Collaborative Research: Lateral weathering gradients typify critical zone architecture in glaciated catchments

地球上土壤的薄皮使生命成为可能。水和岩石的相互作用，或矿物风化，是土壤发育的原因，土壤维持生命并调节水质和水流。因此，矿物风化是一项关键的自然服务，它提供植物生长所需的营养，并控制营养在环境中的循环及其向下游河流、湖泊、河口和海洋的运输。在这项研究中，美国东北部山区提供了一个天然实验室来研究矿物风化的过程和速率，那里的土壤相对年轻(不到1万年)，基岩通常很浅，提供了整个景观中矿物消耗和积累的空间梯度。这个土壤年轻的地区支持着该国一些森林密度最高的州的森林，也是东北部主要河流的源头，为下游的主要大都市地区提供了充足、清洁的水。这些森林土壤还提供其他重要服务，包括野生动物栖息地和由森林产品的可持续收获和娱乐机会推动的区域经济。这些服务中的许多依赖于岩石分解以补充土壤养分供应的速度与通过采伐树木移走材料或向下游运输的速度之间的平衡。矿物风化速率的不确定性，以及这种风化速率是否与损失保持同步，是森林集约化采伐的可持续性和理解森林如何应对干扰的长期问题。这个问题倍受关注，因为除了更传统的森林产品外，对森林的需求日益增加，以提供可再生的生物质能源。这些营养元素中的许多已经被数十年的空气污染所耗尽。准确计算矿物风化速率是临界负荷模型的核心，北半球各国越来越多地将其作为指导制定空气污染政策和管理可持续生态系统的工具。这项研究在新罕布夏州的哈伯德·布鲁克实验森林进行，将在分水岭尺度上引入矿物风化的新范例，描述可用于在临界负荷模型中重新制定矿物风化算法的空间变化，并解决森林管理中的养分消耗问题。在Hubbard Brook工作，拥有北美最长的溪流和雨水化学记录之一，为这项研究提供了一个框架，通过这个框架，这项研究可以展示流域的不同部分如何相互作用，产生由森林输送到下游河流和湖泊的物质流。这将帮助林学家和流域管理者评估如何应用管理决策来确保美国东北部旱地森林的持续生产力。通过测量美国东北部多个地点和哈伯德·布鲁克实验森林的水文通量、溶质通量和固相特征，将沿着从裸露的基岩到深层土壤的横断面研究矿物风化和风化土层的发育过程，这是冰川地区常见的景观梯度。在Hubbard Brook的三个仪器样点收集的数据将用于测试山坡和分水岭尺度上矿物风化梯度的变化。分布在美国东北部的四个地点的单一样带将在更广泛的气候、基岩岩性和土壤类型范围内测试区域规模的适用性。矿物风化过程，包括原生矿物溶解和次生物质在假定为代表集水区功能不同部分的区域中的聚集，将通过对风化层(土壤、底土和岩石碎片)和基岩的固相研究，使用重叠的分析方法来研究，包括光学岩石学、扫描电极显微镜、电子探针测绘、大宗化学分析和次生物质提取。风化层研究表明了长期的风化进程，将结合使用被动流量测量和更传统的水文地质监测来测量风化衍生元素中的当前水通量。流域内四个不同区域的归一化水通量将与流域规模的化学剥蚀进行比较，以确定研究区域对整个流域质量平衡的重要性。这将有助于重新解释Hubbard Brook 50多年的记录，这将通过识别高地集水区内不同区域的不同速度和进程，为评估森林可持续性提供必要的信息。

The thin skin of soil on Planet Earth is what makes life possible. The interactions of water and rock, or mineral weathering, are responsible for the development of the soil that sustains life and regulates water quality and flow. As such, mineral weathering is a critical natural service that provides nutrients required for plant growth and controls the cycling of nutrients through the environment and their transport to downstream rivers, lakes, estuaries, and the ocean. In this study, mountainous areas of the northeastern United States provide a natural laboratory to examine processes and rates of mineral weathering where soils are relatively young (less than 10,000 years) and bedrock is often shallow, providing spatial gradients of mineral depletion and accumulation across the landscape. This region with young soils supports forests in some of the most densely forested states in the country and is the source of the major rivers of the northeast, providing major metropolitan areas downstream with abundant, clean water. These forest soils also provide other vital services including wildlife habitat and a regional economy driven by sustainable harvest of forest products and recreational opportunities. Many of these services are dependent on the balance between the rate at which rocks break down to recharge soil nutrient supply versus the rates at which materials are removed via tree harvest or are transported downstream. Uncertainty in mineral weathering rates, and whether such rates keep pace with losses, is a long-standing question in the sustainability of intensive forest harvest, and in understanding how forests respond to disturbances. This question has redoubled interest due to increasing demands on forests to provide renewable biomass energy sources in addition to more traditional forest products. Many of these same nutrient elements have been depleted by decades of air pollution. Accurate calculation of mineral weathering rates is the centerpiece of critical loads models, increasingly adopted by countries across the northern hemisphere as a tool to guide development of air pollution policy and for the management sustainable ecosystems. This study, taking place at the Hubbard Brook Experimental Forest in New Hampshire, will introduce a new paradigm of mineral weathering at the watershed-scale, describing spatial variation that can be used to reformulate mineral weathering algorithms in critical loads models and address nutrient depletion concerns for forest management. Working at Hubbard Brook with one of the longest records of stream and rainwater chemistry in North America provides a framework by which this research can demonstrate how different portions of watersheds interact to produce material flows transported by forests to downstream rivers and lakes. This will help foresters and watershed managers evaluate how management decisions may be applied to ensure continued productivity of upland forests of the northeastern U.S.Through measurement of hydrologic fluxes, solute fluxes and solid phase characterization at several sites in the northeastern U.S. and at the Hubbard Brook Experimental Forest, mineral weathering and regolith development processes will be examined along transects from exposed bedrock to deep soil, a common landscape gradient in glaciated regions. Data collected at three instrumented transects at Hubbard Brook will be used to test variation in mineral weathering gradients at the hillslope and watershed scale. Single transects at four sites dispersed across the northeastern U.S. will test applicability at the regional scale, with a broader range of climate, bedrock lithology and soil type. Mineral weathering processes, including both primary mineral dissolution and secondary material accumulation in zones hypothesized to represent functionally distinct portions of catchments, will be examined through solid phase studies of regolith (soil, subsoil, and rock fragments) and bedrock using overlapping analytical approaches, including optical petrography, scanning electrode microscopy, electron microprobe mapping, bulk chemical analyses, and secondary material extraction. The regolith studies, indicating long-term weathering progression, will be complemented by measurements of current aqueous flux in weathering derived elements using a combination of passive flow metering and more traditional hydrogeologic monitoring. Area normalized aqueous fluxes at four distinct zones within the catchment will be compared to catchment scale chemical denudation to determine the importance of the study zones to overall catchment mass balance. This will facilitate reinterpretation of a 50+ year record at Hubbard Brook, which will provide essential information for the assessment of forest sustainability by discerning the distinct rates and progression of processes at differing zones within upland catchments.