2.2 Chamber-Based Methods
2.2.4 Chamber Types
There are many types of closed/static chambers, typically developed by researchers for specific purposes (De Klein and Harvey2012; Oertel et al.2016; Saggar et al.
2007). The chambers may be made from various materials, including metals, plas- tics, and glass, and can have different designs, sizes, shapes, and volumes; chambers as small as 50 cm3and as large as ca 1 m3have been used for field flux determination from the soil surface. Obviously, chamber materials should be chemically inert, and thus, neither react with the gases being measured nor emit any contaminants. Recom- mended materials therefore include stainless steel, aluminium, and glass, while the use of polycarbonate, polyethylene, methyl methacrylate, and polyvinyl chloride should be checked for their suitability before use. The schematic diagram of the most common metal chamber is shown in Plate 2.1. This is similar to the design proposed by De Klein and Harvey (2012).
Metal chambers, as shown in Plate2.1, represent the best choice for many reasons.
Metals are not permeable to gases and are inert if materials such as stainless steel are used and can be manufactured in local workshops. The type of material used is important because galvanised steel or normal steel may alter the soil conditions by releasing zinc and iron ions that have the potential to affect microbial activity;
thus, it is recommended to use stainless steel. However, when compared to plastic
Extension with trough for a water seal, note, this is only used if higher plants have to be enclosed
Soil frame with trough for a water seal, the bottom part is inserted into the soil up to the gutter
Closed chamber with holes in the lid for gas sampling via a septum, note, the chamber should be insulated with available insulation material (e.g. styrofoam)
b a
Plate 2.1 A schematic diagramaof three parts base frame, extension or enlargement and top lid with GHG sampling ports, a complete metal closed chamber in the paddy field (b), and specially designed chamber for maize plants (candd)
2 Methodology for Measuring Greenhouse Gas Emissions … 19
c
d
Plate 2.1 (continued)
chambers, they can be more expensive, heavier, and less available. Also, insulation is required to minimise temperature fluctuations inside the chamber which in turn would affect fluxes of GHG’s, and microbial processes that drive their production.
Metal chambers typically consist of two or three parts: the bottom part (also called the base, frame, or soil collar/gutter), the top part (i.e. the chamber), and perhaps a suitable extension (Plate2.1). The bottom part (frame) should be inserted into the soil at least 2 weeks before the first sampling and permanently installed to minimise soil disturbances effects. Chambers shall be insulated (e.g. using foam or polystyrene with reflective foils) to avoid unnatural heating during chamber closure. This allows repeated gas flux measurements in the same place, e.g. during the whole season. The two/three-part chamber design is strongly recommended, as the disturbances of soils prior to the measurements are eliminated. This, however, means that the frame should ideally be placed into the soil a few weeks before the measurements commence (De Klein and Harvey2012; Oertel et al.2016). Precautions should be taken in grazed sites so that neither the chamber nor the animals are endangered.
Plastic chambers have often been used for GHG flux determination (Plate2.2a, b) (Zaman et al.2009). The most critical issue for this type of chamber is the nature of the plastic used for making the chamber. Most plastic materials show permeability for gases, as well as the ability to emit some gases or react with them, e.g. hydrocarbons.
The advantages of using plastic are (among others) the general availability, easiness to work with, low weight, and able to easily glue different parts together. Plastic chambers consist of two parts (a plastic vessel without a base/collar/gutter) and a lid. The plastic vessel is usually inserted into the soil at least 2–3 days prior to the
b a
Plate 2.2 Plastic chamber made up of two parts: vessel and lid for measuring greenhouse gas emission fromapasture soil andbarable and vegetable croplands
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Plate 2.3 Glass chamber consists of only one part (Šimek, personal communication) for measuring GHG emission from pasture soils
flux measurement. For gas sampling, a lid containing a gas sampling port (rubber septum) connected to a three-way valve is carefully placed on top of the vessel using a gas-tight seal (Plate2.2a, b). After gas samples are collected, the lid is removed.
Plastic chambers can be either transparent or opaque.
Glass chambers have been used less frequently, although the material (glass) is probably the best material to use, considering the inertness and the low gas perme- ability. However, glass is fragile which makes it very problematic to work with.
Therefore, there are more disadvantages than advantages to use glass chambers. Still, one type of glass chambers has been tested for gas flux measurements (Plate2.3). The major disadvantage of this chamber type is the limited size of the bottles available.
Bottle volume is usually between 100 and 2000 cm3, and surface area covered by the chamber is less than 100 cm2, which is too small for most uses. Glass chambers consist of a single part (without a base/collar/gutter) and are transparent.