The soil of the Amazon is extremely poor in nutrients thanks to continual leaching of nutrients. The leaching of nutrients is the drawing them down to below the layer of clay. Once there, plants can seldom reclaim them. The majority of the nutrients in the Rainforest ecosystem are actually in the plants. In the rainforest itself, the plants don't depend much on soil composition, as nutrient cycling is largely kept in the plant matter. This means that when the land is cleared, the nutrients are cleared too. The soil that remains is only usable for a few years before it is completely exhausted of nutrients. Farmers must then clear a new section of forest making a self perpetuating problem.
History of erosion and deposition in the basin has caused
extensive leaching of nutrinets from the soil
- including K+, Ca 2+, Mg 2+,
and inorganic phosphate
- especially in central and northern
areas of the Amazon basin
Primarily growth is limited by the absence of usable phosphorus,
but is also limited by calcium and magnesium.
Plants in the Amazon rainforest are tolerant of high
soil concentrations of mobile aluminum
Biomass development and regeneration capacity are not
always clearly associated with differences in soil fertility, but are definately
related to rates and patterns of nutrient cycling
Nutrient cycling tightly regulated mainly by biotic processes
- nutrients get relayed through litterfall.
Soils in the Amazon region
- except quartz, minerals have been
weathered to low activity
- labile aluminum is present
- soils acidic
- depletion of cations and low phosphorus
availabilty
Soil drainage plays an important role in species occurance.
When the density of tree species is analyzed in proportion
to stem density, there was a higher species density in the relatively richer
soils
Biomass differences explained by differences in water
and nutrient availability (although cannot rule out some influence from
succesional stage)
Distribution of carbon, phosphorus, and nitrogen in biomass follow those of soil C, N, and available P
60% of C, 65% of N, and 50% of P occured in particular
organic matter, with a mean residency of 4 years
27% of C, 29% of N, and 33% of organic P was mineral
associated, with a mean residency of 50 years
Stability is dependent on maintenance and cycling of the faster pool of organic matter
Measurement of litterfall and its associated fluxes of
nutrients is a practical way to evaluate the production capacity and availability
of nutrients in tropical forests
- Phosphorus /Nitrogen ratios of leaf
littterfall are inversely related to matter/P ratios.
- There is no relation between p/N
ratios and efficiency of N use
- P/N ratios mainly affected by availabilty
of P, not of N
- N is not limiting
Seasonality can affect nutrient availability
- seasonality of rainfall causes pulses
in decomposition of soil organic matter, and subsequent nutrient release
Ageotropism - lack of growth toward the soil can result from low Ca levals in soil system
Mg difficiency severely limits root growth, reduces drought resistance, and reduces adaptation to nutrient poor sites
References
Chemistry of the Amazon: Biodiversity, Natural Products, and Environmental
Issues
Peter Rudolf Seidl, Otto Richard Gottlieb and Maria Auxiliadora
Coelho Kaplan (Eds.)
ACS Symposium Series No 588. 1995
Lucas, Y., F. J. Luizao, A. Chauvel, J. Rouiller, D. Nahon. "The Relation
Between Biological Activity of the Rain Forest and Mineral Composition
of Soils"
Science. New Series. Volume 260. Issue 5107 (Apr. 23, 1993). 521-523
Laurence, William F., Philip M. Fearnside, SUsan G. Laurance, Patricia
Delamonica, Thomas E. Lovejoy, Judy M. Rankin-de Merona, Jeffrey Q. Chambers,
and Claude Gascon.
"Relationship between soils and Amazon forest biomass: a landscape-scale
study"
Forest Ecology and Management. 118 (1999). 127-138
Biogeochemistry of the Amazon Basin. Edited by Michael E. McClain, Reynaldo
Victoria, and Jeffrey E. Richey
Oxford University Press. 2001
Vitousek, P. M.: Sanford Jr., R. L. "Nutrient cycling in moist tropical
forest"
Ann. Rev. Ecol. Syst. Vol 17, 1986 p137-167