• Carbon turnover in an ecosystem is not a cycle but an open exchange with the atmosphere. Ecosystem balance is determined from assimilation and respiratory processes in plants and soil. Fire and harvesting also determine the C turnover.
• Distinction is made between photosyn-thetic C02 fixation, identified as gross primary production (GPP), the growth of all plant organs, or net primary production (NPP), the net ecosystem productivity (NEP), the balance between assimilation and all respiratory processes, and the net biome productivity (NBP) that also takes into account non-respiratory carbon losses, from processes such as fire, harvesting and grazing.
• Litter couples plant productivity to heterotrophic processes in soil. It is only with great difficulty that the turnover rates of fine roots (<2mm diameter) can be measured. Fine roots are about 28% of the total root mass and are assumed to turn over in woody plants about once a year, which is comparable to the turnover of leaves. Fine roots may attain 4 km m~2 of soil surface area in tropical forests and up to 120 km m~2 in grasslands. The projected area of fine roots is about that of the leaf area index.
• Litter degradation takes about 1-20 years for leaves and needles and up to 100 years for wood. Degradation can be significantly reduced by various local factors such as anaerobic conditions in moors, clay content (which leads to the formation of clay hu-mic complexes) and low pH. Litter degradation is dependent on the chemical com position of the biomass with proteins, amino acids, cellulose and hemicelluloses directly degraded by - and incorporated into - microbes, i.e. they disappear rapidly from litter, whereas lignin and lipids are only slowly degraded and in the case of lignin are degraded without a gain in energy for the organisms (fungi with laccase activity).
• Plant polymers are degraded into monomers, and in part are recycled into microbial biomass (amino acids and pyridine) and are passed from generation to generation and from species to species (recycling).
• During the course of decomposition, organic substances decrease the O/C and H/C ratios, i.e. the proportion of cyclic aromatic substances increases.
• Fire releases C02 but also creates charcoal and crystalline carbon which due to the graphite-like structure are difficult to degrade. Black earth soils are enriched with such forms of black carbon.
• In European woodlands, about 46% of the litter is decomposed in 20 years, 39% in 100 years and only 15% remains after 100 years.
• Mathematical models of litter decomposition show the constant recycling of soil carbon, even for substances that are difficult to degrade in the microbial C cycle. This leads to an upper threshold of C storage in the ecosystem, determined by the clay content, duration and environmental temperature. Only about 1% of the assimilated C is held as recalcitrant carbon in the soil.
that the ecosystem reaches, in the long term, a constant value of soil carbon, specific to the system, but dependent mainly on the clay content of the soil (Fig. 3.3.11 B) and the mean annual temperature (Fig. 3.3.11 C; Lloyd and Taylor 1994; Schimel et al. 1994; Bird et al. 2001).
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