Cycling of NO3-N in Northern Hardwood Forests: Regulation and Consequences of Nitrogen Saturation.
Principal Investigators:
Kurt S. Pregitzer, Michigan Technological University
Donald R. Zak, University of Michigan
Andrew J. Burton, Michigan Technological University
Graduate student:
Eugenie Euskirchen, Michigan Technological University
student research
After six years of nitrate (NO3-) additions to four northern hardwood forests in Michigan, we have detected a variety of changes indicating chronic N additions have altered ecosystem processes in these forests. These changes include much greater leaching losses of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and NO3-, and a decline in soil respiration. Over the next three years we will investigate the specific mechanisms behind these changes and determine if they will persist over time or if they are only temporary responses that will fade as the ecosystems adjust to higher levels of N availability. Our overall hypothesis is that chronic N additions have fundamentally altered the flow of carbon (C) through the soil foodweb. We theorize that continued N additions will cause a reduction in root biomass, an increase in root lifespan, and a decrease in the incidence of mycorrhizal infection. We also predict that long-term N additions will alter the biochemcial composition of fine-root and leaf litter. It is felt these changes in litter quantity and quality will cause shifts in microbial community composition and function. As N additions continue, the residence time of added NO3- within the ecosystem is hypothesized to decline, whereas total C and N export (DOC, DON & NO3-) should continue to increase, magnifying the possibility that N saturation of forests will have significant ecological impacts on aquatic ecosystems.
The research is being conducted at four northern hardwood forests across the state of Michigan (the Michigan Gradient study). At each location, three control plots have existed since 1987, and three NO3- amended plots were installed in 1993. Natural differences among the sites in mean annual temperature and length of growing season, year-to-year variability in climate, and comparisons between control and N-amended plots at each site have allowed us to learn much about the interacting effects of temperature, moisture availability and N availability on ecosystem processes in northern hardwood forests. Over the next three years, we will test a series of specific C and N cycling hypotheses and their alternatives by: 1) quantifying changes due to NO3- additions in the amounts and chemical composition of fine-root and leaf litter; 2) quantifying changes in microbial community composition, the metabolism of root-derived C, and the activity of enzymes involved with lignocellulose degradation; 3) determining the age and chemical composition of DOC on the control and fertilized plots; 4) following 15N added to an entire ecosystem in 1998 to determine the long-term sinks of N deposition; and 5) conducting several laboratory and field experiments to determine what controls the processing and residence time of N at the ecosystem level. These experiments will be complemented by field measurements of soil solution chemistry and leaching losses, NPP, soil and root respiration, root biomass, and root longevity. The goal is a better fundamental understanding of the effects of chronic N additions and the mechanisms that regulate C and N transformations in the soil of northern hardwood ecosystems across an entire geographic region.
A wide variety of opportunities exist within this study for quantitative analysis. Our long-term measurements (up to 13 years of record) would allow a student to model C and N cycling and forest growth for our sites. The student could test existing models in order to assess their accuracy for Lake States northern hardwoods, and then revise these models or develop new models based on the weaknesses discovered during the initial analyses. A quantitatively oriented graduate student could also model in detail the processing of C and N by the soil foodweb, and this will become the most likely target for the new Ph.D. student. The measurements planned for our study will allow the student to quantify not only the effects that environmental factors have on ecosystem C and N ouputs, but also the changes in food-web dynamics that control the broader responses.
Student Research:
Cycling of NO3-N in Northern Hardwood Forests: Regulation and Consequences of Nitrogen Saturation
Eugenie Euskirchen
Forested landscapes can be managed to sequester carbon at rates that are dependent on the management regimes applied to that landscape, and modeling of this carbon storage within a forested landscape is an important objective of forestry research. Although our understanding of how management options can change the carbon sequestration within a landscape continues to improve, we are still faced with questions concerning the magnitude of change in various components of forest productivity following a disturbance (e.g., timber harvest or fire) under various management regimes. Oftentimes basic knowledge of the variables required in highly-parameterized simulation models is too limited for indiscriminate use of such models. However, a simple, parameter-scarce dynamic model can potentially exhibit a plentiful range of behaviors that may aid in characterizing landscape-level carbon storage in response to various disturbances.
The model that we are developing applies generic exponential and decay functions to the components of the forest carbon cycle (e.g., heterotrophic and autotrophic respiration, root death and decay, litter decomposition), and quantifies the magnitude of change in the associated pools and fluxes following the export of biomass. The model is driven by assimilation (photosynthesis) which begins at a value near zero as the leaf area of the stand at age 1 is low. As the stand ages, the woody biomass (and foliage) grow until a predefined biomass is attained, at which point the foliage and biomass are "disturbed" (e.g., harvested) and exported from the site. Carbon is allocated belowground at a rate that increases as the stand ages until an asymptote is attained. Some of this carbon is given off in root respiration, but most of it enters the "soil carbon" pool. In this pool, carbon enters into either long-term, intermediate, or labile storage. To demonstrate the utility of the model, we are parameterizing it with published data from pure, mature stands of aspen, jack pine, red pine, and northern hardwoods growing in northern Michigan, Wisconsin, Minnesota, and Canada. We will then run simulations over a period of 150 years and estimate the carbon fluxes of the components of the carbon cycle across a hypothetical landscape.
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