supply energy for erosion, and soils act as object. Though many soil properties affecting soil erosion directly or indirectly, soil structure was thought as the most important one. Soil structure not only aversely affects soil by reducing infiltration rate, water-holding capacities, soil biota, and crop productivity, but also act as the foundation of soil quality assessment. To study the relationship between soil structure changes and soil erosion process, and soil structure stability mechanisms is helpful for the determination of soil erodibility factor and the prevention of soil erosion in this region. In this research, 10 soils derived from shale, Quaternary red clay and granite were studied. Soil structure stability and its mechanisms were studied by different methods, and the relationship between soil structure changes and erosion process were also studied by simulated rainfall experiment. The main results as followed:First, In the erosion process, erosion broke down soil structure and then the breakdown structure promoted the development of erosion, which couldn't be separated. But, in the past researches, the two parts were often studied individually, and it was not suitable for us to understand the interaction between erosion and soil structure changes. In our study, the changes of soil structure and erosion process during rainfall were studied deeply, and their relationship were studied, which were help to promote the combination of soil erosion process and mechanisms.Our research studied the pore characteristic of three soils by using diagonal image. According the pore characteristics, the soil surface lay were divvied into three microlayers. And changes of the pore characteristics, such as pore size, pore shape with time and space were studied firstly, which were important to the quantitative analysis of the soil structure changes duiring rainfall.The ranking of the soil aggregate stability differed with different measurement method, soil aggregate size, stability parameters and breakdown mechanisms. In the wet-sieve method, based on parent materials, the aggregate stability of the soils were ranked in the order, granite> Quaternary red clay > shale. Based on erosion degree, the rank for the soils derived from granite was sight >medium>severe, While for the soils derived from Quaternary red clay, the rank was medium>severe>slight. Thefragment-size distribution (FSD) of soil aggregates differed with samples. More than 40% of the fraction of soils derived from granite(TGl,TG2) were at the size of >5mm and 5-2mm , and for the soil derived from shale(HS), more than 65% of the fractions were at the size of <0.25mm. The FSD of soils derived from Quaternary red clay were quite different themselves. The MWD of those six soils distributed from 0.41mm to 2.0mm, and the percent of >0.25mm ranged from 36% to 86%.Differed with parent material, the structure stability of the soils derived from granite were represented by coarse gravel, while the structure stability of the soils derived from shale and Quaternary red clay were represented by the stability of aggregates. The mechanisms of aggregate breakdown of the two latter were ranked in the order, slaking>mechanical breakdown>micro-cracking. The effect differed with soil and the initial size of aggregate. The small the aggregate was the higher the aggregate stability according NMWD.The aggregate stability of the soils were not related to SOM, pH, CEC, and the activity, of Fe(AL, Si)oxide, but were significant correlated to Fed, Ald, Alo, Sio and clay%. The results indicated that to the soils rich in sequioxide but poor in SOM, SOM could not play an important role on the stability of aggregate.Second, according the structure changes and the development of microlayers of soil surface, aggregate stability was important to maintain the structure. During the simulated rainfall, the surface structure of soils had a series of changes. For the soil derived from shale, at first, the unstable aggregate broken down in a large amount of finer particles, then the particles were compacted and formed a dense crust layers, and the density and the depth developed with time. The surface structure changes of soils derived from Quaternary red clay differed with soil aggregate stability. For the sample HQ1and HQ2 of which the stability were relatively unstable, aggregates were easier to be broke down and form soil crust than which of the samples HQ3 and HQ4, but the degree of soil crust development were far from which of soils derived from shale. The surface structures of HQ3 and HQ4 showed high stability. Which could not be observed obvious aggregate breakdown and crust formation. The soils derived from granite were very unstable, and crust could only be examined in TGI, and the fine particles of TG2 and TG3 loss seriously which caused the sandification significantly during erosion process.According to the microlayers development of soil surface, the soil surface divide into three layers: compacted layer or crust layer(Ll), transition Iayer(L2)and undisturbed layer(L3).Based on the percent of the pore area of the layer area (PA%) and the pore size, the ranking of the three layers were LKL2500 μ m. Usually, the shapes of smaller pore were round, while the larger size pores were irregular and elongate except HQ3. The pore characteristics also changed with time. The PA% of L1 of HS and HQ2 were declined with time, which declined from 2.25% and 3.72% to 0 and 0.37% in 30min. But the PA% of L2 didn't declined obviously contrast to L1.During the erosion, the pore shapes also had some changes: the composite pore shapes of round, irregular and elongate turned to be round only in L1.Third, the erosion process was affected by raindrop kinetic energy and soil aggregate stability. Getting rid of raindrop kinetic energy by covering, the runoff yield and sediment yield reduced 5%~30% and 27%~76% respectively. The relationship between runoff yield and sediment yield were power function with covering, and the relationship between runoff yield and sediment yield were simple liner correlation with out covering . The sample TG3 had the highest sediment yield, and then HQ1and TG2, the sediment yields of the other four samples were lower relatively, the ranking of which were similar to the ranking of soil aggregate stability.The sediment from high-clay soils had more stability aggregates and less primary particles than high-sand and high-silt soils, therefore it was much coarser. The runoff electively transported 0.002-0.02mm particles, and contained 26.68%- 60.33% of this class particles in sediment. The transportability of thin-flow and its selection to sediment particles decrease when splash was eliminated. Fed, Ald, Alo and Sio decreased with the increase of sediments size of soils derived from granite. They were depleted in bigger particles and enriched in smaller particles. It made small particles more stable than the bigger ones. Fed, Ald and Alo increased with the increase of sediments size of soils derived from shale and Quaternary red clay. They were enriched in bigger particles make big particles stable.Soil aggregate stability was important to soil erosion. For the soils derived from shale and Quaternary red clay, most of the parameters of microaggregate and macroaggregate showed significant liner relationship with soil loss. And in the given condition of presetting the soils, the main mechanisms of aggregate breakdown and soil erosion were mechanical breakdown and transportation. The Fed, Ald, Alo, Sio and clay% affected soil erosion indirectly.
|
PDF Full Text Request