Statement about Tropical Rainforests

Question

I made a statement about tropical rainforests, and I want to know if it's somewhat true or not:

The soil in tropical rainforests is not exceptionally fertile, because it contains few minerals. The reason that a tropical rainforest has a huge amount of vegetation is because of the quick mineralisation. If a dead leaf falls onto the ground, it immediately gets turned into minerals, which the plants immediately use for sustaining theirselves

Answer 1

There are many websites which describe this phenomenon. They all seem to confirm the basic premise of the question: in tropical rain forests most of the minerals are held in the biomass and rapid decomposition contributes to the recycling of these nutrients for new growth. One example is here.

Tropical rainforests are noted for the rapid nutrient cycling that occurs on the ground.  In the tropics, leaves fall and decompose rapidly.  The roots of the trees are on the surface of the soil, and form a thick mat which absorbs the nutrients before they reach the soil (or before the rain can carry them away).  The presence of roots on the surface is a common phenomenon in all mature forests; trees that come along later in succession win out in competition for nutrients by placing their roots over top of the competitors, and this pattern is seen in the temperate rainforest as well.  What does not occur in the temperate rainforest, however, is a rapid cycling of nutrients.  Because of the cold conditions and the acidity released by decomposing coniferous needles on the forest floor, decomposition is much slower.  More of the nutrients are found in the soil here than would be the case in a tropical forest, although like the tropical forest most of the nutrients are held in the plants and animals themselves.

I looked for actual evidence of these differences in rates of decomposition and I found this:

Salinas, N. et al. (2011) The sensitivity of tropical leaf litter decomposition to temperature: results from a large-scale leaf translocation experiment along an elevation gradient in Peruvian forests. New Phytologist 189: 967-977

These authors reported a comparison of rates of decomposition at various elevations in Peruvian forest. The parameter k is the decay rate of dry mass, year^-1. The following data are taken from Table 3.

site          elevation (m)   mean k (yr-1; 15 species)
Tambopata     210             1.753
Tono          1000            1.424
San Pedro     1500            0.968
Trocha Union  2720            0.546
Wayquecha     3025            0.424

and this:

Jacob, M. et al. (2010) Leaf litter decomposition in temperate deciduous forest stands with a decreasing fraction of beech (Fagus sylvatica) Oecologia 164:1083–1094

These authors studied leaf decomposition in a deciduous forest in Germany, with emphasis on the effect of proportion of beech trees upon overall rates of decomposition. Using data from Table 1 and Figure 2 (values converted from mg^-1 g^-1 day^-1 to yr^-1)

% beech yr-1
  96    0.2555
  67    0.657
  21    1.022

My Conclusion

Rates of decomposition can be much higher in tropical rainforest (k=1.753 yr^-1 for the Tambopata site in Peru, k=0.256 yr^-1 for a beech-rich forest in Germany). However, at higher elevations tropical rain forests are characterised by much lower rates of decomposition (Wayquecha k=0.424 yr^-1), comparable to those in deciduous forests.

These are all measurements of loss of dry mass from leaf litter. This will be dominated by loss of carbon, and I would expect that minerals will leach from the decaying litter more rapidly than these values. Nevertheless it seems plausible that relative rates of mineral recycling will reflect these k values.