Environmental Engineering Reference
In-Depth Information
7.8.2.2
Indirect Assessment: IRIS Tubes
During the last decade, another approach has been developed to assess reducing
conditions in soils known as IRIS tubes (Indicator of Reduction In Soils) (Castenson
and Rabenhorst 2006 ;Jenkinson 2002 ). In this approach, an Fe oxyhydroxide paint is
applied to (usually 60 cm) sections of 1.25 cm schedule 40 PVC tubing and allowed to
dry. After making pilot holes in the soil, the tubes are inserted into the soil for a period
of approximately 1 month. If the soils are saturated and microbes are actively
oxidizing soil organic matter, some of the Fe oxide paint functions as an electron
acceptor, becomes reduced and solubilized, and stripped from the PVC tubing. The
degree to which the paint is removed from the tubes is an indication of the degree or
magnitude of reduction in the soil (Castenson and Rabenhorst 2006 ;Rabenhorst
2008 ). As expected, soil temperature has also been shown to affect the rate at which
paint is removed from the tubes with less removal occurring during periods when soil
temperatures are low and approaching biological zero (Rabenhorst 2005 ; Rabenhorst
and Castenson 2005 ). Although the paint is sometimes referred to as being comprised
mainly of ferrihydrite (Jenkinson 2002 ; Jenkinson and Franzmeier 2006 ), it is impor-
tant in the synthesis of the Fe oxides to ensure that 40-60%of the ferrihydrite has been
converted to goethite, as otherwise, the paint will not adhere well to the PVC tubing
(Rabenhorst and Burch 2006 ).
One of the perceived benefits of using IRIS tubes for assessing soil reduction is that it
integrates the conditions of the soils over the period during which it is installed. So while
Eh measurements or indicator dyes can tell you what is happening at the moment, the
IRIS tubes provide a better indication of what the redox status of the soil has been over
the course of several weeks. They also have the benefit of not requiring specialized
lab equipment such as electrodes and volt meters. IRIS tubes are available commer-
cially, and one possible limitation to their use might be the costs involved in purchase
of the devices. However, the paint for making the tubes can also be manufactured fairly
easily in a laboratory following published methods (Rabenhorst and Burch 2006 ).
Because the soil redox properties are considered to be quite variable, normal
protocols for using IRIS tubes call for using five replicate tubes (Rabenhorst 2008 ).
If a majority (three-fifth) of the tubes exhibits a minimum level of paint removal,
the soil is considered to be reducing. To meet the requirement for reducing
conditions that is specified in the Technical Standard of the NTCHS, at least
30 % of the paint must be substantially removed from a 15 cm zone, somewhere
along the upper 30 cm of the tube (representing the upper 30 cm of the soil)
(National Technical Committee for Hydric Soils 2007 ). However, other work has
suggested that a less dramatic removal of the paint might still indicate reducing soil
conditions (Castenson and Rabenhorst 2006 ; Rabenhorst 2008 ).
7.8.2.3
Indirect Assessment: Alpha-Alpha Dipyridyl Dye
Various dyes can be used as indicators of particular soil chemical conditions and
alpha-alpha dipyridyl dye can be utilized for demonstrating the presence of Fe 2+ in
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