Degradation plant detritus

Estuarine fungi contribute substantially to plant detritus due to their abundance and potential for degradation. Fungi are known to accumulate soluble atrazine from seawater through sorption, and release up to 2.2% as hydroxyatrazine and other atrazine metabolites another 4.6% is more tightly associated and less available to the external environment. The combined processes result in atrazine accumulation, and may contribute to its transport and redistribution through the estuary (Schocken and Speedie 1982, 1984). 

The example of a total extract composition of a tropical soil from the Amazon, Brazil, shows mycose as the major compound, numerous other monosaccharides, lipid components such as fatty acids and fatty alcohols, and natural product biomarkers (Fig. 9a). The mycose and elevated levels of the other saccharides reflect the efficient fungal/microbial degradation of plant detritus in the tropics. This can be compared to the saccharides in the soil from an almond orchard in California, where glucose and mycose are the main sugars with lipids, sterols and triterpenoids (Fig. 9b, ). 

Moran, M.A., and Hodson, R.E. (1989b) Bacterial secondary production on vascular plant detritus relationships to detritus composition and degradation rate. Appl. Environ. Microbiol. 55, 2178-2189. 

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). 

Humus is an amorphous, hydrophillic, acidic, partly aromatic, generally dark colored, and structurally complex material, resulting from the microbial degradation of plant detritus. Humus can be further classified as follows (a) humic acids a fraction that is soluble in alkali but precipitates on acidification of the solution, (b) fulvic acids a fraction that remains in solution after the extraction is acidified, and (c) humin a fraction that cannot be extracted by either alkali or acid (see Chapter 5 for details). 

Plant detritus ean, therefore, eontain labile compounds (eg. sugars), relfaetoiy eompounds to deeomposition (e.g. lignin) and decomposers inhibitor compounds (e.g. phenolic compounds). The relative proportion of these compounds in the litter will influenee its rate of decomposition because decomposer organisms will have different abilities to degrade each type of compound. Also, microbial decomposers ean generate new compounds from their transformation activity (e g. tarmin-protein eomplexation). 

Organic matter accumulates wherever continual or seasonal dominance of water inhibits degradation and humification of plant residues by excluding the air needed for the complete microbial oxidation of the detritus. Even when the net production of phytomass exceeds mineralization only marginally, over thousands of years this condition can result in the development of Histosols. Humic substances occur in both the solid and liquid phases of Histosols, but they have not received the same attention as the corresponding humic substances in soil. 

The growth rate of typical crop plants in compost to which partially degraded plastics have been added is shown in Table 6 [36] for the same plastics. Again there is no evidence that fragmented plastics have any effect on plant growth and within the limits anticipated from year to year, the loading of plastics detritus appears to make little difference to crop yields. 

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