刘金涛(1977—),男,河北唐山人,博士,教授,博士生导师,主要从事水文土壤学研究。E-mail:
全球平均土壤厚度仅约为1 m,但土壤厚度的空间分布信息在地貌、生态及水文科学等领域的研究和实践中具有重要价值。由于其具有显著的空间异质性,基于现有土壤制图产品、地球物理勘测及经验统计模型难以获取流域尺度土壤厚度分布信息,亟待发展土壤厚度预测的过程机理模型。本文回顾了土壤厚度演化模型理论方法的研究进展,评价了不同土壤生成及输移模型的适用性。研究指出土壤化学风化成土等机理仍不清晰是制约模型发展的理论瓶颈。此外,模型的方法体系有待完善,亟待进一步发展土壤生成率和输移率的函数形式及其参数的估计方法等,指出物理与随机结合的模拟方法以及基于数学物理途径的参数确定方法等有望解决模型应用中遇到的难题。最后,在土壤厚度演化模型基础上,提出发展基于流域协同演化理论的土壤发生学模型是定量预测土壤理化全要素发生所亟需突破的难点之一。
Global soil thickness is only about 1 m. Its spatial distribution is nevertheless crucial in many hydrological and ecological processes, and it also determines hillslope stability and channel initiation in geomorphological fields. Due to its significant spatial heterogeneity, it is difficult to obtain the soil thickness distribution on a catchment scale based on existing soil survey databases, geophysical investigations, or empirical models. Therefore, it is urgent to develop a process-based model for soil thickness prediction. In this study, methodologies and theories were comprehensively reviewed, and the applicability of different soil production and soil transport models were evaluated. This study pointed out that the mechanism of soil production by chemical weathering is still unclear and is a theoretical bottleneck restricting the development of soil thickness evolution models. Moreover, the methodology of the model still needs to be further developed, and it is urgent to develop and improve the parameter estimation methods and the adoption of equation forms for describing soil production and soil transport in such models upon applications. From our analysis, we inferred that a hybrid model combining stochastic and process-based models as well as mathematical physically-based methods for determining parameters may help solve many difficulties faced in model applications. Finally, we discussed the possible integration of soil thickness evolution models and soil pedogenesis models based on the theoretical frame of catchment coevolution for predicting soil thickness, texture, layering and organic carbon content variation in the landscape.
全球约有94%的陆地被土壤覆盖,但土壤的平均厚度仅有1 m左右。土壤构成了地球关键带脆弱且浅薄的表层[
土壤厚度(或活动风化层,Mobile Regolith),即地表到腐泥岩(Saprolite)的垂直距离,影响着植被覆盖度、生物多样性及水循环过程。土壤厚度的空间分布决定了流域水源划分、植被可利用水量和土壤碳及其他物质蓄积的下边界条件,对于判断山坡稳定性、河道源头点等具有重要的地貌学意义[
因此,有必要改进土壤厚度的表征方法并发展预测模型,以获取较高精度的土壤厚度空间分布数据。现有的土壤厚度预测模型按照模型建立的途径和模拟的方法可大体分为随机模型和过程机理模型两类[
在气候、地质构造等因素的作用下出露基岩逐步风化,风化前锋不断向下延伸并发育,形成具有一定厚度和层次结构的土壤、腐泥岩层(合称“风化层”),风化层中成土与输移作用的动态平衡塑造了地貌景观形态和土壤厚度空间分布(
山坡风化层剖面演化过程 [
Evolution process of weathering layer profile on hillslope [
随着土壤输移理论的发展,最早有关地貌演化的过程机理模型均引入土壤输移方程来模拟山坡地貌的演化过程,如Ahnert[
式中,
