Environmental Engineering Reference
In-Depth Information
without macrophytic vegetation and 150 cm for sampling points with macrophytic
vegetation. Polyethylene bags were in place only during the sampling process.
The bags were pulled down over the chamber frames and attached to the base
with elastic straps. The bags should be constructed so that they fit snugly over the
frame so that the volume of air sampled is consistent. The top of each bag was
equipped with a butyl rubber sampling septa and a 3 m Tygon vent tube (1.6 mm
inside diameter [i.d.]).
7.4.6.3 Scaling CH
4
Fluxes
Measurements of CH
4
fluxes from wetland soils typically reveal high spatial and
temporal variability. The number of chamber measurements needed and the labor
required to carry out this degree of sampling may preclude this approach to
characterizing seasonal CH
4
fluxes on a landscape scale. Another approach is to
rely on modeling or remote sensing methodologies that link CH
4
fluxes to more
easily measured ecosystem properties or processes. For example, soil temperature,
water table depth and range, community structure, and net primary productivity
have been used in CH
4
flux models for wetlands (Potter
1997
; Potter et al.
2006
).
7.4.7 Dissolved Organic Matter
7.4.7.1 Overview
Dissolved organic matter (DOM) is operationally defined as the fraction of organic
matter that passes through a 0.45
m filter and is a heterogeneous mixture of
compounds including carbohydrates, proteins, lignins, organic acids and other
humic substances (Herbert and Bertsch
1995
; Kalbitz et al.
2000
). The fractions
of DOM that contain functional groups with C and N molecules are generally
classified as dissolved organic C (DOC) and N (DON). Wetlands have typically
been identified as the largest sources of DOM in watersheds (Aitkenhead-Peterson
et al.
2003
; Mulholland
2003
). Concentrations of DOC for wetlands have been
observed to range from 3 to 400 mg/L with an average of 30 mg/L (Thurman
1985
).
Among wetland types, bogs (3-400 mg/L) have been found to yield the highest
DOC values, while marshes represent the lower range (3-15 mg/L) (Thurman
1985
). The elevated contents of DOM in wetlands can be attributed to a variety
of factors including: (a) high primary productivity of wetlands compared to upland
and aquatic ecosystems (Thurman
1985
); low decomposition rates of organic
matter in wetlands due to acidic conditions and anaerobic or low O
2
contents of
wetland soils and surface waters (Kalbitz et al.
2000
); reducing redox conditions
that result in reductive dissolution of Fe and aluminum (Al) oxides that could
otherwise have served as sorption sites for DOM (Kalbitz et al.
2000
); flooding
and hydrologic conditions that facilitate the slow continuous leaching of DOM; and
μ
