SCIENTISTS TO STUDY EFFECTS OF SOIL NITROGEN ON TREE ROOTS
HOUGHTON, MI--Scientists at four universities have received National Science Foundation grants totaling more than $1 million for a collaborative study of the effects of soil nitrogen on plant growth in forest ecosystems. Research teams from Michigan Technological University, San Diego State University, University of Alaska-Fairbanks, and the University of Georgia are involved.
"In forests, primary productivity is typically limited by essential soil resources such as water, nitrogen, and/or phosphoorus," says Project Coordinator Dr. Kurt Pregitzer of Michigan Tech's School of Forestry and Wood Products. " Nitrogen, in particular, limits the growth of many forests. Since trees take in these necessary elements through their roots, they expend considerable energy producing and maintaining small diameter roots and associated fungi that facilitate this process. How this below-ground allocation of carbon responds to the altered availability of necessary growth resources such as nitrogen is fundamental to our understanding of how ecosystems function."
Pregitzer says there are currently two competing theories regarding the effects of nitrogen on tree roots. One premise states that increased nitrogen availability decreases carbon allocation to roots; the other says more nitrogen increases the amount of carbon available to roots. "We would like to resolve this difference of opinion once and for all," says Pregitzer.
Pregitzer and his colleagues at Michigan Tech will be joined in the study by research teams headed by Dr. Michael Allen of the Department of Biology and Soil Ecology at San Diego State University, Dr. Roger Ruess of the Institute of Arctic Biology, University of Alaska-Fairbanks, and Dr. Ronald Hendrick of the Institute of Ecology at the University of Georgia. Participants from each university will visit the experiments set up by each of the other institutions. Test plots will be located near Fairbanks, Alaska and Albuquerque, New Mexico, and in the mountains of North Carolina and the northern hardwood forests of Michigan.
"Understanding fine root production and mortality is at the heart of this conflict," says Pregitzer. "Our overall hypothesis is that the type of root-fungi association is a dominant factor controlling carbon allocation to roots, root longevity, and the response of root systems to changes in nitrogen availability. We propose to directly and independently quantify root production and mortality, fungal community composition and abundance, root respiration, soil carbon dioxide flux, and above-ground litterfall at each of the four long-term study sites."
Pregitzer says the study sites emphasize the contrast between the different types of symbiotic fungi-root associations found in conifer and hardwood forests over a wide range of environmental conditions.
Previous attempts to define the factors controlling root longevity and below-ground carbon allocation have typically drawn on published data from many individual studies conducted at different times using a variety of methodologies, according to Pregitzer. Such studies, he says, often confound species, geography, and fungus types in efforts to broadly define responses to resources. "There may not be universal relationships between root longevity and nitrogen availability or root longevity and fungal association that can be widely applied across ecosystems and geographic regions," says Pregitzer. "But if universal relationships do exist, our use of standardized methodologies and planned contrasts under a variety of conditions should clarify them."
Another strength of the study is its ability to separate differences between ecosystems from those within the systems.
"A portion of the controversy over whether fine root longevity increases as nitrogen availability changes," he speculates, " may be a result of some studies comparing root longevity in different ecosystem types adapted to sites of widely different nitrogen availability while other studies look at changes in root longevity within a single ecosystem type at locations having natural or artificially created differences in nitrogen supply. For example, it is possible that root longevity decreases with increasing nitrogen availability when one compares across ecosystems, while root longevity increases with increasing nitrogen within a single species or ecosystem type. Our design enables us to assess such possibilities, hopefully resolving some of the controversy regarding the factors controlling root life span."
Finally, says Pregitzer, various fine root studies have looked at relative production, absolute production, turnover, and longevity. But the observed effect of resource availability on the root system can vary depending on which of these response variables is used.
"We intend to measure all of these variables, allowing us to understand what is driving below-ground carbon allocation without confusing terms and/or processes."
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Contact Kurt Pregitzer at 906-487-2396 or e-mail: [email protected].
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