The structure and function of ecosystems is governed by patterns of nutrient limitation of plants and those organisms that consume plant remains, such as soil microbes. Often, plants and consumers are limited by the same nutrient. However, increasing evidence indicates that different nutrients can be limiters in some ecosystems, a situation known as differential nutrient limitation. This study examines why differential nutrient limitation occurs in some ecosystems but not others, and what the consequences are with respect to the utilization and storage of carbon. These questions will be tested using a network of fertilized plots in four wetlands from Rhode Island to Georgia, including both freshwater and saline systems. Standardized sampling at all sites will ensure comparable measurements of plant and microbial productivity, phosphorus cycling and ecosystem metabolism. It is expected that differential nutrient limitation will occur in ecosystems with higher rates of phosphorus mineralization and will yield less carbon storage.
This study has implications for ecosystem management and provides a conceptual framework to integrate ecological studies at multiple scales by understanding how nutrient ratios affects the biogeochemical cycles that govern ecosystem energetics. It includes a commitment to students from under-represented groups, including American Indians and African Americans, through a research mentoring program.
Summary The flow and storage of carbon in ecosystems is governed to a large degree by the patterns of nutrient limitation on the primary producers (e.g., plants) and decomposers (e.g., soil microbes). For over 150 years, it has been dogma that producers and decomposers in the same ecosystem were limited by the same nutrient (that is, adding a single nutrient will stimulate the growth of both producers and decomposers). Results from a three-year nitrogen (N) and phosphorus (P) fertilizer experiment in four wetlands along the east coast of the U.S. suggest that different nutrients can limit primary producers and decomposers in some ecosystems, as clearly shown at our South Carolina salt marsh site where N limited plant growth and P limited microbial decomposition. Final analysis and comparison of experimental response data across all our wetland sites may provide key evidence of where and why differential nutrient limitation (DNL) occurs and its effects on carbon flow in ecosystems. If DNL is a common feature of ecosystems, this will have significant ramifications for our understanding of how systems respond to nutrients and store soil carbon, key components of assessing and managing the role of soils and plants on global climate change. Introduction, Hypothesis and Objectives Wetlands are important ecosystems that provide a multitude of services for humanity. For example, in addition to serving as a reservoir for freshwater, they control floods on the landscape, serve as key nursery habitat for inland and coastal fisheries, and even moderate local weather conditions. Importantly, wetlands store almost one-third of the world’s carbon while covering a mere 3-4% of the total terrestrial land area. Soil carbon storage in wetlands is governed by the balance between the accumulation of carbon through plant growth and losses of carbon by microbial decomposition, both of which can be affected by the availability of nutrients such as nitrogen and phosphorus. The goal of this project was to better understand these processes and their responses to nutrients, while promoting ecosystem science through community outreach efforts. This study examined why DNL occurs in some ecosystems and what the consequences of DNL are with respect to the utilization vs. storage of carbon. Experimental Design, Research Sites and Methods We altered the nature of nutrient limitation through selective fertilization (no fertilization, +N, +P, and +N+P) in twelve experimental plots at each of four wetland sites. Early in 2009, we established research sites in North Carolina (pocosin freshwater bog), Georgia (tidal freshwater marsh), South Carolina (tidal salt marsh), and Rhode Island (tidal salt marsh). We collected baseline data on nutrients (C, N and P) in soils, plants, and porewater, measured rates of photosynthesis, and determined how much carbon soil bacteria decomposed. The relative proportion of these variables indicates if there is a net storage or loss of carbon in the ecosystem. Results Our results show very interesting trends in soil fertility that influence the responses of plants and soil microbes to nutrient additions and therefore affect the ability of wetlands to store soil carbon. Soils from the four wetlands differed in their ability to bind P and as a consequence had different amounts of P available for plants and microbes. Similarly, these sites differed in the amount of soil N and C. Overall, the plants in Georgia grew more when N was provided and did not respond to P additions, suggesting that N availability controlled their growth. In South Carolina, the wetland plants responded positively to N additions, and then to P, indicating that [unlike the plants in Georgia] once the need for N was met, the plants needed P to grow more. In contrast, plants in North Carolina showed a response to P additions suggesting that they were starved for P and not N. Analyses of the relationship of the microbial communities and soil variables indicates that bacterial communities are primarily linked with some components of phosphorus (total P, or extractable P) at each site but not with nitrogen components. Similarly, soil phosphorus was the primary control on the soil C cycling components (carbon dioxide emissions) that were driven by microbial decomposers. While soil carbon dioxide emissions did not respond to soil C or N content across sites, it was significantly related to soil P content, as was microbial biomass. These features of C cycling were not related to other attributes such as soil C:N:P ratios or the availability of nutrients in porewater. Overall these analyses indicate a significant influence of P on microbial biomass across sites. As part of the broader impacts of this project, we directly engaged middle schools students and teachers from South Dakota and minority undergraduate students from North Carolina to provide them with hands on experience with research and experimentation. We also provided training to multiple graduate students, technicians, and a postdoctoral researcher.
