The Terrestrial Biosphere

Large amounts of carbon are found in the terrestrial ecosystems and there is a rapid exchange of carbon between the atmosphere, terrestrial biota, and soils. The complexity of the terrestrial ecosystems makes any description of their role in the carbon cycle a crude simplification, and we shall only review the most important aspects of organic carbon on land. Inventories of the total biomass of terrestrial ecosystems have been made by several researchers, and a survey of these is given by Ajtay et al. (1979). Primary production maintains the main carbon 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goudriaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e. glass house) increases the assimilation rate of some plants however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect, since most plants in natural ecosystems are growth-limited by other environmental factors, notably light, temperature, water, and nutrients. 

Terrestrial biomass is divided into a number of sub-reservoirs with different turnover times. Forests contain 90% of all carbon in living matter on land but their NPP is only 60% of the total. About half of the primary production in forests is in the form of twigs, leaves, shrubs, and herbs that only make up 10% of the biomass. Carbon in wood has a turnover time of 

Humus is a group of organic compounds in terrestrial ecosystems that is not readily decomposed and therefore makes up a carbon reservoir with a long turnover time. There are also significant structural differences between the marine and terrestrial substances (Stuermer and Payne, 1976). The soil organic matter of humus is often separated into three groups similar in structural characteristics but with differing solubility behavior in water solutions. Humic acids, fulvic acids, and humin are 

The A content of newly formed terrestrial biota is close to 0%o. Transformation of living plants into the various carbon reservoirs with longer turnover times is not associated with fractionation processes (5 C is close to — 25%o for all terrestrial organic material, whereas A C declines with the age of the material. Humus typically has a A C near —50%o. 

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Prentice, K. C. The Influence of the Terrestrial Biosphere on Seasonal Atmospheric Carbon Dioxide An Empirical Model Ph.D. Dissertation, Columbia University, New York, NY, 1986. 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

Carbon monoxide emissions from the terrestrial biosphere are small, but forest fires produce 0.02 Pg C/yr. Degradation of chlorophyll is dying plant material seems to be the largest CO-producing mechanism at 0.04-0.2 Pg C/yr (Freyer, 1979). 

Fung, I. Y., Prentice, K. C., Matthews, E., Lerner, J. and Russell, G. (1983). Three-dimensional tracer model study of atmospheric CO2 Response to seasonal exchanges with the terrestrial biosphere, /. Geophys. Res. 88,1281-1294. 

Assuming the current emissions and sinks remain about the same, estimate the global atmospheric CO2 mixing ratio in the year 2050. Now repeat this calculation, but this time assume that the terrestrial biosphere no longer continues to sequester some of this anthropogenic carbon. 

Hudson R. J. M. et al. (1994). Modeling the global carbon cycle Nitrogen fertilization of the terrestrial biosphere and the "missing" CO2 sink. Global Bio-geochem. Cycles 8, 307-333. 

Including also uptake by the terrestrial biosphere. The mean residence time for exchange with the sea alone is expected to be larger than this value by about 25%. 

A number of other interesting possibilities for utilizing the excess 14C in the atmosphere as a tracer of natural processes come easily to mind. Not much is known about the rate of turnover of humus in the soil. Measurements of 14C in soil humus over the next several years, while the terrestrial biosphere continues to fix carbon with significant amounts of excess 14C, should help to determine the rate of turnover of carbon in the reservoir of humus. Some work along these lines is already in progress (41). 

The required reduction therefore amounts to only a tiny fraction of the theoretical sequestration potential of the world soils. Although evaluation and certification of emission credits for sequestration of C in the terrestrial biosphere is certainly difficult (Marland et al., 2001), it is worthwhile to consider C sequestration in developing possible mitigation plans. 

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

Figure 2 Seasonal and long-term trends in concentrations, and 5 0 values of atmospheric CO2 measured at high latitudes in the northern (Barrow, Alaska) and southern (American Samoa) hemispheres by the NOAA/CMDL-CU flask network (www.cmdl.noaa.gov/ccgg/index.html). The terrestrial biosphere, concentrated in the northern hemisphere, dominants the seasonal cycle. Fossil fuel emission dominates the long-term trends in CO2 and in...

Ito A. (2003) A global-scale simulation of the CO2 exchange between the atmosphere and the terrestrial biosphere with a mechanistic model including stable carbon isotopes, 1953-1999. Tellus 55B, 596-612. 

Kaplan J. O., Prentice I. C., and Buchmann N. (2002) The stable carbon isotope composition of the terrestrial biosphere modelling at scales from the leaf to the globe. Global Biogeochem. Cycles 16(4), 1060. 

Wittenberg U. and Esser G. (1997) Evaluation of the isotopic disequilibrium in the terrestrial biosphere by a global carbon isotope model. Tellus Ser. B Chem. Phys. Meteorol. 49(3), 263 -269. 

Figure 13 Distribution of bomb testing between the atmosphere, ocean, and terrestrial biosphere as reconstructed by Broecker and Peng (1994). The ocean contribution is obtained by a model constrained by the inventory based on the GEOSECS survey. The contribution of the terrestrial biosphere is based on estimates of the biomass and turnover times for trees and active soil humus.

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