DC o N C N C

( ATMOSPHERE )

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Figure 14.3. A highly simplified model of the terrestrial carbon cycle. Here carbon flows from the atmosphere into terrestrial vegetation, then into litter and soil organic matter. Along the way, carbon is lost back to the atmosphere through microbial respiration (or decomposition) of litter and soil organic materials. Adapted from Foley (1995).

Figure 14.3. A highly simplified model of the terrestrial carbon cycle. Here carbon flows from the atmosphere into terrestrial vegetation, then into litter and soil organic matter. Along the way, carbon is lost back to the atmosphere through microbial respiration (or decomposition) of litter and soil organic materials. Adapted from Foley (1995).

where C^ is the carbon content of the litter pools (dead leaves, dead roots, and dead wood) and T is the average residence time of carbon in litter.

As litter decays, a fraction of this carbon is immediately lost to the atmosphere as CO2, while the remaining carbon is converted into soil organic matter (as humus). The carbon balance of the soil carbon pools can be represented as:

dCs,total f) ^ C ^ C s,k where C is the carbon contained in soil organic material (or humus) pools, (1 — f) is the fraction of the litter that is converted to humus (f is how much of the litter is respired to the atmosphere), and T is the average residence time of carbon in soil pools. When evaluating the overall carbon balance of the terrestrial biosphere, we must consider the carbon budget of the vegetation, litter, and soil pools altogether. Looking at these pools together, we see that maintaining a carbon sink in terrestrial ecosystems requires that there must be a net increase in combined carbon mass in the vegetation, litter, and soil:

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