Litter from above-ground and below-ground is the source of nutrients to soil organisms. A certain amount of minerals is also contributed from the weathering of clays. The nutrient composition of SOM reflects that of cells of the living organisms from which it came. Cells lyse soon after death of the organism or tissue. When a cell wall is present, the lysate is not released into the soil immediately. Digestion of the wall by extracellular enzymes secreted from some bacteria or protists is necessary to release the nutrients. Alternatively, the litter is fragmented and digested internally. These and soil trophic interactions will be dealt with in Chapter 4. However, it is worth remembering that the proteins, lipids, nucleic acids, polysaccharides and derivatives of these macromolecules are the initial source of SOM. Some protective or defensive macromolecules in living cells are digestible only by specialized species. These include sporopollenin of plant pollen; chitin of invertebrate exoskeleton, fungal cell wall and some protozoan cysts and tests; lignin, tannins and cellulosic polymers of plant cell walls; as well as cutin and suberin which waterproof plant cell walls.
As litter is fragmented and digested, it releases nutrients which are used for growth and reproduction of interstitial species. In turn, these species are consumed by their predators. The successive rounds of litter digestion and excretion and predation in the soil are responsible for decomposition. The fragments are eaten by organisms that ingest different particle sizes. Some of the ingested matter is excreted and becomes food for other species. In turn, these species are prey for predators and grazers which digest part of the food vacuoles and excrete the remains. The microscopic digested and excreted organic matter forms the amorphous SOM or humus.
The top organic horizons provide most of the labile nutrients. With depth, the organic matter becomes more recalcitrant as the more labile nutrients are removed and less soluble, less nutritious molecules remain. The labile SOM consists of molecules readily removed from the soil by living organisms and readily soluble molecules. These have a short halflife in soils as they are removed easily by living cells, but consequently they have a higher turnover rate. This pool is reduced over several years when soil is used for agriculture. As much as half the labile organic matter, or the light fraction, is used in the first year of growing a crop on a field. Therefore, it is essential that nutrients be replenished regularly with plant debris, manure, compost or chemical fertilizers.
Nutrients released from litter provide the essential chemicals required for growth and division of cells in the soil. These nutrients include species-specific vitamins, dissolved organic matter and mineral elements (Table 2.2). As SOM is decomposed, the amount of non-carbon atoms (especially nitrogen) decreases. They are accumulated in living organisms and released as soluble molecules and ions. The abundance of essential elements is critical to sustain cell growth. Liebig's law of the minimum states that the growth of a cell, or an organism, is limited by the 'most limiting nutrient', i.e. the required chemical which is least abundant. Therefore, in deeper soil horizons where the litter is most decomposed, the SOM may not support cell growth because one or more essential chemical is limiting, even in the presence of suitable carbon sources. The abundance of various nutrients or elements can be expressed in moles per soil dry weight, or in ^g/g dry soil (Tyler and Olsson, 2001). They can also be expressed as ratios of total organic carbon to particular atoms, ions or molecules, such as nitrogen, sulphur or phosphate content of soils. The ratios (e.g. the C:N ratio) can be used as an index of SOM quality or extent of its decomposition. A useful illus-
Table 2.2. Typical concentration of selected ions and compounds in soil solutions. These values change with depth and with soil types.
Molarity (10-2-10"4) Molarity (10"4-10"6) Molarity (<10-6)
Anions No charge Natural organic molecules in organic horizon
Amino acids and peptides, simple sugars, nucleic acids, carboxylic acids
Only if accidental spillage (many organic compounds)
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