冻融交替后不同尺度黑土结构变化特征

冻融交替是改变黑土结构、加剧土壤侵蚀的重要因子。以典型黑土区耕作土壤为研究对象,采用野外季节性冻融循环与室内模拟冻融循环相结合、X射线计算机断层摄影(CT)与扫描电子显微镜(SEM)相结合的方法,通过水分物理性质、团聚体破坏率、孔隙数目、孔隙面积、孔隙成圆率、孔隙Feret直径的测定与分析,研究了冻融交替后0—40 cm、40—80 cm和120—160 cm3个土层以及田间季节性冻融环刀、室内模拟冻融CT扫描和室内模拟冻融SEM3种方式下黑土结构特征的变化规律。结果表明:冻融交替能够对不同土层和不同尺度的耕地黑土结构产生不同程度的影响。季节性冻融后,表层土壤容重升高,非毛管孔隙度和持水能力显著降低(P0.05),40—80 cm土层团聚体破坏率增加40.97%(P0.05),土壤抗蚀性有所削弱,120—160 cm土壤没有受到季节性冻融的显著影响。CT扫描尺度上,3个土层均以1—2 mm径级的孔隙数目为最多,形状也相对规则、接近圆形;冻融循环没有对表层土壤大孔隙结构产生影响,却能够显著降低40—80 cm土层范围内大孔隙面积以及Feret直径(P0.05)。SEM扫描显示冻融后土壤表面粗糙度增加,颗粒松散、脱离,孔壁断裂,证明了冻融交替对土壤微结构的破坏作用;同时结合电子能谱的元素分析可知冻融交替能够改变土壤颗粒表面化学特征。

Effects of freeze-thaw cycles on black soil structure at different size scales

Alternating freezing and thawing is a critical factor associated with soil structure change and accelerates soil erosion in the black soil region of Northeast China. Based on the soils sampled from fields of the black soil region, the effects of freeze-thaw cycles on soil structure at different soil depths (0-40 cm, 40-80 cm, 120-160 cm) and size scales (field core sampling scale of seasonal freeze-thaw cycles, computerized tomography CT] scale of artificial freeze-thaw cycles, and scanning electron microscope SEM] scale of artificial freeze-thaw cycles) were studied. We measured and analyzed the hydro-physical properties, percentage of aggregate disruption (PAD), pore number, pore area, pore roundness, and Feret diameter of pores in the field and lab by using CT and SEM methods. Results showed that: freeze and thaw alternation affected black soil structure from different soil depths and different size scales both in seasonal field condition and artificial freeze and thaw cycles condition. At the scale of seasonal freeze-thaw cycles, bulk density of top soil (0-40 cm) increased, total porosity decreased significantly, with non capillary porosity decreasing dramatically. Water holding capacity decreased consequently(P < 0.05). Within the soil depth of 40-80 cm, bulk density and porosity did not change significantly, but PAD increased by 40.97%(P < 0.05)resulting in weak erosion resistance. Hydro-physical properties and aggregate breakdown characterization of soils in the depth range of 120-160 cm were not changed significantly by seasonal freeze and thaw cycles. At the size scale of CT, the alternation did not change the characterization of macro pores in 0-40 cm but significantly reduced the average pore area, roundness and diameter after freeze and thaw cycles. While the average area and diameter decreased significantly in the 40-80 cm depth, this might result in poor infiltration and water movement to the subsoil. Pore area of 1-2 mm and > 5 mm decreased 9.58% and 42.19% (P < 0.05), respectively. Structure of soils from 120-160 cm underwent by artificial freeze-thaw cycles showed similar results as field scale did, were not affected significantly by alternate artificial freeze and thaw cycles. The number of pores with diameter between 1 and 2 mm was predominant the pore size distribution through three soil depths. At the scale of SEM, the roughness of the soil surface and alveolate cells were significantly increased; we also observed micro-aggregate displacement and rearrangement, and disruption of the pore walls. Integrating electron spectroscopy results of 40-80 cm depths, carbon and calcium did not change after 6 freezing and thawing cycles, but oxygen group elements decreased. Magnesium, silicon, aluminum, potassium and iron elements increased to different degrees, and the total increment equaled to the decrease quantity of oxygen group element. The packing arrangement of oxygen group elements and metallic oxide are the key factors to determine the type and surface chemistry of soil clay minerals, so the amount of change in oxygen group and other elements gives indirect evidence that freeze and thaw cycles can affect soil microstructure. But to determine whether or not freeze and thaw cycles would change the crystal lattice structure of clay minerals, further research will be needed and must be approached from the view of soil chemistry and minerology.