The Soil Production Function: The Key Linkage Between Erosion Rates, Climate and Soil Thickness on Landscapes Developed on Bedrock

迪特里希9527006。100多年来，人们一直认为，基岩分解成可擦除土壤的速度取决于上覆地幔的厚度。这一假设支持了这样一种信念，即那里有风化受限和交通受限的地貌。然而，模拟真实景观而不是假想景观，以及处理与气候、地质、生物和人类行动相关的复杂问题的新趋势，要求对这一假设进行调查。土壤生产计划在控制土壤深度的空间变化和确定在什么条件下景观成为基岩主导方面发挥着至关重要的作用。然而，目前还没有田间数据可以定量定义土壤生产函数的形状。建议将重点放在凸脊上发育的薄土壤(1米)，那里的侵蚀是由于扩散输送过程造成的，生产受到生物活动造成的机械破坏的强烈影响。我们发现土壤和部分风化的基岩之间的边界是陡峭的，使得土壤厚度很容易确定。将使用两种新的、独立的方法来确定土壤生产函数。一种方法是从简单的场关系中获得生产函数的估计。这种方法源于一项理论分析，该分析表明，在土壤生产力与扩散(斜率相关)过程的去除平衡的情况下，扩散系数和曲率的乘积等于土壤生产率。因此，如果土壤生产力取决于土壤厚度，那么土壤厚度应该随地形曲率而变化，如果扩散系数已知，则土壤生产规律可以完全量化。另一种方法是通过分析在土柱底部发现的基岩中积累的宇宙源核素来估计基岩到土壤的转化率。这种方法类似于以前使用宇宙成因核素在裸露的基岩上进行的侵蚀速率估计，但对于目标岩石在地表以下的情况来说，这是一种新的应用。虽然用这种方法估算的基岩-土壤转换率也假定土壤厚度稳定，但可以通过比较26Al和10Be浓度的比率来评估这一假设的长期变化。这种化妆品核素方法完全独立于第一种方法，可以提供长期下降率和扩散率的估计。每种方法的初步结果表明，它们产生的可比结果与预期的生产率随土壤深度指数下降的结果一致。通过应用这两种新方法，提出了一个为期三年的项目，以检验土壤生产中的深度依赖假设。根据之前关于地貌过程、气候差异和长期侵蚀速率的巨大差异的工作，选择了六个地点进行研究，其中两个地点被灰岩覆盖，四个地点被花岗岩覆盖。这两个灰瓦克遗址已经成为大量地貌研究的对象，其扩散性、气候历史以及大型动物在运输和土壤生产中的作用是众所周知的。在这里，我们将重点讨论的问题是，这种方法是否给出了合理的结果，并定义了一个清晰的生产函数。被花岗岩覆盖的四个地点的长期侵蚀率相差1000倍，年降雨量相差2倍，其中三个地点显示出大型动物活跃的洞穴活动的证据，而在澳大利亚的一个地点的土壤中没有大型动物。对所有六个地点的生产函数进行比较，应该能够揭示生产函数的潜在控制因素，并使我们能够解释侵蚀速率差异很大的景观如何具有土壤厚度。成功地确定土壤生产函数对于模拟景观演变，以及对基岩下丘陵地带土地利用加速土壤侵蚀的长期后果提供洞察和指导将具有重要价值。

Dietrich 9527006 For over 100 years it has been assumed that the rate of disintegration of bedrock into erodable soil depends on the thickness of the overlying soil mantle. This assumption underlies the belief that there are weathering-limited and transport-limited landscapes. The emerging trends toward modeling real landscapes rather than hypothetical ones and toward tackling complex problems coupling climate, geology, biology and human actions, however, requires that this assumption be investigated. Soil production plans an essential role in controlling the spatial variation in soil depth and in determining under what conditions landscapes become bedrock dominated. No field data currently exists, however, that permits quantitative definition of the shape of the soil production function. Propose to focus on the thin soils (1m) developed on convex ridges where erosion is due to diffusive transport processes and production is strongly influenced by mechanical disruption caused by biologic activity. Her we find the boundary between the soil and partially weathered bedrock to be abrupt, making the soil thickness easily defined. Two new, independent methods will be used to determine the soil production function. One method obtains an estimate of the production function from simple field relationships. This method arises from a theoretical analysis which shows that where soil production is in balance with removal by diffusive (slope dependent) processes, the product of the diffusion coefficient and curvature equals the soil production rate. Hence, if soil production rate depends on soil thickness, then soil thickness should vary with topographic curvature, and if the diffusion coefficient is known, the soil production law can be fully quantified. The other method estimates conversion rate of bedrock to soil from the analysis of cosmogenic nuclide build up in the bedrock found at the base of the soil column. The approach is analogous to erosion rate estimates previously done on expose d bedrock using cosmogenic nuclides, but this is a novel application to a case where the target rock is beneath the surface. Although estimates of bedrock to soil conversion rates by this method also assume steady state soil thickness, this assumption can be evaluated for long term variation by comparing the ratio of 26Al to 10Be concentrations. This cosmegenic nuclide method is entirely independent of the first, and can provide both estimates of long-term lowering rates as well as diffusivities. Preliminary findings for each method show that they yield comparable results consistent with an expected exponential decline in production rate with soil depth. A three-year project is proposed to test the assumption of depth dependency in soil production by applying these two new methods. Six sites, two underlain by greywacke and four by granite rocks have been selected for study based on previous work on geomorphic processes, on climatic differences, and by the very large differences in long-term erosion rates. The two greywacke sites have already been the subject of substantial geomorphic studies and the diffusivities, climatic history and role of macrofauna in transport and soil production are well-known. Here we will focus on the question of does the methodology give sensible results and define a clear production function. The four sites underlain by granitic rocks vary in long term erosion rates by as much as a factor of 1000, have annual precipitation that varies by a factor of 2 and three sites show clear evidence of active macrofauna burrowing while one, in Australia, has no macrofauna in the soil. Comparison of the production function for all six sites should shed light on the underlying controls on the production function and enable us to form an explanation for how landscapes with widely varying erosion rates could have the soil thickness. Successful determination of soil production functions will be of great value to modeling landscape evolution as well as to providing insight and guidelines about the long-term consequence of accelerated soil erosion due to land use in hilly lands underlain by bedrock.