| Abstract: | Degradation and transformation of plant residues and their components by the microflora of the soil This review describes recent results of publications in this area. It condsiders the dynamics of degradation under field and laboratory conditions and indicates some of the problems of simulation models. The preponderant part of transformation processes apparently takes place in a relatively small fraction of soil organic matter which also includes the biomass. Methods for the quantitative measurements of the biomass have been considerably improved. The turnover rates of plant residues are effectively influenced by their chemical composition, for instance by their C/N ratio and their content of lignin or polysaccharides. C/N ratios also seem to influence the socalled priming effect and the transformation of increasing amounts of plant residues added to the soil. Some progress has been also made in the transformation of plant, residues under different climatic conditions. Experiments with polysaccharides and glucose have indicated that a major portion of the residual carbon residues in soil are contained in the biomass or N-containing microbial metabolites. A small part is also present in the phenolic constituents of humic compounds. Root excretions also contribute carbohydrates. Recent experiments indicated the transportation of a considerable amount of photosynthetic products from sprouts through roots into the soil. They cause intensive microbial turnover processes in the root zone. N-containing compounds are stabilized in the soil biomass. Proteins and aminopolysaccharides are furthermore stabilized by sorption on humic compounds or clay particles. They are even more effectively stabilized by linkage into humic compounds. Plant residues contain appreciable amounts of free and polymerized phenols which are degraded by microorganisms or incorporated into humic compounds. Easily oxidisable phenols are more slowly degraded in soil than the more stable ones. This is especially true for low concentrations of phenols added to the soil. The pH-values of the soil and its content of humic compounds influence essentially the degradation rate. Lignins belong to the most important natural biopolymers. Methods for their specific labelling with 14°C have essentially promoted studies about their degradation and transformation in soil and have enlarged the knowledge about lignin degrading organisms. Beside lignins and other plant polyphenols, melanins may play an important role in the formation of humic compounds. These melanins are synthesized form carbohydrates by some microscopic fungi through secondary metabolic reactions. They are formed from phenols, quinones, polyenes, aminoacid and aminosugar derivatives. Similar to humic acids they are relatively stabile against rapid microbial degradation. Analytical methods developed in the last years indicate that both groups of polymers contain similar building blocks. |
