The role of the root mat in conserving soil phosphorus at San Carlos de Rio Negro in the Venezuelan Amazon

In an Amazon forest on a very nutrient-poor soil (an Oxisol) near San Carlos de Rio Negro, Venezuela, Herrera et al. (1978) showed that nutrients moved directly from decomposing litter through mycorrhizae into the leaf before they could be leached into and through the mineral soil. Herrera et al. (1978) took P32-labeled leaves and placed them on the forest floor. Fine roots of nearby trees grew onto and over the leaves within a matter of weeks. They then harvested the leaves and roots. An electron micro-autoradiograph showed P32 in the leaf, the mycorrhizae, and the root hairs, confirming that direct cycling of nutrients from leaves to roots could increase the efficiency of P cycling at this site (Went and Stark 1968). In a follow-up experiment at the same site, Stark and Jordan (1978) showed that when they applied radioactive calcium and phosphorus to the top of a root mat, 99.9% of the activity applied was retained by the roots through a combination of adsorption and uptake. Only 0.1% of the activity leached through the mat of roots, confirming the finding that roots and mycorrhizae prevented most P and Ca leaching in these nutrient-poor Amazon soils.

Table 2.7. Relationships between tropical regions, naturally occurring vegetation, and soils (three systems of classification). (Reprinted from Jordan 1985)



Soil characterization

Great soil group

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