Terrestrial ecosystems store about 2500 PgC: 500 PgC in vegetation and 2000 PgC in soil. The amount of carbon stored in vegetation varies significantly depending on vegetation type. Forests generally store ten times more carbon than grasslands. However, forest soils do not necessarily contain more carbon than grassland soils, for carbon stock in soil depends on the factors that control the rate of organic matter decomposition.
The global net production of organic matter by plants, net primary production (NPP), is about 60 PgC yr- . This input to the pool of living organic matter is compensated by litter fall. The residence time of the pool is 1 year in case of annual grasslands, but it may be 100 years in case of pristine forests.
The litter fall in its turn is the input to the pool of nonliving organic matter, which is compensated by heterotrophic respiration (i.e., by CO2 released with the respiration of soil biota decomposing organic debris). The net accumulation of carbon by ecosystem, including both soil and vegetation, is called net ecosystem production (NEP).
NEP of an undisturbed ecosystem should be close to 0. However, most of ecosystems are disturbed in some way (harvesting, fire, etc.). Therefore, global NEP is estimated at 10PgCyr- . This value characterizes nonrespiratory losses such as release of carbon due to forest fires, or relocation of carbon to wood products and other components of urban metabolism.
The net accumulation of carbon that includes both respiratory and nonrespiratory losses is called net biome production (NBP). NBP for a relatively short period of time may differ from 0, reflecting continued effect of the losses occurred in the past. Thus, global NBP (i.e., net terrestrial uptake) for the decade 1990-99 has been estimated to be positive (1.4 ± 0.7 PgC yr-1) due to decrease in the nonrespiratory losses.
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