| Fig. 3.3.5. Schematic presentation of the decomposition of lignin. The first step is breakage of the ether bonds, then oxidation of the side chains and, finally, opening of the double bond of the benzene ring, as long as the molecule is still hydrophilic. (Zech and Kogel-Knabner 1994)

fied lignin fractions react, forming new polymers, the humic substances.

Waxes and lipids are long-chain hydrocarbons which form only 1-10% of plant dry matter but they are the most difficult material to decompose, as short-chain lipids are formed in the reaction which may be toxic to microbial metabolism. However, long-chain lipids are decomp osable, e.g. by methanogenic bacteria living under anaerobic conditions in small spaces in soil aggregates, and convert alkanes into acetate and then into methane (Fig. 3.3.6; Parkes 1999; Krii-ger et al. 2000).

At the end of this material degradation, substances remain which are either derived from plant metabolism or are newly synthesised by microbial metabolism. This means that degradation of plant substances consists of a series of several cycles (Fig. 3.3.7):

• conversion of polymers into monomers;

• cleavage of heterocyclic compounds via the side chains;

• uptake and incorporation of plant material into microbial biomass (recycling of carbohydrates, amino acids, pyridine);

• degradation of carbohydrates or terminal groups (e.g. R-COOH or R-CH2) to C02 or CH4;

• binding of polar groups to clay minerals. This last step is the actual stabilisation (protection from further degradation) of the organic substance in soil. Thus the clay content determines the ability of soils to store C (Bird et al. 2001);

• even if plant substances can no longer be produced in the original form (e.g. lignin) it does not mean that the substance has been mineralised to C02, but rather that some parts of the molecules (e.g. benzene) can remain for a long time in the soil, whilst other molecular parts (pyridine, amino acids) are passed on from generation to generation and from organism to organism;

• binding to clay minerals occurs via an ester or ether bond, hydrogen bridges or in the form of ionic interactions.

Methanogenic microorganisms

Methanogenic microorganisms

Sulfate-reducing microorganisms (1 type)

| Fig. 3.3.6. Schematic representation of the degradation of long-chain carbohydrates


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