黄土区坡耕地土壤结皮对入渗的影响

黄土高原地区,坡面土壤水分是生态建设的关键问题。以黄土高原坡耕地人为管理方式为背景,在室内人工模拟降雨条件下采用等高耕作和人工掏挖两种措施,并且设计直线坡作为对照,研究不同耕作措施下土壤结皮的形成特征,同时从降雨-入渗的角度研究两种类型结皮(结构结皮和沉积结皮)对坡面土壤水分入渗的影响。研究结果表明:土壤结皮阻碍坡面土壤水分入渗,结皮坡面产流时间早,且土壤累积入渗量明显低于无结皮坡面;采用Kostiakov模型、Horton模型、蒋定生模型对坡面土壤水分入渗过程进行优化模拟的结果表明蒋定生模型适用于描述本研究坡面土壤水分入渗的特征;耕作措施造成的微地形对土壤结皮的类型有很大影响,在洼地径流携带泥沙堆积形成沉积结皮,地势较高处降雨雨滴直接打击形成结构结皮。研究两种类型结皮发现,沉积结皮相对于结构结皮密度高且孔隙度低,并且两种类型结皮对坡面土壤水分入渗的影响存在差异,沉积结皮平均减渗效应为37.13%,结构结皮平均减渗效应为19.79%,因此,沉积结皮更大程度影响坡面土壤水分入渗。

Effects of soil crusts on infiltration in slope land in the loess area

Infiltration is a process of water penetration through the soil surface. During rainfall events, the amount of infiltration determines the quantity of water available for plant uptake, and abundant water helps increase crop yield. In the arid and semi-arid regions, water availability is one of the greatest factors affecting vegetation restoration and ecological environments, especially in the Chinese Loess Plateau area. Proper tillage practices can significantly alleviate certain soil-related constraints to crop production, such as compaction, crusting, low infiltration, poor drainage, and unfavourable soil moisture and temperature regimes. Traditional tillage practices used in the Loess Plateau, include contour tillage and artificial digging, can increase soil roughness and decrease runoff and soil erosion. However, the soil in the Loess Plateau has been reported to be poor in structure, susceptible to erosion and liable to form soil crusts during rainfall events. Soil crusts can reduce the amount of infiltration and hence soil moisture availability. The objective of this laboratory study is to explore characteristics of the formation of different types of soil crusts on slopelands different in soil micro-topography and effects of structural crusts and depositional crusts on soil water infiltration from the rainfall-infiltration perspective. For that end, an indoor experiment was designed to have simulated slopelands adjustable to 5°, 10°, 15° and 20° in slope gradient, three different tillage treatments (contour tillage, artificial digging and straight slope as control) and artificial rainfall events. Two rainfall events were simulated in the experiment. After the first rainfall event, structural crusts and the depositional crusts were formed on the slopeland 0° in gradient after 30 min of 80 mm h-1 rainfall, and then the slope lands were adjusted to 5°, 10°, 15° and 20° in gradient, step by step and each subjected to 30 min of 80 mm h-1 rainfall, to investigate rain water infiltration rates in the slopelands as affected by tillage pattern and slope gradient. Results show that soil crusts retarded water infiltration into the soil; consequently, runoff formed earlier on slopes with crusts than on slopes without crusts, and cumulative rain water infiltration was much lower on the former than on the latter. Infiltration rate decreased with increasing slope gradient regardless of the effect of soil crusts. Regression analysis of model simulation indicates that the Jiang Dingsheng model was optimal to describe variation of soil infiltration rate in this study. What type of soil crusts would be formed was significantly affected by tillage treatments. The structural crusts tended to form on ridges, which were directly impacted by rain drops while the depositional crusts formed in furrows between ridges as sediments in runoff deposited. Meanwhile, the structural crusts were lower in bulk density, but higher in porosity and in shear strength than the depositional crusts. The effects of the two types of soil crusts on soil water infiltration rate were different. Depositional crusts reduced accumulative water infiltration by 37.13%, while structural crusts did by 19.79% compared with the accumulative water infiltration on uncrusted slopes. It is quite obvious that the effect of depositional crusts reducing soil water infiltration is much higher than that of structural crusts.