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Fig. 11-10 The A14C values of the cores of the North Atlantic, Pacific, and Indian Oceans deep waters. The oldest waters are encountered near 40°N in the Pacific Ocean. (Modified with permission from M. Stuiver et al. (1983). Abyssal water carbon-14 distribution and the age of the world oceans, Science 219, 849-851, AAAS.)

by radioactive decay. The radiocarbon distribution is an important tool for determining the replacement times of the deep oceans. Great care has to be taken when interpreting the 14C distribution to take into account mixing between waters of different origin. This is especially true in the Atlantic, since the degree of isotopic equilibrium reached between the air and surface waters is different in the two source areas of Atlantic deep water, the Arctic and Antarctic surface waters (Broecker, 1979). This complication makes the apparent 14C age in seawater not simply a measure of the time elapsed since isolation from the atmosphere but a complex blending of the effects of water-mass mixing and uneven degrees of isotopic equilibrium in the ocean.

Care must also be taken to not confuse the 14C perturbation, e.g. from nuclear weapons testing with the AC02 perturbation lifetime. The former is largely due to fast isotopic exchange while the latter is controlled by a slower mass flux.

11.3.3 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 some of the most important aspects of organic carbon on land. Inventories of the total biomass of terrestrial ecosystems have been made by several researchers, a survey of these is given by Ajtay etal. (1979).

Primary production maintains the main carbon flux from the atmosphere to the biota. In the process of photosynthesis, C02 from the atmosphere is reduced by autotrophic organisms to a wide range of organic substances. The complex biochemistry involved can be represented by the formula

Assimilation

respiration

Gross primary production (GPP) is the total rate of photosynthesis including organic matter

Fig. 11-10 The A14C values of the cores of the North Atlantic, Pacific, and Indian Oceans deep waters. The oldest waters are encountered near 40°N in the Pacific Ocean. (Modified with permission from M. Stuiver et al. (1983). Abyssal water carbon-14 distribution and the age of the world oceans, Science 219, 849-851, AAAS.)

consumed by respiration during the measurement period, while net primary production (NPP) is the rate of storage of organic matter in excess of respiration. There are two main routes taken to estimate the world NPP and standing phytomass. The first method is to classify the biosphere into ecosystems in which, from measurements of estimates, values for the primary productivity and phytomass are assigned. The alternative method is to use estimates made by prognostic models simulating the effects of environmental factors on productivity and phytomass.

The possible effects of increased atmospheric C02 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing C02 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with C02 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.

Estimates of terrestrial biomass vary considerably, ranging from 480 Pg C (Garrels et al, 1973) to 1080 Pg C (Bazilevich et al, 1970). Bazilevich et al attempted to estimate the magnitude of the biomass before mankind's perturbation of the ecosystems. The latest work that undoubtedly had the most data available estimates the total terrestrial biomass, valid as of 1970, as 560 Pg C (Olson etal, 1983).

Terrestrial biomass is divided into a number of subreservoirs with different turnover times. Forests contain approximately 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 yields twigs, leaves, shrubs, and herbs that only make up 10% of the biomass. Carbon in wood has a turnover time of the order of 50 years, whereas turnover times of carbon in leaves, flowers, fruits, and rootlets are less than a few years. When plant material becomes detached from the living, plant carbon is moved from the phytomass reservoir to litter. "Litter" can either refer to a layer of dead plant material on the soil or all plant materials not attached to a living plant. A litter layer can be a continuous zone without sharp boundaries between the obvious plant structures and a soil layer containing amorphous organic carbon. Decomposing roots are a kind of litter that seldom receives a separate treatment due to difficulties in distinguishing between living and dead roots. Average turnover time for carbon in litter is thus about 1.5 years, although caution should be observed when using this figure. For tropical ecosystems with mean temperatures above 30°C the litter decomposition rate is greater than the supply rate so storage is impossible. For colder climates NPP exceeds the rate of decomposition in the soil. The average temperature at which there is balance between production and decomposition is about 25°C. The presence of peat, often treated as a separate carbon reservoir, exemplifies the difficulty in defining litter. The total amount of peat is estimated at 165 Pg C (Ajtay et al, 1979). Figure 11-11 illustrates this very strongly. The tropics have an extremely high NPP but very little carbon in the soil; whereas all higher latitude areas have the opposite relationship. The dynamics of the carbon reservoirs is very different on either side of the balance isotherm. Also thought provoking is the fact that a very large proportion of the areas that are covered today with carbon-rich soils have appeared in areas covered by ice shields during ice ages; much of the carbon in these soils today has probably been deposited since the last glaciation. A climatic change that moves the balance isotherm polewards would most likely give rise to a net flux of carbon to the atmosphere from regions that today are close to balance or in carbon accumulation zones. The zones of soil carbon accumulation are also the zones most likely to experience growth limitation due to lack of nutrients since the continuous deposition of carbon will always retain some nutrients (e.g., N and P). Another observation regarding Fig. 11-11 is that land-use changes occurring today in the tropics mobilize carbon to the atmosphere by the decrease in standing biomass; the companion flux of soil carbon oxidized upon plowing of virgin land is much smaller than for opening new agricultural land in temperate regions. This is something that occurred in Europe and North America during the nineteenth century. Many of

