Environmental Engineering Reference
In-Depth Information
Laboratory Exercises
Laboratory Exercise 1: Measurement of Soil Respiration
Introductory Comments
Laboratory investigations of soil respiration generally monitor carbon dioxide
production in either gas-tight, static microcosms or by using a dynamic, flowing-
gas system (Zibilske
1994
). The former approach is simpler and recommended
except when experimental considerations dictate use of a dynamic system. Static
systems make use of microcosms containing soil and possessing sufficient head-
space to permit gas accumulation and simultaneously avoid the development of
excessively low oxygen levels if maintenance of aerobic conditions is desired.
If anaerobic conditions are to be investigated, the soil can be flooded with water
and the headspace purged with N
2
or another suitable gas prior to incubation.
Carbon dioxide produced by aerobic or anaerobic respiration, or by certain anaero-
bic fermentative processes, either is captured in alkali traps for measurement by
acid-base titration or is measured by collecting gas samples for analysis by gas
chromatography. The choice between the 2 CO
2
measurement methods is a matter
of preference and instrument availability, as both provide good sensitivity.
The reader is directed to Zibilske (
1994
) for details and a complete discussion of
the various options.
Containers to be used as microcosms must be air-tight. Canning jars work well,
and their lids can be equipped with septa for gas sampling if CO
2
is to be
measured by gas chromatography. The nature of the soil placed in the microcosms
may range from mixed, sieved soil to intact soil cores. In any case, the ratio of
headspace volume-to-soil mass is an important consideration in aerobic studies; a
ratio of 10:1 is recommended (e.g., 500 ml container for 50 g of soil), although
lower ratios can be used, provided the jars are opened more frequently to allow for
aeration. Regardless of the ratio used, it is important in aerobic studies that the
oxygen levels of the microcosm be replenished on a regular basis. Presence of
volatile fatty acids or reduced S-containing gasses in the headspace (e.g., a rotten
egg smell) is an indication that the microcosms need to be opened more fre-
quently. Mixing a bulking agent such as vermiculite or perlite with the soil can
help prevent the development of anaerobic microsites (Thien and Graveel 2003).
Additionally, for aerobic studies the moisture content of the soil should not
exceed field capacity, and it may be desirable especially with fine-textured soils
to use a lower moisture content that is still conducive to microbial activity.
Regardless of the moisture content chosen, it should be standardized across
treatments unless moisture content is being examined experimentally. Another
variable affecting aeration status is the rate of any organic matter addition to the
soil. A commonly used rate is 1 % by mass, although this will vary with the
experimental objectives.
