It has also been shown that the rate of decay decreases with leaf age (see Singh, 1969). The population density (number/m2) of micro- arthropod populations of tropical forest soils and litters range from as low as 0.4 in the case of a dry season secondary forest litter of Nigeria to as high as 72 in the moist evergreen forest soil of Zaire (Madge, 1969; Maldague, 1970). Ultimate decomposition is mediated not by invertebrates but by microorganisms. Litter seems to be attacked first by bacteria and fungi, and insects become important decomposers only later (Madge, 1965).
A close relationship exists between litter decomposition and biogeochemical cycles. One important cause of the difference between the biogeochemical cycles of the various elements is their relative rates of release as the litter decomposes. Very rapid cycling of elements is a characteristic feature of most tropical forests. The factors contributing to the rapidity include high temperatures and humidities and the absence of a resting period which permits high productivity; except for nitrogen, levels of minerals in the vegetation are generally not greater in tropical as compared to temperate forests. The apparent high fertility of tropical forests is not due to any higher mineral contents in the ecosystem or to high contents in the soil but is due chiefly to this rapid recycling. In general, tropical forests tend to differ from most ecosystems in three ways, viz., their high vegetation biomass, the high element stores in this biomass, and their rapid rates of cycling. It has been estimated that the biomass and the superficial soil layers in tropical rain forests may have as high as 75 per cent of the mineral nutrients present, and that the rapid recycling allows these forests to thrive on poor soils, due to deep root systems.
Some run-off generally occurs between the litter layer and the soil proper. In terrestrial ecosystems, mineral turnover involves the circulation of minerals between organisms and soil, and mineral transport to and from the ecosystem. Under natural conditions, mineral cycling between the organisms and the soil is a mostly closed system. The production of organic matter and its mineralization go hand in hand. In tropical rain forests the higher productivity results from the rapid mineral turnover; in these forests the nutrients in the soil are only briefly tied up in organic compounds and soon become available to plants again as inorganic minerals (Larcher, 1975). The microbial-mediated mineralization rate in the soil is not the only determinant of mineral availability. Plants, microorganisms, and soils are regularly supplied with inorganic salts from external sources.
There is also a continuous leaching or loss of these salts from the ecosystem since mineralized nutrients are highly mobile in the soil and hence are subject to ready leaching. The inorganic salts entering the ecosystem mainly originate from the underlying rocks, water, air, and in agricultural areas, the added fertilizers. Weathering is an important source of nutrient input in raw soils. Inorganic salts may be lost from an ecosystem by wind erosion, by seepage into the ground, or by drainage. Most of the seeped and drained salts eventually discharge into the sea. Erosion from the watershed brings in dissolved salts and other solutes such as nitrogenous, phosphatic and silica compounds, which are important for plant growth. In fact, all suspended inorganic matter in rivers originates from the erosion and weathering of rocks.
Rocks may be composed of only a single mineral species or of several different kinds. Some important rocks, arranged in order of decreasing solubility, and their elemental composition, are listed here. Rock salt, NaCl; gypsum, CaS04; calcite, CaCO3; dolomite, MgCO3 + CaCO3; feldspar, KAlSi3Oj; soda-lime feldspar, mainly NaAISi3Os and CaAl2Si2Os; basalt, mixture of various aluminium silicates of Na, K, Mg or Ca; and granite, e.g., silica quartz and kaolin, Al2Si2Os(OH)4.