Toward an Understanding of Global Change Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program (1988) / Chapter Skim
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Biogeochemical Dynamics
Biogeochemical Dynamics
Pages 47-68
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From page 47... ...
Through the active intervention of the biota, each of these four elements follows a closed loop or cycle, passing through molecular species of increasing energy content as the elements are incorporated into living tissue, and then moving through decreasing energy levels as the organic matter is returned to inorganic form. These cycles significantly influence atmospheric and oceanic chemistry and the global energy balance.
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From page 48... ...
For example, in cold-dominated, wet tundra ecosystems, carbon fixation exceeds decomposition; there is net carbon and nitrogen storage in soil, and plant growth is limited by nitrogen. In contrast, tropical forests on old infertile sails cycle large amounts of carbon and nitrogen, but biomass accumulation is limited by phosphorus.
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From page 49... ...
However, if the decomposition and nutrient release in the soil are sufficiently delayed, nutrient limitation could become more severe, decreasing rates of carbon fixation and hence decreasing carbon storage. In the context of changes in biogeochemistry in terrestrial systems, five geographic areas are judged as critical foci for experimental ecosystem studies; wet tundra, boreal forests, temperate forests in areas receiving nitrogen deposition, tropical forests, and semiarid ecosystems.
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From page 50... ...
clarify their probable feedbacks to the greenhouse effect. The low physical stature of tundra ecosystems makes them particularly suitable for such an experimental approach, although their remoteness and the harshness of the environment pose a significant challenge.
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From page 51... ...
modeling and measurement programs across natural gradients will be needed to supplement enclosure experiments. Ecosystem-level enclosure experiments, as for the tundra, are essential since the added nitrogen may decrease tissue C/N and thereby increase decomposition/nitrogen release.
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From page 52... ...
It would be useful to undertake measurements to examine the question-of whether increased CO2 increases photosynthesis in a range of tropical forests. It would be of particular interest to examine whether the increase in carbon fixation could cause increased nitrogen fixation, given the abundance of nodulated legumes in many tropical forests.
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From page 53... ...
Nitrogen is particularly important because, while NO3- is the primary form of nitrogen Toss in the temperate zone, N20 and NOT fluxes are much greater in tropical forests than they are in temperate forests. Most importantly, the mechanisms controlling pathways of Toss or gain must be analyzed in order to extrapolate the fluxes over the range of land uses/ecosystems that are being affected.
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From page 54... ...
Integrative, coordinated studies of a broad areal extent over natural ecosystems are needed in an overall research strategy, as are experimental modifications, including enclosure experiments, of systems that can provide invaluable insights. Finally, a comprehensive strategy must include a commitment to a Tong-term observing system both from space and from the ground.
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From page 55... ...
Fluctuations in the terrestrial fluxes on nitrogen and phosphorus, due to variations in weathering and estuarine processes, and exchange with coastal sediments, could have an effect on temporal variations in oceanic nutrients, and consequently global climate, through changes in oceanic productivity. Thus previous models of the geochemical cycles of nitrogen and phosphorus that assume steady state behavior may need to be modified to explore implications of nonsteady state models for ocean nutrient cycles.
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From page 56... ...
As a consequence of this "biological pump," the concentration of dissolved inorganic carbon is not uniform with depth: the concentration in surface waters is 10 to 15 percent less than that in deeper waters. There is a corresponding depletion of phosphorus and nitrogen in surface waters, even in areas of intense upwelling, as a result of biological uptake and loss of detrital material.
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From page 57... ...
The concentration of preformed nutrients may be expected to reflect physical processes, and it can be influenced also by biological activity to the extent that this activity can result in packaging of carbon, nitrogen, and phosphate in fecal material that can fall to the deep, providing a path for transfer of nutrients from the surface to the deep independent of the physical processes such as those responsible for the formation of deep water in high latitudes. There is a need for careful, coordinated studies of the processes responsible for transfer of nutrients from the surface to the deep.
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From page 58... ...
For example, we infer past temperatures of the ocean from counts of the relative abundance of the fossils of organisms preferring cold and warm ocean waters, or from measuremeets of the oxygen isotope composition of the fossils. While the empirical and theoretical justification for these inferences is generally accepted, there is a distinct lack of direct gIobalscale documentation of the relationship between the sedimentation and geochemistry of fossils and the physical and chemical properties of the modern ocean.
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From page 59... ...
In principle, we expect that we should be able to monitor past changes in high-latitude i3C using measurements of carbon in the shells of fossils that grew in surface waters. But it is reported that high-latitude planktonic fossils reveal a Tower abundance of i3C in glacial times than would be indicated by theoretical expectations.
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From page 60... ...
The magnitude and timing of past changes in the phosphorus content of the ocean will be key to obtaining an understanding of the global phosphorus cycle. Because of the Tong time constants involved (approximately 105 years)
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From page 61... ...
Study of the past phosphorus content of the ocean is also a key in testing some models of past changes in atmospheric CO2. The phosphorus content of the deep ocean is one of the most significant factors in setting the CO2 content of the atmosphere.
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From page 62... ...
This has clear implications for productivity in impacted areas and may be expected to significantly affect biogeochemical cycling over extensive regions. The chemistry of tropospheric O3 assumes additional importance in that the abundance of OH may be expected to change in response to changes in lower atmospheric O3.
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From page 63... ...
There is a clear need for resources to be directed to these activities to stimulate the pace of research. The objectives are to understand the processes regulating the composition of the troposphere with particular attention to oxidants and to define paths for removal of biospherically formed gases.
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From page 64... ...
For example, it should be possible to estimate rates for production of CH4 in the past using measurements of CH4 in ice cores in combination with data on NOX en c! other relevant species.
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From page 65... ...
The Tong-term record of change is equally illuminating. Studies of gases trapped in polar ice cores have shown that the level of atmospheric CO2 is Tow, about 200 ppm, in glacial times, rising to about 280 ppm during interglaciais.
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From page 66... ...
This will require carefully crafted experimental strategies using a variety of approaches, including passive observations of natural systems, selected manipulation of natural systems, studies of large and small enclosures, and selected laboratory investigations. Experimental strategies should be designed to enhance understanding of how cycling of biogeochemical elements in specific terrestrial ecosystems might respond to changes in physical and chemical climate.
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From page 67... ...
Studies of atmospheric chemistry in combination with ecosystem investigations are needed, as are integrated studies of the troposphere and stratosphere. · To use the archives of the paleoenvironment preserved in ice and sediments to help develop and test models of the cycling of major biogeochemical elements and the feedbacks and linkages.
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From page 68... ...
McCarthy, Harvard University John Torrey, Harvard University Peter Vitousek, Stanford University
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