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The Biogeochemistry of the Amazon Basin$
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Michael E. McClain, Reynaldo Victoria, and Jeffrey E. Richey

Print publication date: 2001

Print ISBN-13: 9780195114317

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195114317.001.0001

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Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

(p.53) 4 Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests
The Biogeochemistry of the Amazon Basin

Elvira Cuevas

Oxford University Press

Terra firme forests are those that by definition are not permanently or seasonally flooded (terra firme meaning “firm terrain”). This type of forest encompasses the Amazon and Orinoco basins, stretching from the lower slopes of the Andes, east to the Guianas, and south to about 15°S in western Brazil and northern Bolivia (Richards 1996). Structural and compositional variability in these forests in the Amazon basin is very wide as a result of climate differences and geomorphological position. The region is not climatically uniform; the central and much of the southern parts have less and more seasonal rainfall than the eastern and western parts (Walsh 1996). These differences have direct and indirect ecological significance, as phenology and biological processes related to nutrient availability will be strongly influenced by both factors (Cuevas and Medina 1986, 1988, 1990, Medina and Cuevas 1989). Periods of two or more consecutive dry days are ecologically significant in a humid area such as San Carlos de Río Negro, in the northern part of the Amazon, because of low water retention capacity in the widespread sandy soils. In lower geomorphological positions, dry spells of 5–10 days may result in fluctuations of the water table from 0.4–1.0 m (Herrera 1977, Bongers et al. 1985). In areas with a more strongly seasonal climate, roots have been found extending to 18 m depth (Nepstad et al. 1995). This may explain the presence of evergreen forest in the seasonally dry eastern Amazon. Structure and physiognomy of terra firme forests is very similar throughout Amazonia, but floristically it is quite variable due to different compositions in the subbasins of the Amazon’s major tributaries. These subbasins are located within geochemical regions that can be differentiated based on the physicochemical properties of drainage waters (Sioli 1975, Fittkau 1971, Fittkau et al. 1975). Blackwater rivers, such as the Río Negro, drain mostly sandy podsolized soils low in most essential nutrients for plant growth. They are characterized by a high content of humic acids, which remain dissolved because of the predominant low concentrations of polyvalent cations, mainly Ca2+ and Mg2+.

Keywords:   ageotropism, base cations, calcium, decomposition bags, ectorganic horizon, fertilization, kaolinite, litterfall, magnesium, nitrification

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