Nitrous oxide (N2O) ranks as the number one ozone-depleting chemical (usurping chloroflorocarbons, which have been successfully controlled by the Montreal Protocol) and is an increasingly important greenhouse gas. Understanding and controlling the flux of N2O is complicated because it is produced by natural systems as well as by human activity that perturbs these natural systems. One of the most important perturbations is the dramatic increase in the amount of fixed nitrogen (N) in the biosphere through the combined effect of fossil fuel combustion and fertilizer application. Much of the excess N applied on land and released to the atmosphere eventually finds it way to estuarine and coastal waters, leading to a fundamental restructuring of the ecology of these habitats. Salt marshes are an essential feature of coastal landscapes and microbial processes in marsh sediments are critical to the exchange of nitrogen between land and sea. The research proposed here will examine the effects of two key environmental variables, N supply and redox conditions, on the fluxes of N2O and N2 in salt marshes. The work will be performed at a marsh in Massachusetts that has been the site of a long running fertilization experiment, in which varying amounts of nitrogen have been added to different test sites for 30 years. Molecular biological methods will be used to examine the microbial community composition, activity and abundance using functional genes that are diagnostic for N2O production, nitrification and denitrification. Rate measurements and gene distribution/expression analyses will examine how N supply and redox conditions independently affect the proportion of N2O and N2 produced at each site and the pathways that are responsible for that production.
This research will improve our understanding of the interaction between the activity of key microorganisms responsible for nitrogen cycling, the gaseous N fluxes associated with those microorganisms, and the environmental factors that regulate both. Three components of outreach and education are proposed in parallel with the research: (1) improve undergraduate curriculum in environmental science at Princeton, (2) increase public awareness of the role that microbiology plays in the maintenance of salt marshes and cycling of nitrogen globally, and (3) train the next generation of microbial ecology researchers. At Princeton University, the lead investigator (Ward) teaches one of the core courses in the Princeton Environmental Institute's Introduction to Environmental Science courses. The CoPI (Bowen) will be starting her teaching career at University of Massachusetts, Boston, in the fall of 2010, and will incorporate this research into her new courses. Both Ward and Bowen will be able to develop experiential learning opportunities for undergraduate courses that grow organically out of this research. In addition a new web site will be developed to provide information on the project's goals and progress, educate the public about the impact of fertilization on coastal ecosystems, and the critical role that wetlands and, particularly, the microbes that reside therein, play in maintaining the productivity of estuarine and coastal waters. Finally both PIs will work as a team to bring socioeconomically and culturally diverse young investigators into the field and laboratory to perform hands-on research that will include undergraduates in class projects, and junior and senior thesis research projects, and graduate students, whose dissertation research will be a part of this project.
Nitrous oxide (N2O) is a potent greenhouse gas, which is produced by microbial nitrogen transformation in soils, sediments and aquatic environments. Salt marshes are important buffers between terrestrial and marine systems and they intercept nutrients in runoff as they move from land to sea. The most important nutrient in this system is nitrogen, which acts as a fertilizer on land and in the ocean – addition of nitrogen to the ocean can cause an increase in primary production and can lead to eutrophication. Microbial metabolism can remove excess nitrogen from water and sediments by the processes of denitrification and anaerobic ammonium oxidation – these pathways convert biologically available nitrogen into dinitrogen gas, effectively removing it from the biosphere. One of the side products of this process is nitrous oxide, which is formed in small amounts alongside the dinitrogen gas. Even at trace levels, nitrous oxide is an important greenhouse gas and is involved in the production of ozone in smog and its destruction in the stratosphere. The goals of this project were to determine the source of nitrous oxide and its control by environmental factors in salt marsh sediments. This included measuring the rates of N2O production, using isotopic methods to deduce the pathways responsible, and characterizing the microbial assemblages in the sediments. The project was a collaboration between Principal Investigators Bess Ward (Princeton University) and Jennifer Bowen (University of Massachusetts at Boston). Graduate and undergraduate students at both institutions participated in the fieldwork, made the analytical measurements and performed diverse sample analysis. The field work was carried out in Great Sippewissett Marsh in Massachusetts, where a long term fertilization experiment has been carried out for the last 40 years. The rates of nitrogen transformations and the composition of the microbial communities were measured in plots that had received no or various levels of fertilizer during the growing season for 40 years. Rates of key processes were measured using stable isotope tracer approaches, and methods for such measurements in sediments were developed and improved. Clear correlations between rates of N cycle processes, functional gene abundance and diversity and level of fertilization in the long term fertilized plots have been demonstrated: At the highest fertilization levels, rates of nitrification, denitrification and nitrous oxide production were highest, and the microbial assemblages involved in denitrification were most abundant and diverse. The main source of nitrous oxide was ammonium, probably through nitrification and coupled nitrification/denitrification. The diversity of the assemblage was assessed using gene sequencing of genes involved in the nitrogen transformation pathways. The sequencing efforts have greatly increased the database for some key functional genes. The marsh contains extraordinary diversity of nirS genes compared to open ocean and estuarine sites. Numerous presentations at national meetings and publications in peer reviewed journals have resulted from this research.
