降雨及聚丙烯酰胺(PAM)作用下土壤的封闭过程和结皮的形成

Sealing Process and Crust Formation at Soil Surface Under the Impacts of Raindrops and Polyacrylamide

封闭、结皮是土壤表面水土相互作用的结果 ,能显著减少土壤入渗、降低水资源的利用率 ;增加地表径流并导致土壤侵蚀。为深入理解封闭的作用过程和结皮的形成 ,利用 PAM能缓解封闭作用并抑制结皮形成的特性 ,进行了一系列的人工降雨模拟试验 ,试验采用 3个重复 ,包括 3个雨强 ( 5 0 mm/ h、1 0 0 mm/ h和 1 5 0 mm/ h)、4个坡度 ( 8.74 %、1 7.6 3%、36 .4 %和 4 6 .6 3% )和 3个 PAM覆盖率 ( 80 %、6 0 %和 4 0 % )。通过对比土壤表面的电子扫描显微镜 ( SEM)照片 ,可以认为土壤结皮是由结构结皮和沉积结皮构成 ,入渗量与时间的历时曲线反映了结皮的 4个形成过程。

Soil seals and crusts are formed at the surface of soil due to the impacts of raindrops and break of aggregates. Soil seals and crusts can significantly reduce soil infiltration rate and subsequently low the utilization of water resources, and increase runoff, which result in soil erosion. Study of the processes of seals and formation of crusts is of very importance to understand the interrelations of runoff, infiltration, and soil erosion under rainstorms. The objectives of this study were for better understanding of crust formation the processes and mechanisms of seal and crust. Experiments were conducted with rainfall simulation under laboratory conditions. These experiments involved three different rainfall intensities (50 mm/h, 100 mm/h, 150 mm/h); four slopes (8.74%, 17.63%, 36.4% and 46.63%) and four soil surface treatments (one control and three PAM coverage rates: 80%, 60%, 40%). The flume used for the experiments was formed platform, which is adjustable from 0% to 46.63% slope. The flume of 3 by 8 m was sub-divided with PVC plate into fifteen 800 cm long, 20 cm wide, and 38 cm deep mini-flumes for different soil treatments under the same laboratory conditions. The rainfall simulator is adjustable for its rainfall intensity, from 20 mm/h to 300 mm/h, and drop sizes from 0.6 mm to 3 mm.Experimental soils used in this study were from Inner Mongolia. A typical loess sampled from top layer of cultivated soils. The soil contents about 60% of silts and about 15% of clays.Soils, air-dried before passing through a 10mm sieve, were slightly compacted into the flumes for 15 cm in depth, over a 20cm thick layer of sand. The bulk density was 1.2 g/cm, about the same as that of the cultivated field. PAM mixed with dry soil was applied uniformly at the soil surface for different coverage rates of PAM accordingly.Each individual experiment run was divided into two consecutive stages. At the first stage, the designed rainstorm was turned on and runoff and sediment samples were collected from the downstream outlets in regular time (1, 3, 3, 5, 5, 5, and 5 min). As soon as the last sample was taken, the first experimental stage ended. After the first stage of experiment, the soil was set for 24 h before the second rainstorm was put on.Three replicates were adopted for each individual experimental treatment.Moist crust samples were taken at lower part of the flume and packed with filter paper and gauze. Samples were dried for at least ten days. After further desiccated, a small piece of the top layer of sample (crust) was severed carefully, then fitted on the top of the microscope stub with a thin layer of gold covering. The prepared sample was put into the scanning electric microscope (SEM). A series of vertical and horizontal photograph were taken at magnification of from 1500 to 3500 times.Crust classification varies in literature but there is in agreement on two major types: structural crust, which is formed with no involvement of any external imported material, and the depositional crust, which always involves external material into its construction.SEM photographs showed that the surface of control soils tended to have more compact pile up of particles, higher bulk density, and smaller pores, but on the other hand that the surface of soils treated with PAM have stable aggregation and porosity.Comparing the micromorphology of soil surface layer of the control with that treated with polyacrylamide (PAM), it can be concluded that soil crust should be consisted of depositional crust and structural crust. The conceptual model of soil crust has two main complementary processes: i), physical disintegration of soil aggregates and soil compaction caused by raindrops, and ii), chemical dispersion and movement of particles that clog the conducting pores and form a less permeable layer below soil surface. From the SEM micrograph of the crust, the effect of rain and runoff on soils was demonstrated. The impact of rainfall caused in situ particles of soils movement to form structural crust, and splash of raindrop and chemical dispersion c