U.S. Geological Survey, Northern Prairie Wildlife Research Center, 26611 U.S. Highway 385, Hot Springs, South Dakota, 57747, USA.
Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado, 80523, USA.
Ecol Appl. 2019 Jul;29(5):e01915. doi: 10.1002/eap.1915. Epub 2019 May 29.
The critical load concept facilitates communication between scientists and policy makers and land managers by translating the complex effects of air pollution on ecosystems into unambiguous numbers that can be used to inform air quality targets. Anthropogenic atmospheric nitrogen (N) deposition adversely affects a variety of ecosystems, but the information used to derive critical loads for North American ecosystems is sparse and often based on experiments investigating N loads substantially higher than current or expected atmospheric deposition. In a 4-yr field experiment in the northern Great Plains (NGP) of North America, where current N deposition levels range from ~3 to 9 kg N·ha ·yr , we added 12 levels of N, from 2.5 to 100 kg N·ha ·yr , to three sites spanning a range of soil fertility and productivity. Our results suggest a conservative critical load of 4-6 kg N·ha ·yr for the most sensitive vegetation type we investigated, badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions, where N addition at this rate increased productivity and litter load. In contrast, for the two more productive vegetation types characteristic of most NGP grasslands, a critical load of 6-10 kg N·ha ·yr was identified. Here, N addition at this level altered plant tissue chemistry and increased nonnative species. These critical loads are below the currently suggested range of 10-25 kg N·ha ·yr for NGP vegetation and within the range of current or near-future deposition, suggesting that N deposition may already be inducing fundamental changes in NGP ecosystems.
临界负荷概念通过将空气污染对生态系统的复杂影响转化为明确的数字，促进了科学家、政策制定者和土地管理者之间的沟通，这些数字可用于为空气质量目标提供信息。人为大气氮（N）沉积对各种生态系统都有不利影响，但用于推导北美的临界负荷的信息很少，而且通常基于研究 N 负荷远高于当前或预期大气沉积的实验。在北美大平原（NGP）北部的一个为期 4 年的野外实验中，当前 N 沉积水平在 3 到 9 kg N·ha·yr 之间，我们在三个跨度从土壤肥力和生产力的范围内添加了 12 个 N 水平，从 2.5 到 100 kg N·ha·yr。我们的结果表明，对于我们研究的最敏感植被类型——荒地稀疏植被，这是一个支持适应低肥力条件的植物物种的群落，添加 4-6 kg N·ha·yr 的 N 是一种保守的临界负荷，在这个速率下添加 N 会增加生产力和凋落物负荷。相比之下，对于更具生产力的两种更具代表性的 NGP 草原植被类型，确定的临界负荷为 6-10 kg N·ha·yr。在这里，在这个水平上添加 N 会改变植物组织的化学性质，并增加非本地物种。这些临界负荷低于目前建议的 NGP 植被 10-25 kg N·ha·yr 的范围，并且在当前或接近未来的沉积范围内，这表明 N 沉积可能已经在引起 NGP 生态系统的根本变化。
