Anaerobic soils are found on natural wetlands, floodplains, swamps, peatlands, and disturbed crop lands or even in our back gardens. Aerobic soils have particle arrangement which allows for free movement of air within its pores (open spaces between soil particles). On the contrary, anaerobic soils have restricted flow of air within its soil pores, owing to a high moisture or water table level. Soils can be temporarily anaerobic- like water logged soils on agricultural land or permanently anaerobic- like the natural wetland soils. Under anaerobic conditions, the amount of oxygen used by soil biological organisms exceeds the amount of oxygen diffused into the soil profile (1). Anaerobic soils have positive and negative effects on its surrounding. This article will enumerate and briefly discuss some interesting facts (both good and bad) to note about soils in this condition.
Anaerobic soils contribute to greenhouse gas emission
Anaerobic soils emit methane gas, a major greenhouse gas. Fossil fuel production, distribution and use are blamed as the chief causes of global methane emission. However, wetlands also have a role to play as about 25% of methane released to the atmosphere comes from wetland soils (1). In highly saturated soils, methane is released when organic matter is decomposed by anaerobic soil micro organisms called Archaea. Apart from the effect of water methane emission in wetlands is significantly influenced by temperature and the presence of organic matter (2, 7). Methane emission from wetlands can be controlled by draining the soil but this must be done with caution, as drainage of wetlands result in the emission of carbon-dioxide another potent greenhouse gas.
Anaerobic soils are natural water stores and filters
Wetland soils collect, store and slowly release water. This allows for ground water recharge (movement of water downwards the soil profile to the ground water level) and an even release of water to neighbouring water bodies (3). This reduces flood heights, nutrient run off and soil loss caused by erosive agents. The storage and slow movement of water within the soil allows for the absorption of nutrients. As well as the dissolving of contaminants from chemicals, sewage sludge, or septic tanks preventing them from flowing down to neighbouring streams and lakes.
Anaerobic soils are useful in agriculture
Some crops are cultivated on anaerobic soils. For example, rice, one of the world’s most consumed cereal crop is grown on paddy soils (paddy rice), or flooded areas (deep water rice) in many Asian countries. Wetlands are also well used for fish farming. Statistics show that two-thirds of fish consumed worldwide is reliant on coastal wetlands (4). Other edible or medicinally beneficial crops grown on wetlands include wild edible berries (blue berries, black currant, black raspberries, cranberries) and the Chinese water chestnut.
Anaerobic soils provide habitation and food for unique/diverse organisms
Wetland soils are home to several aquatic plants and micro-organisms which have unique features for survival in low oxygen conditions. In anaerobic soils for example aerobic bacteria (bacteria which need oxygen to survive) become dormant or die out giving room for bacteria which will thrive in oxygen starved conditions. Anaerobic soils are also a habitat for micro algae and hydrophytes (aquatic vegetation) some of which are used to beautify gardens. Aquatic plants have developed adaptations in their morphology (water roots, adventitious roots), their anatomy (intercellular air spaces) and their physiology (capacity to respire in anaerobic conditions) (1).
Bibliography and further reading
1. Inglett et al (2005) Anaerobic soils. In Encyclopedia of Soils in the Environment. https://soils.ifas.ufl.edu.
2. Methane Sources – Wetlands https://www.ghgonline.org/methanewetland.htm.
3. Osmond et al (1997) Watershedss: A decision support system for watershed-scale nonpoint source water quality problems. Journal of the American Water Resources Association 33: 327-341.
4. Barbier et al (1997) Economic Valuation of Wetlands: A Guide for Policy Makers and Planners. Retrieved from: https://www.ramsar.org/.
5. Functions and values of wetlands- https://www3.epa.gov/.
6. Parashar et al (1993) Effect of soil temperature on methane emission from paddy fields. Chemosphere 26: 247-250.
7. Bringham et al (2013) Methane emissions from wetlands: biogeochemical, microbial, and modeling perspectives from local to global scales. 19: 1325-1346.