Litterfall, the plant matter that falls to the forest floor, plays an important role in the nutrient cycling of rainforest plants. Nutrients spend almost no time in the soil of the Amazon, as the soil tends to leach nutrients from the topsoil to beneath a layer of clay, preventing the plants from retrieving them. So the Plants directly recycle the nutrients in the litterfall by reabsorbing them through their roots. This means that the ammount of litterfall and the concentrations of various nutrients - and toxins - in the litterfall, are highly indicative of the current state of the plants in an area, and thus highly indicative of the health of an area in general. 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.
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. There is no determined method to how the litterfall traps are spread, they can either be randomly spaced in an area marked for observation or spaced regularly. Both the troughs and the conical traps will work equally well for observational purposes, but I would favor the conical traps as they are slightly less cumbersome to empty. Both have mesh bottoms (or in the case of the cones, mesh sides) to prevent the accumulation of water in the bottom of the trap. If you desire to test the water as well, a tub can be placed beneath the trap to catch the 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, or even bimonthly in certain areas. In collecting samples, include only the portions of plant matter that are within, or directly above 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 measured. 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 speaking, 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. If the concentration fluxuates, there is probably a problem in the area. For example, an influx in Mercury is poisonous to the plants in an area, and might be an indication that there is mining in the area - as mining often involves the release of mercury into a system.
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. Again, you want to check for fluxuations in the concentration. Concentrations should never stray to far from your established base line. If they do, there is an outside influence distorting the concentrations and affecting the plants.
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).
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