Environmental Engineering Reference
In-Depth Information
and randomly retrieved at predetermined intervals. Bag size is commonly
20 cm
20 cm. Nylon mesh or fiberglass screening material is used so the
bags themselves are not subject to decomposition. Harmon and Lajtha ( 1999 )
recommend fiberglass bags for light intensive sites where UV light can degrade
nylon. Litter bags typically have a mesh size of 1-2 mm (Robertson and Paul
1999 ), although mesh sizes from
<
>
10 mm have been used. If the
mesh is too small, access to some macroinvertebrates may be denied. Large
mesh sizes facilitate the loss of small particulate matter. One option is to use a
small mesh, but staple the edges of the bags at relatively large intervals (e.g.,
5 cm) to provide openings along the periphery for access of macroinvertebrates.
Regardless of mesh size, low molecular weight organic compounds can be lost
through leaching. The practitioner must be aware of implications to leaching
losses and the bag deployment period should reflect that. Decomposition rates
will be highest when the soil is moist but not saturated. Leaching losses will be
greatest during periods of soil saturation. Therefore, the bags should not be
deployed in continuously saturated soils.
In general, at least five replicate litter bags are collected at each sampling
interval during year 1 of the study (Karberg et al. 2008 ). Sites displaying significant
microclimate variability may require more replicates. For example, wetlands with
pit and mound topography exhibit spatial variability in soil moisture and soil
temperature and would require greater replication. Certainly, the researcher has
the option of adjusting replicate number in subsequent years. Sample material is
chopped into 2-5 cm lengths and a known amount of fresh plant tissue is placed in
the bags. Subsamples of the plant material are dried (70 C, 48-72 h) to obtain their
water content. The organic matter of interest (e.g., leaves or fine roots) is placed in
nylon mesh bags. The bags are then placed where the organic material would
normally be found. For example, abscised leaves would be placed on the soil
surface, fine roots would be placed in organic soil horizons or in the topsoil,
and detrital tissue would be placed on the soil surface or in the detrital layers.
One advantage of this system is that material can be collected from the site in
question and returned to its natural environmental conditions. The filled bags
should be returned to the site soon after sample collection to ensure representative
environmental conditions. Bags are pre-weighed (tared) so that sample weight can
be determined in the bag. Loss of biomass due to decomposition is calculated as the
difference between initial biomass and remaining biomass. All values are expressed
on a dry weight basis. Average rate of decomposition (per day) is determined by
dividing biomass loss by the incubation period. However, since biomass decreases
over time, a more accurate estimate of decomposition rate is produced with
exponential decay equations (see Karberg et al. 2008 ). If a chemical analysis is
conducted on the tissue before and after the incubation period, N and P mineraliza-
tion rates can also be determined.
The litter bag method also can be used to estimate fine root decomposition rates
(Fahey et al. 1988 ). Fine roots can be collected by the sequential coring method
(above) or with a spade. Soil residues are removed by rinsing, and root samples
(2 g fresh weight per bag) are placed in litter bags (nylon, 10
1mmto
10 mm, mesh size
1.2 mm). Subsamples of the root material are dried (70 C, 48 h) to obtain the water
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