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.

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.

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, th...

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