Denitrification is a critical, natural environmental process carried out by various groups of bacteria. Denitrification removes reactive nitrogen from aquatic ecosystems and has important implications for water quality. Quantifying the factors that control denitrification rates in streams, and their spatial and temporal variability, has long been a challenge. The goal of this study is to determine the extent to which patterns in denitrification can be explained by the genetic structure of bacterial communities and the interactions between bacteria and other environmental factors, such as the chemistry of dissolved organic matter. This project will be conducted in an agricultural landscape in the midwestern U.S. where nitrogen pollution is a pressing concern. The study will combine modern molecular biology with recent advances in use of isotopic tracers to measure in-stream denitrification rates. Nitrogen enrichment of aquatic ecosystems is a persistent environmental problem, particularly in agricultural areas. However, current understanding of nitrogen cycling in streams and rivers is based largely on studies in watersheds with low levels of nitrogen pollution. By focusing on a nitrogen-rich agricultural landscape, this study will expand understanding of nitrogen cycling in human-dominated ecosystems. This project will advance scientific understanding of how bacterial communities interact with the environment to produce an important ecosystem function (denitrification). It will also contribute to a growing body of knowledge addressing the question of how ecosystem function is related to biodiversity, and promote interdisciplinary graduate research training and undergraduate education.
Nitrate is a common pollutant in many streams and rivers due to runoff from cropland and urban areas. Excess nitrate can contribute to undesirable algal blooms in receiving waters and is directly harmful to humans at high concentrations. The goal of this project was to further our understanding of the factors that control denitrification, a critical ecosystem process that removes nitrate from streams and rivers (see Figure). Our study was conducted in an agricultural watershed, where high nitrate is common due to fertilizer runoff. We investigated the extent to which diversity of the bacterial community responsible for denitrification influenced the rate at which denitrification occurred within the stream. Historically, water temperature and the amount of nitrate in the stream have been considered the primary controls on denitrification rates. We found that we could explain variation in denitrification better if our models included not only nitrate concentration but also information about the bacterial community. Both the number of denitrifying bacteria and the diversity of denitrifying bacteria were important. We also discovered that these bacteria rely much more heavily on organic matter in the stream bed than on organic matter dissolved in the stream water. This was surprising and has changed how we view the connection between denitrification and organic matter in streams and rivers. Improved water quality (including reduced nitrate concentrations) is a common goal of stream restoration. Our results suggest that restoring natural accumulations of organic matter in a stream bed could be crucial to promoting in-stream denitrification. Our study therefore has implications for stream and river management, in addition to advancing basic knowledge in the field of aquatic ecology. Water quality in streams and rivers is affected both by human activities and also the natural cycles of important elements such as carbon, nitrogen, and phosphorus. The cycles of these elements are coupled through the activity of organisms, mainly algae and bacteria. We conducted whole-stream experiments that revealed how these three elements were coupled and how the stream ecosystem responded to changes in carbon and nitrogen levels in the stream. Thus another outcome is a better understanding of how coupled elemental cycles affect nutrient concentrations and, ultimately, water quality in streams and rivers. To date, we have published four peer-reviewed scientific articles based on results from this research, with several additional publications in progress. Lastly, this project contributed to the nation’s human resources through the training of a post-doctoral researcher and more than a dozen graduate and undergraduate students who have subsequently obtained employment or gone on to graduate school.
