|Alternative Title||Characteristics of Macropores and Preferential Flow in Farmland Purple Soil|
|Place of Conferral||北京|
|Keyword||土壤大孔隙 水力学参数 微ct扫描 穿透曲线 Cxtfit 生物炭|
本论文以川中丘陵区紫色土耕地为研究对象，通过圆盘入渗仪野外原位监测、原状土微CT扫描（Micro-Computed Tomography，Micro-CT）和土柱入渗穿透曲线试验和模型模拟，系统研究了紫色土耕地土壤大孔隙结构特征及水分优先迁移规律，同时采用CXTFIT软件模拟出流动态变化，并运用优先流定量评价指标PFSP，阐明大孔隙对优先流的发生机理。主要研究结果和结论如下：（1）对耕地翻耕前后耕作层与非耕作层原状土壤样品进行微CT扫描（Micro-CT）分析，发现不同深度的土壤中存在丰富的、大小不同、形状各异的大孔隙。耕作层表层2~7cm、耕作层亚表层10~15cm、非耕作层20~25cm和30~35cm的大孔隙度分别为0.058、0.017、0.025和0.020m3 m-3。翻耕后，耕作层2~7cm和10~15cm土壤的大孔隙度分别减少42.3%和49.1%，平均孔隙形状因子分别降低2.4%和1.2%。尽管翻耕活动改变了土壤大孔隙结构特征，但对土壤持水性能和导水特性均无显著性影响，说明每年两次翻耕活动所形成的土壤孔隙结构能够满足土壤水分保持和快速下渗的需求。（2）通过实验测定和模型估算获得土壤基质和大孔隙的饱和导水率。2~7、10~15、20~25和30~35cm土壤基质的饱和导水率分别为2.7、1.8、1.6和1.3 cm h-1，对应土层深度大孔隙的饱和导水率分别为28.9、3.3、4.2和4.1 cm h-1，大孔隙对土壤饱和导水率的贡献率分别为91.5%，64.6%，72.9%和76.4%。这表明，虽然大孔隙只占总孔隙度的一小部分（4.0%~13.5%），却是土壤中水流通量的主要运输通道。（3）室内土柱模拟降雨试验结果表明：原状土柱的Br-穿透曲线具有明显的不对称性和拖尾特征，说明土壤中优先流和基质流共同发生。以填装土柱水流为平衡基质流计算，耕作层0~20cm原状土柱中优先流的导水贡献率为66.2~68.5%，而Br-累积淋出量占总淋出量的62.3~66.1%。对于非耕作层20~40cm，土壤优先流导水贡献率为0.2~1.7%，而Br-随优先流运移的比例却达14.5~20.5%。说明耕作层土壤中优先流现象远比在非耕作层土壤中更为显著。（4）采用CXTFIT拟合不同土柱实验中的优先流参数。结果表明：0~20cm原状土柱的孔隙流速V、水动力弥散系数D明显大于20~40cm原状土柱的对应值；阻滞因子R则呈现相反规律，表明优先流越发育的土柱，R值越小，这是因为优先流的存在使部分Br-直接通过大孔隙优先流出，阻滞作用较弱；对于动水区比例因子β，0~20cm原状土柱的动水比例大于20~40cm原状土柱的对应值，说明优先流发育的土柱中可动水区的比例较大；0~20cm翻耕后原状土柱的两区质量交换系数ω最小，其他土层深度的ω值比较接近。（5）基于室内实验结果和CXTFIT软件模拟结果，计算得到PFSP综合指标值。PFSP值可以将优先流对穿透曲线的贡献与其他机理的贡献区分开来。结果表明：优先流作用对穿透曲线延展量的贡献率最大，其次是两区交换运移作用，最小的是水动力弥散作用。这一结果与实验分析结果一致，由此说明PFSP值可以作为定量刻画优先流的指标，PFSP值越大，优先流的作用越大。（6）通过野外坡耕地小区试验，研究施用秸秆生物炭一年后对川中丘陵区紫色土耕作层（2~7cm和7~12cm）土壤水力学参数、土壤大孔隙度以及不同孔径大孔隙对饱和导水率的贡献率的影响，结果表明：施用生物炭降低紫色土中植物无法利用的滞留水（θr）和易流失的结构性孔隙水（θstr）的含量，增加了对植物有效的基质性孔隙水（θtxt）含量（由0.058±0.003 m3 m-3增加至0.085±0.002 m3 m-3）。生物炭施用后，2~7cm和7~12cm土层中对产流起主导作用的直径>250 μm的有效大孔隙度分别平均增加0.007和0.001 m3 m-3，其中直径>1000 μm的大孔隙增加最为明显，平均增幅高达0.003 m3 m-3，提高了土壤导水率，有利于减少地表径流及土壤侵蚀的发生。;
A combined application of methods including disc permeameter, Micro-Computed Tomography (Micro-CT), Bromide tracer test with numerieal modeling was conducted to analyze soil macropore characteristics and water transport processes in a farmland purple soil in the hilly area of central Sichuan Basin, China. The breakthrough curves (BTCs) obtained from lab-scale soil column experiment was also analyzed to characteristize the preferential transport and evaluates the extent of preferential flow based on model results of CXTFIT sofeware. The main conclusions of this study are as follows:(1) The undisturbed soil cores taken from different layers of the farmland soil prior to and after tillage were scanned using Micro-CT. It indicated that macropores at different depths have a large variety of distributions, amount and shape. The macroporsity at depths of 2~7, 10~15, 20~25 and 30~35 cm was 0.058, 0.017, 0.025 and 0.020 m3 m -3, respectively. After plowing, the macroporsity at the depths of 2~7 and 10~15 cm was decreased by 42.3% and 49.1%, with the average pore shape factor decreased by 2.4% and 1.2%, respectively. In spite of such changes, there is no significant difference in the water retention capacity and hydraulic conductivity of the soil after plowing activity. This indicates that soil pore structure formed through two non-inversion plowing operation every year can hold plant-available water and thus enhance water permeability.(2) Saturated hydraulic conductivity (Ks) for soil matrix and macropores was obtained from lab-scale measurment and model fitting. The Ks values of the soil matrix at the depths of 2~7, 10~15, 20~25 and 30~35 cm were 2.70, 1.81, 1.56 and 1.27 cm h-1, respectively. The corresponding Ks values of soil macropores at four depths were 28.88, 3.30, 4.20 and 4.11 cm h-1, respectively, which have very high contributions (91.5%, 64.6%, 72.9% and 76.4%, respectively) to the Ks of the undisturbed soil. This indicates that macropores perform as the major contributing pores to the fast drainage at four depths of the soil, even thourgh they constitute a very low percentage of the total porosity (4.0%~13.5%).