INTRODUCTION
Litter decomposition is one of the key biogeochemical processes in forest
ecosystems ( Swift et al. 1979). It is estimated that the nutrients released during litter
decomposition can account for 69-87% of the total annual requirement of essential
elements for forest plants (Waring &Schlesinger 1985). The study of litter decomposition
is also an important part of the most intensively studied nutrient cycling processes in
forest ecosystems. The rate of litter decomposition is largely a determining factor for
productivity or biomass of every terrestrial ecosystem in general, and of forest
ecosystems in particular.
Experimental studies on leaf litter decomposition in forest ecosystems are
obviously easier than on the decomposition of any other parts of the plant. The reasons
can be summarized as follows. First, leaf litter decomposition is faster, and thus less
time-consuming, especially in the tropics and subtropics. The decomposition of twig and
stem litter takes significantly longer due to the higher content of more recalcitrant
compounds contained in these highly lignified plant parts, and therefore long-term
observations need to be considered. Second, the concentration of nutrients contained in
leaf litter is generally higher than in any other parts of a plant.
MATERIALS AND METHODS
Collection of litter
A leaf litter decomposition experiment was carried out during a 5 week period from
February 18 to Mar 24, 2004. Sapium species was selected to study the litter
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decomposition as this is a very fast decomposing species (Cameron and Spencer, 1989).
Hence, it was convenient to use this species to study litter decomposition over a short
time period. Entire leaves of Sapium were harvested directly from adult trees, just before
abscission. Then they were air dried for about 30 days. Leaf litter was used to determine
decomposition because this plant organ represent substantial portion (50-80%) of the
total above ground litter production in terms of biomass and represent a major nutrient
input because of their relatively high nutrient concentration (Toky and Ramakrishnana,
1983; Alvarez et al., 1992; Montenaz, 1998; Sundarpandian and Swamy, 1999).
Furthermore, leaves decompose faster than twigs and leaf petioles (Anderson and swift,
1983), the petioles were removed from air dried leaves so as to get a uniform
decomposition for the purpose of study.
Litter Bag Technique
Leaf decomposition was evaluated through the litter bag technique described by
Swift and Anderson (1989) and Alvarez et al. (1992). This is useful technique to analyze
and delineate differences in decomposition rate (Coleman and Crossley, 1996). Mesh
bags 12x15 cm in size with 2 mm mesh were filled with about 2g of air dried leaf litter of
Sapium. Each sample was weighed separately and it was recorded as the initial weight of
each sample labeled numerically. The 2mm mesh size was sufficiently small to prevent
losses of litter due to breakage, but sufficiently small to permit the access of decomposers
(Sundarpandian and Swamy, 1999).
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Experiment Layout
On Feb 18, 2004, 5 spatial replicates each for above and below ground
decomposition study were placed in natural areas of the Natural Ecosystems Lab. Three
time points viz 7days, 21days and 35days, were selected for study and hence the litter
bags were laid according to three sampling dates( Feb25, Mar10 and Mar 24). Each time
point had 5 spatial replicates for above ground decomposition and 5 replicates for below
ground decomposition. For the study of below ground decomposition, the litter bags were
placed at a soil depth of 8-10 cm. All the litter bags for one time point were appropriately
labeled and tied to a central flag using a fishing line. Hence, we had 10 litter bags for
each time point or sampling date and a total of 30 litter bags.
Decomposition Study
Decomposition was measured by rate of mass loss and fitted with an exponential
decay function to estimate the decomposition rate (k). The litter bags collected at their
respective sampling date were washed with deionized water and picked for some
extraneous material in the decomposing leaves. These were then oven dried for 48hrs at
60°C and weighed after a constant weight was obtained. The final weight represents the
mass remaining after decay and hence from this the percent of initial mass remaining was
calculated. The percent of the initial mass remaining was plotted against time (days) and
the difference in decomposition between above and below ground samples was assessed.
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Carbon and Nitrogen Analysis
The oven dried litter samples, were ground to a fine powder in a grinding
machine. Each of these was then weighed 3mg and rolled in tin cups for the carbon and
nitrogen analysis. Peach in range from 0.5 mg to 3mg was used a standard for this
purpose. A decay constant was estimated for % carbon and %nitrogen.
STATISTICAL ANALYSIS
Analysis of Variance to Examine decomposition Data
This was a comparative study to assess the effect placement ( above and below
ground) litter samples by examining the differences in treatment means in the proportion
of original mass , carbon or nitrogen remaining at various times. Hence, analysis of
variance using one-way classification of data was used (Weider and Lang, 1982). The
analysis was
performed on the proportion of percent of the initial
amount of X
remaining at time t , where X was percent initial mass, percent initial carbon and percent
initial nitrogen remaining at time, t =7, 21 and 35. Hence for each analysis of variance, I
had 2 treatments, ( above and below), 5 replicates of each and hence a total of 10
observations. The analysis of variance produced an F-statistic that is used to test a null
hypothesis of no differences between 2 treatments. Only large F-values indicate that null
hypothesis is false. The p-values obtained indicated that mean above and below ground
decomposition are statistically
different at a
significance level ‘p’. Additional
comparison of means using Tukey’s test was also done.
Fitting Decay Functions to Mass Values Using the Single Exponential Model
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The second approach to analysis of decomposition data is the fitting of mathematical
models to estimate constants that describe the loss of mass overtime. This approach gives
us considerable insight into the biology of decomposition. Single exponential decay
function, first proposed by Jenny et al (1949) and discussed a considerable detail by
Oslon (1963). The expression for this is
ln (Xt/X0) = -kt
where X0 is the initial mass, Xt is the residual litter mass at time t expressed as a
proportion of initial dry mass, and K is the decay constant expressed in days-1. The
regressions of ln (Xt/X0) overtime were preformed separately for each replication within
each treatment.
The appeal of this exponential model arise from the fact that a single constant ‘k’,
characterizes the loss of mass, thereby facilitating comparisons with other data sets and
simplifying attempts to model the accumulation of organic carbon in soils ( Oslon
1963; Oohara et al 1971). The assumption underlying this single exponential model is
that either the absolute decomposition rate decreases linearly or the relative
decomposition rate remains constant. As decomposition proceeds, soluble components
and relatively easily degraded compounds such starch, sugar, and proteins will be rapidly
utilized by the decomposers, while the more recalcitrant materials such as cellulose, fats,
waxes, tannins, and lignins will be lost at relatively slower rates. Thus, with time, the
relative proportion of these recalcitrant materials will progressively increase and the
absolute decomposition rate should decrease, while the relative decomposition rate may
remain constant.
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References
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Whitmore, T.C., Chadwick, A.C. (Eds.), Tropical Rain Forest. Blackwell
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Cameron GN, SN Spencer.1989. Rapid leaf decay and nutrient release in a chinese tallow
forest oecologia 80 (2): 222-228
Coleman and Crossley, 1996. Coleman, D.C., Crossley, D.A., 1996. Fundamentals of
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Jenny, H., S. P. Gessel, and F.T. Bingham. 1949. Comparative study of decomposition of
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Oslon, J. S. 1963. Energy storage and the balance of producers and decomposers in
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Cameron, G. N. and T.W. Lapoint. 1978. Effects of tannins on decomposition of
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Swift, M.J., Anderson, J.M., 1989. Decomposition. In: Lieth, H., Werger, M.J.A. (Eds.),
Tropical Rain Forest Ecosystems. Biogeographical and Ecological Studies.
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Weider, K.R. and G.E. Lang 1982. A critique of the analytical methods used in
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