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Carbon Turnover in Ecosystems

The term carbon cycle is often used for ecosystems. This is incorrect, as it does not take into account that C02 from the free atmosphere used in photosynthesis mainly. Only a small amount originates from respiration at the same site. Atmospheric turbulence leads to a rapid mixing in the atmosphere. Thus the C02 which is assimilated enters the ecosystem by advection via the atmospheric transport of air masses and is lost again in the same way (see Chap. 2.1).

The plant cover assimilates C02 from the atmosphere by photosynthesis (Fig. 3.3.1). The sugars synthesised are either used immediately for growth, to form or maintain existing structures, or are stored in the form of starch (e.g. see Chap. 2.4). The sum of photosynthetic C bound over a certain period (e.g. 1 year) is called gross primary production (GPP) neglecting light-induced respiration (photorespiration, see Loreto et al. 1999).

About 50% of the photosynthetically assimi lated carbon is consumed by the plant itself for maintenance and synthesis of biomass. The balance of assimilation and respiration of plants is called net primary production (NPP). A distinction is drawn between the more frequently measured above-ground primary production (ANPP) and the below-ground primary production (BNPP) because it is easier to measure ANPP. Depending on the organisation of individual plant structures, there are allometric relations between ANPP and BNPP (for trees, on average, BNPP/NPP = 0.3). Quantitative assessments of plant biomass on earth and of NPP were made in the 1960s, within the International Biological Programme (IBP). These data are still the basis for many model calculations, but there are still, according to the subject area, many different interpretations about the "productivity" of plants. For the farmer it is the grain yield, for the forester the increase in stem growth and wood volume, for the biologist the total increase including fruit and fine roots. There is no direct connection between biomass and NPP. Biomass formed by NPP remains for very different periods in the organism or in the stand, thus leading to different values of biomass (see Table 3.3.1). For annual species the total biomass, except seeds, dies off at the end of the growing season and is again supplied to the heterotrophic organisms in the soil as litter. Disregarding rhizomes, bulbs or corms which are permanent organs in perennial herbaceous plants, almost all the total biomass resulting from the NPP also dies and becomes litter. In woody plants about 50% of the NPP is wood. In the lig-nified organs, part of the sapwood is changed to dead heartwood, depending on the C assimilated, and this is converted into litter with a long delay, because the heartwood forms the tree's structure (see Chap. 2.4.5.4). Even for trees about 50% of the NPP reaches the soil each year

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