Nutrient cycles

All the plant nutrients are in continuous circulation between plants, animals, the soil and the air. The processes contributing to the production of simpler inorganic substances, such as ammonia, nitrites, nitrates, sulphates and phosphates, are sometimes referred to as mineralization. Mineralization yields chemicals that are readily taken up by plants from the soil solution. The formation of humus, organic residues of a resistant nature, is known as humification. Both mineralization and humification are intimately tied up in the same decomposition process, but the terms help identify the end product being studied. Likewise it is possible to follow the circulation of carbon in the carbon cycle and nitrogen in the nitrogen cycle, although these nutrient cycles along with all the others are interrelated.

The carbon cycle

Green plants obtain their carbon from the carbon dioxide in the atmosphere and, during the process of photosynthesis, are able to fix the carbon, converting it into sugar. Some carbon is returned to the atmosphere by the green plants themselves during respiration, but most is incorporated into plant tissue as carbohydrates, proteins, fats, etc. The carbon incorporated into the plant structure is recycled and eventually released as carbon dioxide, as illustrated in Figure 18.4.

All living organisms in this food web release carbon dioxide as they respire. The sugars, cellulose, starch and proteins of succulent plant tissue, as found in young plants, are rapidly decomposed to yield plant nutrients and have only a short-term effect. In contrast, the lignified tissue of older plants rots more slowly. Besides the release of nutrients, humus is formed from this fibrous and woody material, which has a long-term effect on the soil. Plants grown in the vicinity of vigorously decomposing vegetation, e.g. cucumbers in straw bales, live in a carbon dioxide enriched atmosphere. Carbon dioxide is also released on combustion of all organic matter, including the fossil fuels such as coal and oil. Organic materials such as paraffin or propane, which do not produce harmful gases when burned cleanly, are used in protected culture for carbon dioxide enrichment (p113).

Fossil Plant Paraffin

Figure 18.4 Carbon cycle. The recycling of the element carbon by organisms is illustrated. Note how all the carbon in organic matter is eventually released as carbon dioxide by respiration or combustion. Green plants convert the carbon dioxide by photosynthesis into sugars which forms the basis of all the organic substances required by plants but also animals and micro-organisms

Figure 18.4 Carbon cycle. The recycling of the element carbon by organisms is illustrated. Note how all the carbon in organic matter is eventually released as carbon dioxide by respiration or combustion. Green plants convert the carbon dioxide by photosynthesis into sugars which forms the basis of all the organic substances required by plants but also animals and micro-organisms

The nitrogen cycle

The nitrogen cycle similarly follows the fate of nitrogen in its many forms in the plant, the soil and the atmosphere (see p366).

The sulphur cycle

Sulphur is an essential constituent of plants that accumulates in the soil in organic forms (see page 370). This sulphur does not become available to plants until aerobic micro-organisms mineralize the organic form to produce soluble sulphates. Under anaerobic conditions there are micro-organisms which utilize organic sulphur and produce hydrogen sulphide, which has a characteristic smell of bad eggs often evident in waterlogged soils in warm conditions.

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