RAPID: Experimental Recovery at the Bear Brook Watershed in Maine

这个研究项目将测量缅因州森林生态系统在长达27年的实验结束后的反应。实验包括每隔一个月用直升机向森林中添加酸、氮和硫。从2017年初开始，在为期一年的时间里，将收集土壤和溪流的样本，并分析其化学成分的变化。研究人员将对流域中的一条小溪进行采样，其中一条小溪受到了添加物的影响，另一条小溪则没有受到添加物的影响。样本将在没有暴风雨的正常流量时期和暴风雨期间从溪流中收集，因为暴风雨改变了水从景观流向溪流的方式，暴风雨增加了溪流中的流量。先前对森林流域的补充是对酸雨影响的长期研究的一部分。然而，在过去的25年里，北美东部和欧洲从大气中沉积到地表的硫和氮的数量急剧下降。在同一时期，森林生态系统也经历了气候变化和其他干扰，包括冰暴、土地开发以及虫害和疾病的爆发。酸降水的减少与其他扰动水平的增加重叠，使得很难单独区分大气输入减少的影响。在所有其他干扰保持相对恒定的情况下，突然停止试验添加氮和硫提供了一个独特的机会来评估森林生态系统对这些输入的反应。因此，这项研究有可能测试关于溶解在地表水中的有机分子的化学性质以及硫在土壤中的运动将如何随着酸雨的减少而改变的假设。对森林生态系统动态的进一步了解有可能影响州和国家一级与森林管理及其生态系统服务（如提供高质量饮用水来源）有关的政策。这个项目利用了一个独特的科学机会，与整个流域操作有关。2016年10月，停止向西熊溪流域添加27年的氮和硫的实验，将使研究人员能够将西熊溪的反应与参考流域东熊溪流域观察到的变化进行比较。研究人员将在基流条件下和风暴期间对河流进行采样，以检验一些河流中溶解有机物浓度增加的假设，即水体褐化，是由于酸性大气沉积减少而发生的。这项研究将使科学家们能够区分由于大气增加的突然停止而观察到的变化，以及可能由气候变化和林地开发造成的长期变化。溶解有机碳的表征将采用最先进的分光光度法、荧光法和分子方法来测试有关土壤中有机物和矿物复合物性质的假设。研究人员还将研究由土壤中硫酸盐出口量下降驱动的恢复机制，将在恢复的第一年结束时对长期土壤地块进行土壤取样，以获取土壤化学变化的潜在初步证据，并最终希望更好地了解土壤和河流中化学变化的顺序，特别是与铝有关的化学变化。该项目将包括本科生和研究生的参与，并与当地一所高中的K-12教师和学生建立联系。

This research project will measure the response of a forest ecosystem in Maine as a 27-year long experiment ends. The experiment consisted of adding acid, nitrogen, and sulfur to the forest by helicopter every other month. Starting in early 2017 and extending over a one-year period, samples of soil and stream water will be collected and analyzed for changes in their chemical composition. The researchers will sample a stream in the watershed that had been subjected to additions and a stream in another watershed that did not receive additions. Samples will be collected from streams under periods of normal flow when there are no storms and during storms as storms change how water moves from the landscape to the streams and storms increase the flow within the streams. The prior additions to the forested watershed were done as part of a long-term study of the effects of acid precipitation. However, over the past 25 years Eastern North America and Europe have experienced dramatic declines in the amounts of sulfur and nitrogen that are deposited on landscapes from the atmosphere. During that same time period forest ecosystems have also experienced changes in climate and other disturbances that include ice storms, land development, and outbreaks of insects and disease. The overlap in a decline in acid precipitation with increased levels of other disturbances make it difficult to isolate the influences of the decline in atmospheric inputs alone. The abrupt end to the experimental additions of nitrogen and sulfur provides a unique opportunity to assess forest ecosystem responses to those inputs while all other disturbances are held relatively constant. As a result, this research has the potential to test hypotheses about how the chemistry of organic molecules dissolved in surface waters and the movement of sulfur through soils will change with reductions in acid precipitation. The increased understanding of forest ecosystem dynamics has the potential to influence policy at the state and national level, associated with the stewardship of forests and their ecosystem services such as providing a source of high quality drinking water.This project takes advantage of a unique scientific opportunity associated with a whole watershed manipulation. Stopping the 27-year experimental additions of nitrogen and sulfur to West Bear Brook watershed in October 2016 will allow the researchers to compare the responses in West Bear Brook to the changes observed in a reference watershed, East Bear Brook watershed. The researchers will sample the streams under base flow conditions and during storms to test hypotheses about why increases in dissolved organic matter concentrations in some streams, referred to as browning of the waters, have occurred as acidic atmospheric deposition has diminished. The research should allow the scientists to distinguish changes observed with the abrupt cessation of atmospheric additions from the long-term changes that could be the result of climate change and forestland development. Characterization of dissolved organic carbon will employ state of the art spectrophotometric, fluorescence, and molecular methodologies to test hypotheses about the nature of organic matter and mineral complexes in the soil. The researchers will also study mechanisms of recovery driven by declining sulfate export from soils, will sample soils from the long-term soil plots at the end of the first year of recovery to capture potential initial evidence of alterations in soil chemistry, and ultimately hope to gain a better understanding of the sequence of changing chemistry in soils and streams with particular reference to aluminum. The project will include participation by undergraduate and graduate students, with linkages to K-12 teachers and students at a local high school.