Environmental Engineering Reference
In-Depth Information
(Butterly et al. 2009 ). The C pulse is thought to result from both the presence of
dead microbial cells that accumulated during soil desiccation and the release of
previously unavailable organic C sources that resided in the interior of soil
aggregates and similarly protected areas. The C released is typically readily avail-
able to soil microorganisms and results in increased microbial respiration. Provided
O 2 diffusion is restricted as a result of rewetting and sufficient nitrate (NO 3 )is
present, these C pulses can produce sharp spikes in respiratory denitrification
(Myrold 2005 ). Rates of denitrification drop rapidly once C or NO 3 availability
decreases or O 2 availability increases. In fact, N removal from soils due to denitri-
fication is typically greatest when alternating aerobic and anaerobic soil conditions
occur frequently. This is because the nitrifying bacteria responsible for converting
NH 4 + to NO 3 are active only under aerobic conditions, whereas denitrification is
dependent on NO 3 availability (produced during aerobic conditions), presence of
easily decomposable C compounds, and lack of O 2 (Vasilas and Fuhrmann 2011 ).
Therefore, the practitioner must be cognizant of these when designing exercises to
quantify soil N or C pools, or to quantify rates of processes that contribute to these
pools. It is recommended that soil redox potential (Eh) be measured when
conducting investigations on biogeochemical processes affected by Eh. The
methodology for measuring Eh is presented in Oxidation-Reduction Processes in
Soils . If nothing else soil moisture conditions during the field assay period should
be noted.
7.4 Carbon
7.4.1 Overview
There are six principal C reservoirs in wetlands: plant biomass C, microbial
biomass C, soil C (both organic and inorganic), particulate organic C in the water
column, dissolved organic C, and gaseous C compounds such as carbon dioxide
(CO 2 ) and methane (CH 4 ) . Often, C in microbial biomass and C in soil organic matter
are combined into the category soil organic C (SOC). Carbon is also a major
constituent of sedimentary rocks such as coal and limestone. In minerals, it is
found predominantly as carbonates, salts of the carbonate ion (CO 3 2 )suchascalcite
(CaCO 3 ). Significant quantities of free carbonates may accumulate in high pH soils in
arid climates. In some soils, extensive quantities of C are stored as carbonates
(CO 3 2 ). Public awareness of the C cycle has recently increased due to concerns
over global warming which is attributed to the atmospheric increase in greenhouse
gases including CO 2 and CH 4 . Wetlands can serve as both a source and a sink for C
(Kayranli et al. 2010 ) depending on their age, type, and condition. Some wetlands
produce CH 4 (see Methane Emissions below). However, most wetlands are
characterized by a net retention of organic matter and plant detritus (Mitsch and
Gosselink 2000 ). As such, a critical wetland service is C sequestration-the removal
Search WWH ::




Custom Search