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Amazonian Biodiversity
The Amazon Rainforest, or Amazonia, provides habitat to what is perhaps the most diverse and rich biome on Earth. Wet, tropical, moist broadleaf forests provide sustainable habitats for not only millions of insects and thousands of exotic animals, but it also provides a suitable environment for tens of thousands of plant species. Thus, the species richness and biodiversity of the forests are extremely vast. The ability to support such vast life has therefore been the subject of much study, as deforestation is a prominent political and environmental concern today. Conservation biology and soil sciences are two fields of study with significant developmental research that attempts to address recuperation of forests after disturbances and deforestation destroy a large portion of the above-ground biomass in the forests. An array of factors that contribute to the overall biomass in Amazonia have been studied, and correlations have been drawn relating the effectiveness and relative importance of many of these factors on overall biomass production of plant species which provide the expansive habitat this is Amazonia.

Soil Characteristics

The soil composition in the Amazon rain forest is extremely variable, though the majority of rainforests in the Brazilian Amazon occur on heavily weathered, nutrient poor soils. Additionally, the Amazonian soils tend to have lower nutrient budgets and a higher proportion of nutrients contained in the living biomass of the ecosystem than rainforests on richer substrates. Some of the Amazon Basin, the area in South America drained by the Amazon River and its tributaries, is composed of Terra preta soils rich in organic carbons, however. These dark, fertile soils have notably high concentrations of nutrients such as nitrogen (N), phosphorous (P), calcium (Ca), zinc (Zn), and manganese (Mn), as well as a relatively high activity of microorganisms. The Terra preta soils are often resistant to leaching, a major problem in rain forests; however, areas characteristic of these soils are often surrounded by acrisols, but also ferralsols, both of which are infertile soils. Ferralsols, relatively widespread in the Amazon Basin, are heavily weathered soils that have low base saturations. They are often well aggregated, porous, and friable, with variable clay contents. The clay particles can form durable aggregations, giving the soil poor water-holding characteristics, even with high clay contents. The Xanthic ferralsols in some areas of the Amazon Rainforest, around the Manaus area, are typically acidic and very poor in certain nutrients, P, Ca, and potassium (K), thought essential for increased biomass according to many studies. Acidities of soils can often reach such levels that relatively few soil organisms can inhabit the soils, granting various bacterial and viral species dominance in various Amazonian soils.

Significant Determinants of Biomass

While it is difficult to pinpoint exact determinants of biomass due to varying environmental interactions and localized ecologies, when relatively isolated and randomly sampled, several key elements, nutrients, and geographical features significantly affect the amount of above-ground biomass of Amazonia. Similarly, the amount and scale of disturbances within a given system has impacts on the above-ground biomass (AGBM). It has been shown previously that, in temperate forests that have relatively uncommon or infrequent disturbances, soil fertility is often negatively correlated with species richness. Additionally, when phosphate levels, whose concentration is thought to be the most significant determinant of plant biomass, are relatively low and constant over a large area, it has been shown that the AGBM is positively associated with total N, total exchangeable bases, K+, Mg2+, clay, and organic carbon (C). Additionally, biomass was found to be negatively associated with Zn cation concentration, aluminum saturation, and find sand content. In studies with relatively constant and low phosphate levels, total nitrogen was the single most important soil variable for biomass production; however, this contradicts many other studies which claim that nitrogen availability apparently constrains productivity at tropical montane sites. It is therefore hypothesized that the observed nitrogen dependence is a direct result of lowered phosphate and phosphorous levels in the soils, which, when relatively constant over a large enough sampling size, have no significant effects on Amazonian biomass.

The relative slope of the land in Amazonia, though marginally non-significant using a Bonferroni-corrected alpha value of 0.006, has been shown to have a negative effect on the total biomass of a given area due to heightened rates of water and nutrient movement. Additionally, the slope of certain regions has impacted various other soil variables, decreasing concentrations of clay, silt, organic C, total N, total exchangeable bases K+, Mg2+, iron (Fe), and Mn2+, and increasing levels of sand and aluminum saturation.

Soil gradients within soils also have a large impact on the overall biodiversity of a region. With increased clayey substrate composition, to a certain level, biomass increases. Clays are very often moist, nutrient-rich, fertile soils; however, without the proper composition of sands and silts, mobility of fluids, organisms, and nutrients within clay can hinder biomass production. Flat areas often have high clay contents optimal for biomass growth (typically 45-75% clay-composition), while sandier soils more often lie on slopes and in gullies. Therefore, this soil "clay-sand continuum" is an extremely important gradient for determination of biomass. Clays have heightened concentrations of nitrogen and many cations as clays are positively correlated with organic matter, which is an important determinant of cation exchange capacity in soils like Amazonian ferralsols.

A second gradient, though less significant than the aforementioned "clay-sand continuum," is related to soil pH. Soils with higher acidity tend to have less total phosphorus and higher aluminum and calcium ion concentrations than soils of more basic nature. Soil pH has been shown to strongly influence aluminum, calcium, and magnesium ions in tropical soils, and these metal ions are often very toxic to plants;  thus, while this soil acidity gradient is less significant and largely independent of the "clay-sand continuum," it is still relatively important to consider.

A third gradient between soil phosphate and available water capacity is also important, though relatively insignificant in areas with low phosphate concentration. Phosphate is positively associated with organic carbon and iron and aluminum oxides in Amazonian ferralsols, which in turn increase in clayey soils.

Soil Effects on Tree Biomass

Soils affect tree biomass primarily in one of two main ways, though this is certainly not an exhaustive list of soil effects on tree biomass. First, the species composition of an area will be largely influenced by the composition and physical characteristics of the soil. Large, emergent species which contain a high fraction of forest biomass, for example, could be associated with the most fertile soils. Second, trees might simply grow bigger on very fertile soils, regardless of the species richness or composition in the area. Soil and drainage factors have been shown to have marked influence on florisitic composition in Amazonian terra-firme forests.