Environmental Engineering Reference
In-Depth Information
wetlands, the dominant pathways switch during the year from anaerobic processes
to aerobic processes. This temporal variability in soil oxygen (O
2
) content promotes
some processes such as denitrification as explained below. Water source and
landscape position influence inputs and outputs. Sediment loading is a dominant
process in riverine wetlands subject to frequent overbank flooding and much of the
P inputs will be in particulate form as opposed to ground water driven slope
wetlands in which most of the P inputs will be soluble orthophosphates. Hydrody-
namics and surface roughness dictate water resonance time. Sedimentation is a
more dominant process in a depressional wetland subject to surface runoff than a
groundwater driven slope wetland. Mineral soil flats are associated with seasonally
saturated hydroperiods and alternating periods of aerobic and anaerobic conditions.
7.3.2 Role of Plants and Microbes
Plants are the dominant source of organic C which supplies the energy for
microbially-mediated processes. Microbes are critical to the decomposition of
detritus and leaf litter and mediate pathways in the C, N, S, Fe, and Mn cycles.
Microbial populations vary with respect to total numbers and species composition
according to soil depth and distance from plant roots. These differences are
primarily in response to a gradient of available C. The rhizosphere refers to the
zone of soil close to and impacted by plant roots. The rhizoplane is the surface of
plant roots. Microbial numbers are substantially higher (10- to 100-fold) (Paul and
Clark
1996
) in the rhizosphere than in bulk soil and are inversely proportional to
distance from the roots. The highest microbial numbers, by far, are on the rhizo-
plane. Plant roots supply most of the C that drives microbial activity in soils. Up to
90 % of fine roots may die and decompose annually in forest soils. In addition, dead
root cap cells slough off and supply organic C, and exudates from live roots include
readily available C sources (sugars, organic acids), a readily available source of
N (amino acids), and growth promoting (and sometimes inhibiting) compounds
(Vasilas and Fuhrmann
2011
).
7.3.3
Importance of Wetting and Drying Cycles
Soil microbes are critical to the development of anaerobic and reducing conditions
in wetland soils. Their activity in turn is impacted by soil moisture conditions.
Following the onset of soil saturation, respiration by plant roots and microbes
produces anaerobic conditions. Further respiration by microbes produces reducing
conditions. For purposes of this discussion we consider reducing conditions to be
present when ferric iron (Fe
3+
) is reduced to ferrous iron (Fe
2+
). Increased microbial
numbers and activity subsequent to rewetting a dry soil are commonly observed and
are thought to reflect a temporary increase (pulse) of readily available organic C
