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Nutrient-Conserving Mechanisms 2.6.3.1

"Direct" Nutrient Cycling

Soils in many regions of the lowland wet tropics are very low in nutrient elements because the processes of leaching and weathering are intense and there has been little geologic activity that would result in fresh parent material for the soils. Many soils in humid tropical regions are classified as Ferralsols in the FAO soil classification system (Zech 1993), a term that reflects the iron remaining in the upper soil horizon after the nutrient elements have been removed. Under other systems of classification, such as the US Soil Taxonomy System, Ferralsols are called Oxisols and Ultisols (Brady and Weil 2002; see also Table 2.7). Because of the very low capacity of these soils to retain nutrients (Sanchez 1976) and the high rate of release of nutrients from decomposing litter and soil organic matter, a high rate of nutrient loss from the system might be expected. However, in fact, losses due to leaching in tropical forests are not great and in some cases they can even show a positive balance (Table 2.6). The high Ca losses in Panama shown in Table 2.6 may be due to the exceptionally Ca-rich soil (Golley et al. 1975).

How is it that lowland moist tropical forests are so conservative of nutrients despite the high potential for nutrient loss? One reason appears to be the high concentration of roots and mycorrhizal fungi intertwined with the decomposing litter and humus on or near the soil surface. An example of the role of the root mat in conserving soil phosphorus in a nutrient-poor rain forest is given in Box 2.7.

Table 2.6. Runoff, atmospheric input, and balance of calcium in various ecosystems, arranged in increasing order of calcium runoff. Values are in kilograms per hectare per annum. (Sources of data given in Jordan 1985)

Formation or association and location

Runoff

Input

Balance

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