Environmental Engineering Reference
In-Depth Information
of C (primarily CO
2
) from the atmosphere and subsequent long-term storage in a
reservoir such as soil organic matter. Disturbance to a wetland, especially in the
forms of artificial drainage or deforestation, reverses the net C flow so that disturbed
sites initially serve as a source of CO
2
.
Because of the impact of O
2
availability on the direction or rate of many
biogeochemical reactions, some of the C processes are compartmentalized in
specific zones in the soil or water column. For example, CH
4
oxidation occurs in
aerobic zones, while methanogenesis is restricted to anaerobic zones (Knight and
Wallace
2008
). Furthermore, since many of these processes are driven by microbial
activity, compartmentalization is further promoted by the availability of organic C
as an energy source. For example, the highest decomposition rates are found in
close proximity to the wetland surface where there are high inputs of fresh litter and
recently synthesized labile organic matter (Sherry et al.
1998
) and the highest
duration of aerobic conditions.
7.4.2 Primary Productivity
7.4.2.1 Overview
Carbon sequestration refers to the removal of C from the atmosphere and
subsequent storage in C sinks such as oceans, forests, and soils. Primary production
is the production of organic compounds from CO
2
(atmospheric or aquatic) princi-
pally through the process of photosynthesis. Therefore, photosynthesis is integral to
C sequestration. The primary producers in wetlands are mainly plants and algae.
Net photosynthesis (gross photosynthesis-respiration) can be approximated by
assessing biomass. In this section we present methods for determining above-
ground biomass for trees and herbs, abscised leaves, and fine roots. Conversion of
biomass to C requires a C content value which is obtained from the literature or by
chemical analysis of the sampled biomass. Direct chemical analysis will be more
accurate as published values will represent averages across species and may not
reflect the specific growing conditions of the individual plants in question. Chemi-
cal analysis for C content of plant tissue is not presented here. We also do not
address biomass and C assessment of shrubs. For this topic we refer the reader to
Chojnacky and Milton (
2008
).
7.4.2.2 Tree Biomass-Allometric Equations
Direct calculations of tree biomass to determine primary productivity or C seques-
tration is not an option as it requires destructive sampling, determination of dry
weight, and chemical analysis for C. However, there are indirect methods that allow
for the estimate of tree biomass and C. Above-ground tree biomass can be estimated
using a single field measurement, published data, and simple allometric equations
