Carbon terrestrial biota

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). 

The exchange of CO2 between the atmosphere and terrestrial biota is one of the prime links in the global carbon cycle. This is seen by studying the variations of C in the atmosphere. Figure 11-14 presents atmospheric A C for the years... 

To solve this problem, we need to make computer calculations on the long-term global carbon cycle including the effect of terrestrial biota, ocean circulation pattern, and metamorphic activities which are not included in Kashiwagi et al. (2000) s computation. 

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. 

Shaver G.R. and Woodwell G.M. (1983) Changes in the carbon content of terrestrial biota and soils between 1860 and 1980 A net release of CO2 to the atmosphere. Ecological Monograph 53, 235-262. 

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. 

The exchange of CO2 between atmosphere and terrestrial biota is one of the prime links in the global carbon cycle. This is seen by studying the variations of C in the atmosphere. Figure 11-9 presents atmospheric <5 C for the years 1956 and 1978. The lines are consistent with addition or subtraction of CO2 with a d C of about — 27%o. This CO2 could be derived from either fossil fuel or plants. It cannot be oceanic, since surface water DIC has a 6 C of about + 2%o (Kroopnick, 1980). This confirms that the annual P Oj variations are primarily due to exchange with the terrestrial biosphere, and not caused by seasonal exchange with the oceans. 

From Fig. 6.1 it can be seen that the annual net primary production for land plants and marine plants is similar (c.60 and 40Gt, respectively), although the biomass of terrestrial plants is much greater than that of marine plants. This is an important demonstration of the fact that biomass is not necessarily a guide to productivity. There is another difference between the marine and terrestrial parts of the biochemical subcycle the residence time of C in the main reservoirs. From Fig. 6.1 it can be seen that the residence time of carbon in the terrestrial biota is c.5.5 years (i.e.600/110yr), and c.26 years (1600/60.6 yr) in soil organic matter. In contrast, the residence time of C in marine phyto-planktonic biomass is only c.2 weeks (1.5/40 yr), but c.338 years (39 000/115.3 yr) in oceanic dissolved carbon. 

Fcarbon from renewable terrestrial biota to the atmosphere (deforestation)... 

Fr Flux of carbon from the atmosphere to terrestrial biota (reforestation)... 

The full compartmental model and numerical values of the coefficients are given in Table 22.1. It is assumed that all reforested land increases the terrestrial biota, so ar = 1.0. The value of ad = 0.23 is that suggested by Schmitz (2002) [Lenton (2000) proposed 0.27]. Initial values for all M, are the preindustrial values given in Figure 22.6. The preindustrial fossil fuel reservoir is assumed to have contained 5300 Pg carbon the actual value is not important, only the emission rate Ff(t). The model requires as input Ff(t), Fd(t), and Fr(t) from preindustrial times to the present. Ff(t) is obtained from the historical record of carbon emissions from fossil fuels Fd(t) is that for deforestation, expressed also in units of Pg C yr-1. Until very recently, reforestation, Fr, can be assumed to have been negligibly small. 

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