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(a) Global distribution of carbon produced annually, i grams of dry matter/m per year, (b) Global distribution of carbon preserved in soils, i in

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Fig. 11-11

(a) Global distribution of carbon produced annually, i grams of dry matter/m per year, (b) Global distribution of carbon preserved in soils, i in kg/m . (Both figures from Box and Meentemeyer (1991), used with permission from Elsevier Publishers.)

these lands are today being abandoned because of changes in agricultural practice; this gives rise to a net flux of carbon from the atmosphere to the standing biomass and soils (Sedjo, 1992). In the perspective of national carbon balances there are several complications; the net uptake today in temperate regions is only possible due to an early release of carbon due to land use changes. This illustrates two aspects of carbon exchanges and the terrestrial biosphere, the time scales of exchange can be long and simultaneously human behavior can alter the reservoirs rapidly upon change of human habits (see Fan et al, 1998).

There is a group of organic compounds in terrestrial ecosystems that are not readily decomposed and therefore make 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 discussed in Chapter 8. Schlesinger (1977) presented an assessment of the various carbon pools for temperate grassland soil (Fig. 11-12). The undecomposed litter (4% of the soil carbon) has a turnover time measured in tens of years, the 22% of the soil carbon in the form of fulvic acids is intermediate with turnover times of hundreds of years. The largest part (74%) of the soil organic carbon (humins and humic acids) also has the longest turnover times (thousands of years).

Soil Respiration

Soil Respiration

Permanent Accumulations in the Lower Profile

Fig. 11-12 Detrital carbon dynamics for the 0-20 cm layer of chernozem grassland soil. Carbon pools (kg C/ m2) and annual transfers (kg C/m2 per year) are indicated. Total profile content down to 20 cm is 10.4 kg C/m2. (Reproduced with permission from W. H. Schlesinger (1977). Carbon balance in terrestrial detritus, Ann. Rev. Ecol. Syst. 8, 51-81, Annual Reviews, Inc.)

Permanent Accumulations in the Lower Profile

Fig. 11-12 Detrital carbon dynamics for the 0-20 cm layer of chernozem grassland soil. Carbon pools (kg C/ m2) and annual transfers (kg C/m2 per year) are indicated. Total profile content down to 20 cm is 10.4 kg C/m2. (Reproduced with permission from W. H. Schlesinger (1977). Carbon balance in terrestrial detritus, Ann. Rev. Ecol. Syst. 8, 51-81, Annual Reviews, Inc.)

11.3.4 The Lithosphere

Although the largest reservoirs of carbon are found in the lithosphere, the fluxes between it and the atmosphere, hydrosphere, and biosphere are small. It follows that the turnover time of carbon in the lithosphere is many orders of magnitude longer than the turnover times in any of the other reservoirs. Many of the current modeling efforts studying the partitioning of fossil fuel carbon between different reservoirs only include the three "fast" spheres; the litho-sphere's role in the carbon cycle has received less attention.

Fossil fuel burning is an example of mankind's ability to significantly alter fluxes between reservoirs. The burning of fossil fuel transfers carbon from the vast pool of reduced carbon in the lithosphere to the atmosphere, and hence to the biosphere, hydrosphere, soils and sediments. The elemental carbon reservoir is estimated from average carbon contents in different types of rocks, ranging from 0.9% elemental carbon in shales to 0.1% in igneous and metamorphic rocks (Kempe, 1979b) and the relative abundance of the rock types. The resulting estimate is 2 x 107 Pg C (Hunt, 1972), a single reservoir several orders of magnitude larger than the sum of all reservoirs discussed so far. Of the 2 x 106 Pg of recycled elemental carbon (recycled carbon has traveled at least once through the lithospheric cycle) in the lithosphere only 104 Pg make up the economically extractable reserves of oil and coal. Most of the reduced carbon species in the Earth's crust are highly dispersed and probably never will be used as fuels. The carbonate minerals distributed in sedimentary rocks represent a carbon reservoir that is even larger than the elemental carbon reservoir. About three-fourths of the carbon in the Earth's crust is present as carbonates. Several forms exist; the dominant biogenic forms are calcite and aragonite. Both are stoichiometrically CaC03 but calcite has six-coordinated Ca atoms and is capable of substituting several percent Mg into its lattice. Aragonite has nine-coordinated Ca atoms and several percent Sr can be incorporated into its lattice. Both forms can precipitate depending on the Ca/Mg ratio in the solution; for the present

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