Environmental Engineering Reference
In-Depth Information
content of the fresh roots. Bags are typically left in the field for 2-3 months,
although the incubation period will depend on soil and air temperature, soil
moisture, and biomass composition. After collection, bags are transported to the
laboratory and the samples are gently rinsed to remove soil residues and fungal
hyphae, and then dried and weighed.
There are two general approaches to analyzing the data. If the intent is to
compare treatments such as litter species composition, or to compare sites, an
analysis of variance is performed on the percentage of initial dry mass remaining
at time
t
. If the intent is to determine decomposition rate constants, mathematical
models are fitted to the data to describe biomass loss over time. Both single
exponential decay models and double exponential decay models have been fre-
quently used to describe organic matter decomposition. The single exponential
decay model is based on the assumption that the relative decomposition rate
remains constant over time. The double decomposition decay model is based on
the assumption that litter has two distinct components, an easily decomposed
(labile) fraction and a more recalcitrant fraction. Therefore, each fraction requires
a separate decay rate constant. Wieder and Lang (
1982
) present a critique of these
analytical methods.
7.4.4.3 Cohort Layered Screen
The cohort
layered screen method is an inexpensive approach to assessing
long-term (
3 years) litter decomposition. As presented by Karberg et al. (
2008
)
aluminum or fiberglass window screening (1 m
1 m, 2-3 mm mesh) is placed
over the forest floor following the major annual litterfall. An additional layer of
screen is placed over the screen from the previous year following each annual
litterfall. The litter is held in place by the screens and decomposes
in situ
. Sections
of the screen are cut out to supply samples of the decomposing litter which is then
dried and weighed. Dry weights are then compared to stand level estimates (see
Quantifying Litterfall
) for the year in question. One benefit to this approach is that
the litter sample is naturally representative of the site, as opposed to the litter bag
method where the practitioner chooses the litter sample. The cohort layered screen
method does have several limitations in common with the litter bag method. Certain
macrofauna may be denied access to the litter which can alter decomposition rates.
Also both techniques do not allow for the separation of true decomposition from
losses attributed to leaching and comminution. Leaching losses are especially a
concern in wetlands. Since the cohort layered screen method is intended for
long-term deployment, it is not appropriate for wetlands that exhibit long-term
periods of inundation.
7.4.4.4 Cotton-Strip Assay
The decomposition of cellulose strips has been used extensively as a surrogate for
plant organic matter decomposition including in a variety of wetlands (Newman
