RAPID: Experimental Recovery at the Bear Brook Watershed in Maine

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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.