It has been suggested that small particles with a relatively large specific surface are particularly effective in stabilizing soil organic matter (Christensen, 1988; Cheshire et al., 1990; Leinweber et al., 1993), and hence in the formation of organo-mineral associations.
Certainly, such fractions can preserve large amounts of soil organic matter in biologically resistant forms and thus provide a pool of moderately available nutrients (Anderson and Paul, 1984). In arid and semiarid zones, water erosion processes lead to a substantial loss of these fine fractions -20 mm, which are the most fertile and, as a consequence, soil fertility declines (Stocking, 1984).
Likewise, the amounts of C and N that may be associated to clay and silt size fractions are affected by many factors including soil texture (Christensen, 1992; Hassink, 1995), the dominant type of clay mineral (Hassink, 1997) and the use to which the land is put (Hassink, 1994; Parfitt et al., 1997). Thus, in arable soils most of the soil organic matter can be found in the clay and silt fractions, whereas in forest and grassland soils the contribution of sand size organic matter to total soil organic matter is greater (Christensen, 1992; Hassink, 1997). On the other hand, soils dominated by clays with a high specific surface area and numerous reaction sites probably adsorb more humic substances than soils dominated by clays with a low specific surface area (Tate and Theng, 1980).
The loss of organic matter from forest soils following disturbance is an important source of CO2 for the atmosphere. Bolin (1977) and Schlesinger (1977, 1983) have estimated the net loss of organic carbon from the world's soil. Bolin (1977) states that if it is assumed that from 25 to 50 per cent of the presently cultivated land has been converted from forest land since the early nineteenth century, the release of organic carbon from the soil to the atmosphere during
53 the last two centuries can be estimated at 10 to 40 X1015g, with an annual loss at 0.1 to 0.5 x 1015 g. Kovda (1974) has estimated the total humus in the earth's soil at 2400 x 1015 g, equivalent to approximately 1400x 1015 g of carbon. Bohn (1976) stated that earlier in this century the organic carbon in the world's soil was estimated at 710 x 1015 g. This estimate was based on the carbon content of nine North American soils. Bohn estimated that there are 3000: 1500 x 1015 g of organic carbon in the world's soil. Although he did not try to estimate the amount of CO2 released to the atmosphere, Bohn (1976), says that the decay of soil organic matter is one of the largest CO2 inputs to the atmosphere'. Both plants and animals provide inputs of organic matter to soils. Once within the soil organic residues can be distinguished on the basis of their chemical structure (e.g., old lignified humic substances that degrade slowly), by their source (plant or animal) or by location.
The standing crop of litter in semi-arid grasslands is usually more than 3 t/ha and in temperate dry steppe may exceed 11 t/ha (Klemmedson 1989). There has been much debate about the relative contents of organic matter in tropical and temperate soils. Within those wet-and-dry climates that have hot summers assisting rapid decomposition, there is no evidence of inherently lower levels of organic matter in the tropics than in comparable temperate regions (Juo and Payne 1993). Kowal and Kassam (1978) and Juo and Payne (1993) review the role of organic matter in tropical soils. Here, it is sufficient simply to state that organic matter has various interrelated effects on soil fertility. In particular it should be noted that both chemical and physical effects are of relatively great importance in the soils of the semi-arid tropics because these generally have low cation exchange capacity.
Schlesinger (1977, 1983) has calculated the mean carbon content in 11 ecosystem types and multiplied the mean by the amount of land included in these ecosystems. He offered a preliminary estimate that the earth's total soil carbon is 1515x 1015 g. The annual release of soil carbon by his estimate is about 0.8 x 1015 g. This figure is based on the assumption that the annual conversion of forest to cultivated land is 15 x 106 ha and that the average carbon content of 131 t/ha in forest soil drops to 78.6 t/ha after conversion to cultivation, a decline of 40 per cent.
Although organic matter is often present in the soil to a depth of 1 or 1.5 m, most is in a surface layer of from 1 to 20 cm. Carbon also exists in the mineral part of soils or in the soil
54 solution, mainly as carbonates and bicarbonates of calcium, magnesium and sodium. Dudal (1978), observed that there is also a slight increase in soil carbon when land is irrigated, when organic manure is added or when cropping is intensified, but annual addition of soil carbon due to these practices is very small in comparison to the net loss.
The organic matter content in soil depends on soil conditions, present and recent vegetation cover, topography, hydrological conditions, and elevation and farm management practices.
Soil conditions in turn are most influenced by the soil moisture and temperature regimes, although the biological and mineralogical regimes are also important. For example, soil derived from rocks that are basic (as opposed to felsic) generally contains more organic matter than soil from felsic rocks. Clay content and type also affect organic matter content.
Furthermore, the oxidation of organic matter is more rapid in calcareous soil than in non-calcareous soil. Various attempts have been made to correlate the organic matter content of soils in a specific region or country with some of these factors. Young (1976) has provided a summary of processes that affect organic matter content.
Under natural conditions the content of organic matter in soil is constant; the rate of decomposition is equal to the rate of supply of organic matter from plants. The equilibrium is disturbed when forests are cleared and the land is used for agriculture. There is also a decline in organic matter when grassland in the tropics and subtropics is transformed into cropland, or when savannahs are burned. The decline is rapid in the first few years after deforestation and gradually slows over the next 10 to 50 years. Organic matter is also lost through misuse or deterioration of land (soil erosion, salinization, alkalization and soil degradation), and because of the increasing non-agricultural use of land (urbanization and highway construction).
On the other hand, there may be an increase in organic matter when good farm management is practiced and organic manure and compost are used, when arid land is irrigated, or where agricultural land is reforested. The proportion of animal and human manure used on cropland is more variable. Some farmers have developed stable systems which strongly emphasize the use of animal manure on crops. For example, Norman et al., (1982) describe how farmers in northern Nigeria managed to apply 4 t/ha of manure to their heavily-cropped crop land though they had only 3 cows each. Many other farmers do not ensure adequate recycling, either because they are more concerned with livestock management or they do not know the importance of maintaining a 'zero nutrient budget' to replace nutrients removed by the crop.
55 Norman et al., (1982) also describe farmers with 10 cows each, who applied only 1.9 t manure/ha to their crops.
Within a cropping system, manuring practice varies with location. There is transference towards the centre of the system. On traditional farms, the area near the household or village is highly fertilized with human and animal manure while more distant fields receive little or no organic matter. Fussell (1992) describes such a traditional 'ring' farming system in semi-arid West Africa. Here, if houses are thatched, the village needs rebuilding or moving every 2 to 4 years. Moving takes advantage of the fertility gradient. Where the huts are not moved, the fertility gradient becomes steeper with time. Rather than trying to even out fertility by labour-intensive transport of manure, farmers vary the cropping of the fields. Continuous cropping of millet is sustainable close to the hut or village where there is plenty of human and animal manure but crop rotations are essential at the periphery.