Wetland soils

Soils are potentially powerful indicators of wetland dynamics because of the specific morphological features that develop in wet environments.

Wetland soils impact directly on other wetland characteristics (for example, water quality, fauna, vegetation), and can be a reflection of the physical processes occurring in the wetland.

The amount of time wetland soils spend saturated, flooded or ponded results in anaerobic conditions in the upper layers of the soil profile. These anaerobic and/or saturated conditions will influence the genesis of the wetland soil as well as the wetland plant species that will grow.

Four conditions are required for a soil to become anaerobic and support reducing reactions (and the development of soil morphological features, if these chemical transformations occur in the soil’s upper layers):

• the soil must be saturated to the point of excluding atmospheric oxygen
• the soil must contain a source of organic matter that can be oxidised or decomposed
• the soil must contain a population of respiring microbes that will oxidise the organic matter
• the water must be stagnant or slow moving.

Wetland soils can be identified using soil morphological indicators, such as:

• the accumulation of organic matter
• gleyed soil colours
• soil mottling
• iron or manganese segregations
• oxidising root channels and soil pore linings
• reduction of sulphur and carbon (for example, acid sulfate soils).

Photo: Wetland soil derived from sands. Note the dark, fibrous surface layer showing clear accumulation of organic matter.	Photo: Oxidised root channel linings in a soil ped. This morphological feature was observed throughout the top 30cm of a wetland soil.	Photo: Aerial roots on a Melalueca spp, suggesting that the surface soils in this wetland remain under anaerobic conditions for many months at a time.

Wetland soils impact directly on other wetland characteristics (for example, water quality, fauna, vegetation), and can be a reflection of the physical processes occurring in the wetland (for example, water inflow, water chemistry, filtering of pollutants). The national soils layer is the Australian Soil Resource Information System, which carries broad scale to point scale information.

Indicators of Reduction in Soils

Indicators of Reduction in Soils reports on the Indicator of Reduction in Soils (IRIS) method for identifying wetlands and demonstrates how to use the method on wetlands in Queensland. Field indicators of ephemeral wetlands can be hard to find in dry times. Wetland soils – identified by reducing conditions – become a major indicator of existence and extent.

The IRIS method uses synthetic iron oxides to indicate anaerobic conditions in soils to indicate the presence of wetlands. Wetland managers and decision makers may use the IRIS method as an additional line of evidence to identify wetland extent.

Soil Indicators of Queensland Wetlands: Statewide assessment and methodology (2008)

The Soil Indicators of Queensland Wetlands: State-wide Assessment and Methodology details and discusses findings of a statewide assessment of wetland soil indicators conducted by the Department of Natural Resources and Water during 2007 and 2008 under the Queensland Wetlands Program (QWP). The definition of a wetland, developed through the QWP is based on the Ramsar definition and includes a component on wetland soil features. To be useful at a finer scale, this definition needs a scientifically robust method for applying wetland soil indicators. This study concludes that the formation of soil indicators in Queensland is influenced greatly by climatic region (tropical/equatorial, subtropical, semiarid and arid), wetland system (palustrine, lacustrine and estuarine) and by inundation frequency (periodically or commonly wet).

A Soil Indicators of Queensland Wetlands:Field Guide has been developed to assist in the field identification of wetland soil indicators in Queensland, and is intended to be read and used in conjunction with the  Soil Indicators of Queensland Wetlands: State-wide Assessment and Methodology.

The proposed wetland soil indicators were tested in 58 wetlands, representing a range of wetland classes, climatic zones and biogeographical regions. Thirty-nine soil information sheets or case studies have been developed.

Soil Indicators of Queensland Wetlands Phase 1: Literature Review and Case Studies (2007)

Soils are potentially powerful indicators of wetland dynamics because of the specific morphological features that develop in wet environments. This review of national and international scientific literature evaluates the ability of indictors often found within soils to reliably predict wetland boundaries. The Soil Indicators of Queensland Wetlands Phase 1: Literature Review and Case Studies (2007) report highlights two case studies of wetlands within Southern and Central Queensland to test if predictable relationships exist between soil indicators identified within the literature and wetland status for Queensland wetland soils.

Other links of interest

Queensland soils and land resource information provides a description of landscapes,  their characteristics, attributes for land management and planning and links.

The Australian Soil Resource Information System, is a national soils layer which carries broad scale to point scale information.