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Litterfall, the plant matter that falls to the
forest floor, plays an important role in the nutrient cycling of rainforest
plants. Plants recycle the nutrients in the litterfall by reabsorbing them
through their roots. A sharp decrease in the amount of litterfall is indicative
of deforestation, and any major changes in the nutrient richness of the litterfall
in an area can act as a sign that the area is in trouble.
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Litterfall is measured both for its biomass and for its chemical composition,
and is collected in either conical traps or long troughs. Neither the
conical traps nor the long troughs are very large (80 – 100 cm in diameter
for the cones), and many of them will be spread throughout the area marked
for observation. Both the troughs and the conical traps will work for
observational purposes, but I would favor the conical traps as they are
slightly less cumbersome to empty. Both are constructed of mesh prevent
the accumulation of water.
During the rainy season, litterfall accumulates more rapidly and needs
to be measured at least once a week. During the dry season, measurements
can be lessened to once a month. In collecting samples, include only the
portions of plant matter that are within the trap – for example, cut long
twigs, so that only the portion of twig that was over the trap is collected.
Once the sample has been collected, separate it into leaves, reproductive
parts, fine woody material, and residue, discarding any animal material
or scarce adhering mineral material. Then dry and weigh the samples - adding
them to obtain the biomass – and store for chemical analysis.
Chemical Analysis
There are three main kinds of tests run on the litterfall samples once
they have been dried and their biomass measures. The first of these, plasma
emission spectrometry is the most widely used and can account for the presence
of most elements and ions that require study. It cannot, however, account
for all of them, which is why Redox potentiometry and ion liquid chromatography
are also employed.
Plasma emission spectrometry is used to detect concentrations of Si,
Al, Ti, Fe, Mn, Ca, Mg, Na, K, P, Ba, Sr, Ge, and Y. Generally, the sample
is exposed to a high energy source, exciting the electrons of the various
atoms to specified higher energy levels. Once the electrons fall back into
their ground states, they emit specific wavelengths of radiation. Determination
of emitted wavelengths demonstrates what elements are present in the sample.
Further comparison of the intensities of the emitted wavelengths with given
standards allows determination of the concentrations of different atoms.
Redox Potentiometry is used to determine the presence of NH4+ By forming
a reaction with oxalacetate and malate, you can determine the concentrations
of NH4+ - setting up the half equations and determining the number of
electrons consumed.
Ion liquid chromatography is used to determine the presence of Cl- and
SO42-The chromatography is the separating of the mixtures into their respective
elements or ions. This separation occurs because the components of the
mixture have different partition ratios between their mobile and solid phases,
and have different rates of travel through the solid phase. All forms of
liquid chromatography use liquids for the mobile phase (as opposed to using
a gas, which cannot separate all the elements).
