Environmental Engineering Reference
In-Depth Information
SOM decomposition sections. Also, soil surface CO
2
efflux has been used to
compare decomposition rates in different soils. But, soil surface CO
2
efflux is an
index of respiration by soil organisms and plant roots (Zibilske
1994
). As such, it is
not directly comparable to SOM degradation.
There are three main approaches to assessing soil respiration-closed chamber
systems, open chamber systems, and flux gradient sensors. Chamber-based
approaches are simple, economical, and portable. A chamber is placed over the soil
to create a headspace of air, which can be sampled repeatedly over a short time
period. Closed chamber systems are the most commonly used and commercially
available. They are classified as “closed” because there is no exchange of air between
the chamber and the outside atmosphere during measurements. Closed chamber
systems may be “active” or “static”. In dynamic systems, air is continuously
circulated between the chamber and an infrared gas analyzer. In static systems, air
samples from the chamber are collected with a syringe for laboratory analysis or CO
2
is absorbed by soda lime in the chamber. In open chamber systems, air is exchanged
between the chamber and the outside atmosphere. In the flux gradient approach,
infrared sensors are inserted into the soil at various depths. The CO
2
concentration
gradient over soil depth and additional soil characteristics are used to calculate CO
2
diffusivity. Bradford and Ryan (
2008
) present an evaluation of the relative benefits
and challenges to each system. In this section, we present the soda lime method, a
common and relatively simple method of measuring soil respiration that utilizes a
static closed chamber approach. In the section
Methane Emissions
,weinclude
methods that can also be used for quantifying CO
2
fluxes.
7.4.5.2 Carbon Dioxide Detection by Soda Lime Absorption
In this method, an open dish containing soda lime is placed on or just above the soil
surface and covered with a container to restrict airflow between the soda lime and the
atmosphere. Carbon dioxide and water vapor released by the soil microorganisms
during decomposition are absorbed by the soda lime. After drying to remove water,
the gain in soda lime dry weight during the exposure period reflects the amount of
CO
2
evolved. The following specifics were detailed by Zibilske (
1994
).
The soda lime jar must consist of oven-safe glass with air-tight screw caps.
A 5.5 cm diameter jar is suggested. Jar supports (for stability) consist of 12 cm
square pieces of galvanized mesh, bent down at each corner to form four 2-cm legs.
For jar covers, they used cylindrical cans (28 cm diam., 25 cm height), open at one
end. The exact dimensions are not critical, but each container should have an
opening at least 600 cm
2
and a total volume of at least 15,000 cm
3
. To prevent
the jar covers from direct exposure to sunlight in the field, they can be painted
white, covered with aluminum foil, or by placing a 50 cm
2
flat board on top of them
in the field. The amount of soda lime (6-12) mesh needed for each jar should be
slightly greater than 0.06 g for each cm
2
of soil surface area covered by the jar cover
container.