土壤的形成关乎全球土壤资源利用的可持续性,故而探寻土壤生成速率及其影响因素一直是国际土壤学界的研究热点。早在19世纪,Gilbert[
土壤生成速率随土壤厚度变化函数[
Variation function of soil production rate with soil thickness [
式中,
然而,基岩向土壤的转化是复杂的物理、化学风化过程,除土壤自身厚度外,水分及其对风化溶液的运移也控制着基岩化学风化的速度[
式中,
为了更准确地反映实际土壤生成速率与土壤厚度之间的函数关系,Dietrich等[
在重力、水力等外因作用下,山坡上的土壤会发生移动,引起地面高程的变化进而改变原有地形。土壤输移的定量表述对于土壤厚度演化过程的模拟起着关键性作用。按照驱动力的不同,土壤输移过程的数学物理模型大体分为二类(
土壤输移模型总结
Summary of soil transport models
模型类别Type of models | 公式Equations | 方法描述Method descriptions | 适用范围Application scopes | 公式Equations |
注:上式中 |
||||
流水沉积物侵蚀输移模型Sediment and erosion transport model | 当集水面积及坡度较大时,水力侵蚀作用对于塑造地貌和土壤厚度有重要作用 | 坡面流或河道径流等水力作用显著区 | (4) | |
线性扩散模型Linear transport model | 在重力驱动下,土壤输移以缓慢的蠕动为主,且输移通量与坡度呈线性关系 | 缓坡、地形发散的凸坡 | (5) | |
非线性扩散模型Nonlinear transport model | 对上覆一定厚度土壤的山坡,土层中的生物活动显著影响土壤蠕动,进而影响着土壤的输移通量 | 陡坡、地形收敛的凹坡、直坡 | (6) | |
在浅层滑坡的作用下,土壤输移通量随坡度接近临界值 |
陡坡、地形收敛的凹坡、直坡 | (7) | ||
对式(7)的改进,在浅层滑坡中考虑土壤厚度产生的影响 | 陡坡、地形收敛的凹坡、直坡 | (8) |
土壤扩散模型描述了土壤颗粒在重力作用下发生顺坡移动的现象,包括土壤蠕动及浅层滑坡过程。土壤蠕动主要由以下过程驱动,包括:雨滴溅蚀、树木倒伏、动物挖洞、土壤干湿及冻融变化等等。在坡度相对缓和且地形发散的凸坡地带,滑坡难以发生,最适宜发生蠕动,此时土壤输移通量与坡度呈线性关系(见
在流域尺度上,土壤厚度演化是水文及生态系统协同演化中的重要组成部分,影响着碳、氮等生源要素的生物地球化学过程。土壤厚度的空间分布还决定了流域水流的路径和水源划分,是水文、生态乃至地球系统模型中的重要参数[
Dietrich等[
近30年来,随着同位素技术(宇生核素、U系同位素)在地貌学领域的发展和应用,使地貌或土壤演化过程及其速率得以定量表达[
水循环是流域内岩石风化、土壤形成和生物活动及其中化学元素循环运动的主要驱动力[
母质在自然环境条件下发生物理风化和化学风化,在生物扰动作用下分散成细小的矿物颗粒,完成基岩向土壤的转化,并在侵蚀作用下达到稳定状态后形成具有一定厚度的土壤层。在这个过程中,可由土层厚度及土壤年龄粗略计算土壤形成的平均速率[
式中,
鉴于上述将流域或者山坡作为集总单元的方法会产生较大误差,研究者们将研究对象细化到土壤剖面上,并引入铀(U)系同位素(238U、234U、230Th)定量计算剖面土壤的生成速率[
随着加速器质谱仪(Accelerator Mass Spectrometry,AMS)分析技术的不断进步,由高精度宇生核素浓度估算土壤生成速率的新方法得以运用,主要有测定土壤中大气成因宇生核素[
式中,
目前对于土壤输移速率的研究方法主要有模型法、3S技术法和放射性同位素示踪法,其中利用放射性核素半衰期互异的特点示踪不同时间尺度上的土壤输移速率是本领域国际研究热点问题之一。土壤中MCN累积量可表征土壤的停留时间及输移通量[
式中,
尽管土壤厚度演化模型具有较强的数学物理基础,然而应用该类模型模拟土壤厚度的时空分布时仍有诸多的不确定性,如土壤生成率和输移率函数形式及其参数的获取等。统计或随机类模型不需考虑土壤演化的复杂机理,具有结构简单、参数较少的优点,但此类模型参数的确定须依赖大量的实测数据,模型的外推能力较差[
上文分析(1.1和1.2节)表明化学风化是控制土壤生成的重要过程,化学风化溶液运动的最小阻抗路径及其输移速度和能力决定了岩石风化及成土作用的快慢[
在土壤厚度演化模型中,土壤生成速率可通过2.1中介绍的方法测定,如采用稳态法测定腐泥岩中的10Be、26Al浓度[
是否有物理途径来确定模型参数并与核素法相互验证?在大多数山坡地貌演化模型中,局部稳定假设是简单适用的[
土壤是开放性系统,其形成演化规律与自然环境条件密切相关,土壤发生学模型就是要定量刻画在环境因素驱动下的土壤厚度、层次和质地等如何形成、演化等过程。早期的土壤发生学模型具有明显的经验和定性色彩,如土壤形成的因素学说将土壤定义为环境因素(如气候、生物、地形、母质和时间)的函数[
由于土壤系统的复杂性,传统的土壤发生学模型(如因子模型[
关键带土壤圈层是水、碳、氮及其他元素与空气、地下含水层发生交换作用的重要场所,土壤也是陆地生态系统中最大的有机碳库,是生源要素积累、封存和生物地球化学反应的前缘。因此,精确的土壤厚度制图是改进流域生物地球化学、水文及地球系统模型中水、碳、氮等元素动力过程模拟的关键。本文系统梳理了土壤厚度演化模型的理论方法研究进展,指出仍需进一步揭示土壤形成的化学变化过程以定量解析复杂的化学侵蚀通量,并帮助建立更为完整的连续性方程。为了推动土壤厚度演化模型在水文、生态和地貌等领域的应用,本文给出了有关模型发展的三点建议:(1)发展物理与随机结合的模拟方法,以解决机理模型应用时存在的诸多不确定性;(2)研究基于数学物理途径的参数确定方法,给出同位素测定方法的替代解决方案,推进模型理论的实际应用;(3)在土壤厚度演化模型基础上,融合土壤发生学模型发展流域协同演化模型,提升对地球生态系统物质循环预测的能力水平。
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