In most terrestrial ecosystems the
bulk of nutrient cycling occurs in the topmost layers
of soil. The main sources of the nutrient inputs to
these soil layers comes from weathering,
rainfall, fertilizers, atmospheric fallout,
and organisms. Organism add nutrient matter via excreted
wastes, shed tissues, and from the decomposition of
their tissues when they die. Under most conditions,
plants are the greatest single source of nutrients
to soils. Plants not only supply nutrients released
by organic decomposition of
shed tissues and dead body parts,
but also substances carried in from the plant leaves
when water flows over them (foliar
leaching). Losses or outputs of
nutrients within ecosystems are by leaching, erosion,
gaseous loss (like denitrification),
and plant root uptake for growth purposes. Within the
soil, nutrients are found attached to the surface of
soil particles by chemical bonds, stored within the
chemical structure of dead organic
matter, or in chemical compounds.
Organic matter decomposition is the main
process that recycles nutrients back into the soil. Decomposition
of organic matter begins with large soil organisms like earthworms, arthropods (ants,
beetles, and termites), and gastropods (slugs
and snails). These organisms breakdown the organic matter
into smaller pieces which can be decomposed by smaller
organisms like fungi and
heterotrophic bacteria (Figure
9q-1).
| Figure 9q-1:
Fungi play an important role in the decomposition
process converting organic matter back into basic
inorganic chemicals. |
Decomposition of organic
matter may take several months to several years to
complete. In tropical regions, the whole process is
quite quick because moist conditions and high temperatures
enhance the rate of biological activity.