Human activity has doubled the amount of nitrogen on the landscape, creating a pollution problem and changing the balance among multiple nutrients that limit biological activity in ecosystems. At the same time, other disturbances, such as acidification, interact with nitrogen enrichment in ways that strongly influence the productivity and health of terrestrial and aquatic ecosystems. This project examines the interactions among multiple elements and disturbances (nitrogen, phosphorus, metals, and acidification) along a continuum from the atmosphere through soils to streams. This project takes advantage of two unique experiments in which entire watersheds have been experimentally enriched with nitrogen and acid for nearly two decades. A series of new studies in those watersheds examine how chemical and biological changes in soils alter the ability of streams to take up, use, and retain nitrogen and phosphorus. These nutrient interactions are then related to important biological processes that affect the productivity and health of streams.
This research addresses an important pollution problem that requires an approach that integrates biology and geochemistry along flow paths that link the terrestrial and aquatic ecosystems. This type of integration is a challenge, but needed for effective environmental management, environmental research, and science teaching. Results from this project and interactions between university and US Forest Service researchers will inform effective management of watersheds faced with multiple pollution problems. A series of collaborative workshops in which high school, undergraduate, and graduate students work with researchers and teachers will promote multidisciplinary learning. The collaboration will seek to develop a computer simulation model for use in teaching integrated biology and chemistry in high school and college science curricula.
’ was a team-oriented approach to investigate how two long-term sources of pollution may be stressing America’s headwater streams. For decades industrial and agricultural activities have been introducing nitrogen (N) to stream networks resulting in increased availability of a nutrient that is typically limiting to growth of algae and microbes. Increased availability influences those living things responsible for the self-purification processes for which streams are well known. Over the same time period, atmospheric deposition has increased acid content in stream draining valleys downwind of industrial activity. These influences co-occur in most places, but we know little about how the interactions between them affect the biota and function of headwater streams. Our work used chemical tracers introduced to a group of streams that differed in their acid and N content. The streams were in experimental forests in Maine and West Virginia that are maintained by the US Forest Service. Scientists have been adding N and acid to these forests for decades to test the combined influences of these forms of pollution. We also worked in Shenandoah National Park, Virginia where variation in atmospheric deposition created gradients in stream acidity. We used a special form of nitrate, a common type of N found in fertilizer used by plants and algae, that weighed more than typical nitrate (i.e., a heavy isotope) to follow how well the streams were able to remove nitrate from the stream water. Our past work and work by forest ecologist have shown that living systems can become saturated with N (i.e., N becomes so abundant that living things can no longer remove it all from soils and water). We developed a hypothesis suggesting that streams in acidified valleys would be less capable of taking up N because increased acid impairs biological activity and decreases the abundance of other important elements (e.g., phosphorus) needed for biological growth. In this way, we proposed direct (influence felt by the organisms) and indirect (influence on resource availability) influences of acidification on N processing and the degree of N saturation. We ran experiments in more acid streams and in streams without enhanced acidity, either with, or without added P, and we also experimentally altered the availability of carbon (C) as a food source to see how interaction among C,N and P were influenced by acid. In the Shenandoah streams of Virginia, we focused on the biofilms that grow on leaves that accumulate in streams. Others have shown that these biofilms are composed almost entirely of fungi (i.e., like mushrooms) that cause the decomposition of leaves. Many have documented decreased leaf decomposition in acid streams. Our studies addressed the link between these biofilms and N processing. Our results show that N uptake in acid streams is impaired by the stress that living things experience when acid is present in excess. The abundance of microorganisms in leaf biofilms declines drastically as acid content increases and the biological activity (i.e., respiration) per gram of biofilm declines as well. The challenges to microbial health that are posed by acid appear to translate to decreased N uptake as a result of physiological stress. The end result is that acid streams have less ability to address increased N than streams not stressed by acid. As a result, these streams are more quickly saturated and simply transport the increased N to downstream rivers and, eventually, the ocean. The work was a collaboration among professors and students at Virginia Tech, University of Maine, and University of Montana. Students were an important part of the effort and we helped train new scientists at the Masters, PhD and post-Doctoral levels. We generated a large and strong data set on N processing, organic matter content, stream metabolism, and water quality across a series of streams that represent the range of pollution typically encountered as a result of atmospheric deposition. Together these efforts helped increase our understanding of streams, the influence of increased N on ecosystem function, and the implications of interaction among stressors like acid and N pollution.