(3) According to the results of simulated rainfall experiment, the breakthrough curves (BTCs) of Br- obtained from undisturbed soil column exhibited early breakthrough, non-symmetry and increased tailing, which indicates the presence of both preferential flow and matrix flow. The preferential flow for Br- accounted for 66.2~68.5% and 0.2~1.7% of total flux in the undisturbed soil columns at depths of 0~20 and 20~40 cm, respectively. On the other hand, the accumulative mass of Br- leaching in the preferential flow was 63.2~66.1%and 14.5~20.5% of total leachate mass for the two depths, respectively. The result shows that the preferential flow accounted for only a small ratio of the total flux, but had caused relatively more solute transport.(4) Transport parameters of Br- were obtained by fitting its breakthrough curves in the undisturbed soil column to a two-region model using CXTFIT software. It was found that the values of pore velocity V and hydrodynamic diffusion coefficient D for the soil at the depth of 0~20 cm were higher than those at 20~40 cm depth, while the retardation factor R, with higher value indicating more preferential flow developed in the soil, showed contrast trend for the two depths. This was due to the fact that Br- quickly migrates with preferential flow through soil macropores which may weaken the retardation effect. As for the dynamic watershed scale factor β, the value was higher at the depth of 0~20 cm than that of 20~40 cm, indicating the higher percentage of flowing water in the soil column that has better-developed preferential flow. The mass transfer coefficient ω value of the undisturbed soil column taken at 0~20 cm after plowing activity was the lowest, while the values were close among other soil columns.(5) The integrated parameter of PFSP, which can ascertain the contribution of preferential flow out of other mechanisms, was calculated based on the results of BTCs and CXTFIT modeling of the undisturbed soil column. The data showed the contribution to the extended quantity of BTCs followed the order of preferential flow > two-region effect > hydrodynamic dispersion. The results also indicate that PFSP can be a useful index in quantifying the extent of preferential flow in the field where BTCs could be measured.(6) A one-year field trail of biochar application was carried out in the sloping farmland plots with aims to investigate effects of bichar amendment on soil hydraulic properties, pore size distribution and its contribution to flow in two layers (2~7 and 7~12 cm) of cultivated soil. As compared to the control treatment, an incease in the soil’s capability to hold plant-available water was observed, which was caused by a significant increase in soil matrix pores (θtxt) that can store water available to plants (from 0.058±0.003 m3 m-3 to 0.085±0.002 m3 m-3, P<0.05). On the contrast, the residual water content (θr), this is unavailable to plants and water content in structure pores (θstr), which is easy to be drained out, were decreased. After one year of biochar application, the effective porosity (pores with R > 250μm) increased by 0.007 and 0.001 m3 m-3 at depths of 2~7 and 7~12 cm, respectively. Particularly, the amount of pores with R > 1000 μm increased most remarkedly, reaching 0.003 m3 m-3 for the two depths, respectively. Such improvement due to biochar application poses a positive effect with the aspect of enhancement of water permeability of the soil which can reduce surface runoff and thus the potential soil erosion.
|王红兰. 紫色土耕地的大孔隙结构特征及优先流研究[D]. 北京. 中国科学院大学,2016.|
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