Environmental Engineering Reference
In-Depth Information
Each jar with the cap on is weighed. Soda lime is placed in the jars and dried to a
constant weight in a drying oven at 100
C for 24 h. The jars are re-capped, cooled,
and reweighed to determine the amount of dried soda lime in each jar. A minimum
of five replicate chambers are used at each location to accommodate spatial
variability. Any vegetation or debris that would interfere with the formation of a
tight seal between the cover and the soil surface is removed, but without disturbing
any leaves and the soil surface under the chamber. The wire mesh legs are pressed
into the soil to produce a stable surface. After opening, each soda lime jar is
immediately placed on a mesh stand and covered. The lip of the cover is forced
into the soil with a twisting motion. A weighted object such as a rock may be placed
on the cover to keep it in place and maintain the soil surface seal. Controls are
needed to account for any CO
2
absorbed during this part of the procedure.
To construct controls, a jar of soda lime is left open for the same amount of time
required to deploy the sample jars (from opening to covering) and then covered.
For every ten sample jars, two control jars are used. The control jars will be used to
produce blanks. Incubation is commonly for 24 hours (h). Retrieved sample jars are
tightly and quickly capped; then dried without caps in a 100
C oven for 24 h,
capped, cooled, and weighed. The absorption of CO
2
generates water which is
removed during drying of the soda lime. To account for this, the weight gain
determined after drying is multiplied by 1.4. All necessary calculations are
presented by Zibilske (
1994
). The data are commonly expressed as mass per unit
area per unit time (e.g., g CO
2
/m
2
/h).
Absorption of water vapor is needed to activate the soda lime after drying.
However, it should not come in direct contact with surface water. For this reason,
this technique is not appropriate for sites with deep inundation. If need be, the jar
supports can be constructed with larger legs to keep the jars above surface water.
In addition, to compare soils they should have similar water content when sampled
as soil moisture content impacts respiration. Therefore, deployment should coin-
cide with a period of stable soil moisture conditions. We recommend that soil
moisture content (dry weight basis) be determined for topsoil adjacent to each
sampling point. Take four soil samples equally spaced from each other and 0.5 m
from each sampling point and combine for soil moisture determinations. Tempera-
ture also affects respiration so air temperature should be taken at a height of 0.5 m
and soil temperature should be taken at a depth of 5 cm.
7.4.6 Methane Emissions
7.4.6.1 Overview
Methane is of environmental concern as it has been implicated in global warming.
Methanogenesis is the utilization of CO
2
as a terminal electron acceptor to produce
CH
4
, sometimes referred to as “swamp gas”. Methane is produced by a distinct
group of obligate anaerobic bacteria (methanogens) only under very reduced (redox
