Carbon cycle decomposition

Dead vegetation also afreets the global carbon cycle. Dead organic matter decomposes, releasing carbon dioxide to the atmosphere. Rates of decomposition vary with material, location, and climate. Non-woody organic matter decomposes rapidly woody organic matter slowly. Decomposition tends to occur faster at the soil surface than below. Decomposition is relatively fast in warm moist climates. In cold climates and in wetlands, decomposition is so slow that there is a net increase of stored carbon in the soil and organic soils called, "histosols, are formed. 

Golchin A, Baldock JA, Oades JM (1998) A model linking organic matter decomposition, chemistry, and aggregate dynamics. In Lai R, Kimble JM, Follett RF, Stewart BA (eds). Soil processes and the carbon cycle. CRC Press, Boca Raton, pp 245-266... 

Figure 6.7. Simplifed soil carbon cycling scheme. Major inputs (plant litter) to and outputs (respiration and erosion) from the soil carbon reservoir. The observed flux of C out of the soil can be modeled by assuming three pools of carbon an active pool with a turnover time on the order of years, an intermediate pool with a turnover time on the order of decades to centuries, and a passive pool with a turnover time on the order of millennia. The decomposition constant is k = 1/t. Subscripts a, i, and p refer to the active, intermediate, and passive C pools, respectively. Adapted with permission from Amundson, R. (2001). The carbon budget in soils. Annu. Rev. Earth Planet. Sci. 29, 535-562.

All models of the C02 cycle need improvement in the way they detail the spatial distribution of soil-plant formations and in the way they specify exchange processes in the ocean and at the atmosphere-ocean boundary. Accurate parameterizations of all studied elements of the biogeochemical carbon cycle should be synthesized into a single system. Such an attempt was made in the block scheme of the model shown in Figure 3.6. The main sources of C02 are the day-to-day activity of land and marine animals, photochemical reactions, decomposition of dead organic... 

Methane clathrate decomposition has been implicated in the Latest Paleocene Thermal Maximum (—55 Ma ago) by an extraordinary injection of isotopically light carbon into the carbon cycle (Dickens, 2000, 2001a) and in Quaternary interstadials as indicated by observations of isotopically light foraminifera in Santa Barbara Basin sediments (Kennett et al., 2000). Dickens (2001a) compares the functioning of the CH4 hydrate to a bacterially mediated capacitor. 

Figure 2 Conceptual model of carbon cycling in the litter-soil system. In each horizon or depth increment, SOM is represented by three pools labile SOM, slow SOM, and passive SOM. Inputs include aboveground litterfall and belowground root turnover and exudates, which will be distributed among the pools based on the biochemical nature of the material. Outputs from each pool include mineralization to CO2 (dashed lines), humification (labile slow passive), and downward transport due to leaching and physical mixing. Communition by soil fauna will accelerate the decomposition process and reveal previously inaeeessible materials. Soil mixing and other disturbances can also make physically protected passive SOM available to microbial attack (passive slow).

The transformation of plant detritus into stabilized humic substances is one of the most complex and least understood biogeochemical processes in the carbon cycle (Stevenson, 1994). Traditionally, decomposition and humification of plant residues was thought to be dominated by the mineralization of labile materials, while more recalcitrant aromatic compounds accumulate in the soil. The application of modem analytical techniques—including solid-state NMR spectroscopy, pyrolysis gas chromatography, and degradative chemical techniques—to the study of decomposition and humification has significantly altered this simple view of carbon transformation in the soil (Baldock et al., 1997 Kogel-Knabner, 1997). 

The turnover and stability of SOM depends mainly on environmental and biological parameters. Either biomass production or decomposition rates are affected. Additionally, soil matrix and litter quality and fire frec uencies stabilize carbon in soils. From the presented results it is obvious that eco.systems have different mechanisms for stabilizing SOM, which lead to different chemistries of the stable compounds. For a better understanding of SOM in the terrestrial carbon cycle and to identify the missing carbon sink, some major points have to be considered ... 

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