The relative nutritional status of autotrophs and heterotrophs influences the structure, function, and health of ecosystems. Sometimes, the nutrient that limits the primary producers and microbial heterotrophs is the same, in some cases it differs, and in still other cases multiple nutrients limit the growth of organisms. This study focuses on differential nutrient limitation (DNL), which occurs when primary producers and soil microbial heterotrophs are limited by different nutrients. It is postulated that DNL is a function of biotic and abiotic interactions within the sediment chemical environment that results in maximization of resource utilization within the autotrophic and heterotrophic communities. When DNL occurs in an ecosystem, the necessary result is an increase in C throughput because reduced C compounds represent the currency of biological energy.

The causes and implications of DNL will be addressed in a network of wetland sites by testing the following hypotheses: (A) DNL in an ecosystem is a function of P turnover rates in the sediment chemical environment such that DNL will occur in ecosystems with greater net P mineralization rates; and (B) a greater proportion of gross primary production (GPP) will be stored in the ecosystem when DNL does not occur. The study sites include a mineral salt marsh, where DNL has been documented, and an organic salt marsh, as well as a mineral freshwater tidal marsh, and a pocosin, which are expected to differ in their patterns of autotrophic and heterotrophic nutrient limitation. The hypotheses will be tested using a standardized approach to study P cycling (including P mineralization and soil P binding), productivity of individual trophic components, and ecosystem metabolism in field-fertilized plots across these sites. Nutrient availability and the throughput of fixed-C in these ecosystems will be related to the nutrient limitations of primary producers and soil microbial heterotrophs.

The intellectual merit derives from a greater understanding of ecosystem development, the effects of nutrient availability on the cycling of carbon between autotrophic and heterotrophic organisms, and the fate of fixed C in ecosystems. Since not all ecosystems exhibit differential nutrient limitation, it is necessary to know: a) Why does DNL occurs in some ecosystems but not others? and b) What are the biogeochemical implications of DNL? The results from this study will have implications for ecosystem management and health. Because DNL has been demonstrated in other ecosystems, this study will be broadly relevant and the findings can be incorporated into contemporary theories of ecosystem development and the emerging theory of ecological stoichiometry. The proposed research provides a framework to integrate ecological studies at three scales by understanding the influence of ecological stoichiometry on biogeochemical cycles, productivity and respiration rates that govern ecosystem energetics.

The broader impacts include a major commitment to training students from under- represented groups as this project provides a platform for synergizing the strengths and resources of the three collaborating institutions to advance the participation of under- represented groups in ecosystem studies. This study commits resources to these students to provide hands-on experiences in scientific research and to promote their participation and retention in the sciences. Each summer, a dozen undergraduate students from minority serving institutions in South Dakota, North Carolina, and South Carolina will participate in research internships that will include an extended visit to USC’s Baruch Marine Field Laboratory for training/education in wetland science, to include participation in the proposed research. In addition to the minority student program, two graduate and multiple undergraduate students will be directly supported by this project. This research will support institutions in two EPSCoR states and will also provide resources to develop and enhance wetland and environmental courses at an Indian tribal college, the South Dakota School of Mines and Technology, and Duke University.