Bioturbation is the rummaging through and mixing ( turbation ) of soils or sediments by living beings. Since soils differ fundamentally from rocks in that they are alive, bioturbation is primarily a research area in pedology . However, bioturbate structures are often preserved in sedimentary rocks and play an important role in the reconstruction of past sediments in geology and palaeontology .
In terrestrial soils, the biogenic mix is primarily responsible for soil animals such as small mammals (e.g. moles ), worms (e.g. earthworms) or insects (e.g. ants). In aquatic sediments or subhydric soils , these are mainly worms (e.g. lugworms), mussels, sea urchins , sea cucumbers , crustaceans that build living tubes, etc. Primarily, these are recent processes of pedogenesis , i.e. the formation of soils and their consequences are also represented in geological dimensions. In addition to bioturbation, hydro (pelo-), cryo- and cultoturbation should be added to the pedogenic mixing processes.
Importance for soil formation (pedogenesis)
Bioturbation plays an important role in soil formation, and even a decisive one in the development of black earth. Above all, the physical properties (the aggregate structure ) are positively influenced by them, as the burrowing Edaphon (soil burrower) contributes to loosening the soil through its burrows, sometimes promoting the formation of a crumb structure with cavities , in the special case of pronounced earthworm activity Creates a worm solution structure . The loosening of the soil can also contribute to better aeration of the topsoil (if the soil material is deposited on the surface, see molehills) and counteract water stagnation. However, the draining effect is to be classified as secondary, as many of the animal tunnels end blind. However, positive effects can still be demonstrated for clayey soils (increase in water capacity).
Finally, through the processes to be included in the bioturbation, the boundaries of the soil horizons are "blurred" in that on the one hand nutrients such as lime that have been washed out are returned to the subsoil and humus-free subsoil material is transported upwards, and on the other hand the humus topsoil is deepened and homogenized learns. Especially those representatives of the Edaphon who hunt (moles) or live in the ground ( geophages such as earthworms) contribute to the soil relocation to a particular extent (mole and earthworm up to 12 kg per m² and year).
The importance of bioturbation in the case of black earth is justified by the fact that the small mammals involved in pedogenesis (here hamsters , ground squirrels and prairie dogs ) are driven to migrate vertically by the winter cold or summer drought and thus cause a deep accumulation of humus (one might think, for example to the pantries of the hamsters), which ultimately creates the powerful, characteristically dark brown to black Ah horizon. The humus soil material is transported down in the corridors, which are known as crotovines , and protected from complete mineralization. The material that is transported upwards by the soil diggers in turn counteracts decalcification.
Since it is primarily the fine soil material that is homogenized, accompanied by a mixture of organic and mineral substances, and larger soil particles such as stones and gravel are hardly transported, bioturbation can in certain cases favor the formation of stone floors in the subsoil - and thus, to a certain extent, a separation.
The macro and megafauna involved in bioturbation can be categorized into different groups according to their role in soil restructuring and the type of digging influence they have on the soil structure (= structure). Thus, moles, beetles and mole crickets were the shovel graves , ants and termites the mouth graves , hamsters, ground squirrels and prairie dog's scratching graves and worms and snakes to Bohrgräbern associate. Other ways of differentiation are also possible.
With bioturbation, soil and sediment components are shifted to a not inconsiderable extent. This can break up the stratification in sediments . In the soil, hydrophobic , strongly sorbent pollutants (e.g. PCB , dioxins ), which are actually fixed in the uppermost soil horizon after atmospheric deposition, can reach deeper soil zones.
Bioturbation of the bent neck
Bioturbation also plays an important role in the soils of waters ( Benthal ). It is also about the degradation processes of organic material that falls to the ground as detritus and is embedded in the sediment. A loosening of the sediment and thus a better supply of oxygen is the most important aspect. In addition, the species composition and diversity of the benthos and thus the food chain, at the top of which are many fish, are influenced.
In the sea, bioturbation is mainly brought about by different groups of worms , primarily from the class of the multibirst , but also by mussels , which, as filter feeders, play a further role in the exchange of substances. Crustaceans burrowing in the ground such as the flea shrimp Monoporeia affinis (10,000 to 20,000 individuals per square meter in the Baltic Sea ) or decapods from the Callianassid family (responsible for fossil burial tunnels of the Thalassinoides - Ophiomorpha group of forms) can also be of importance for the bioturbation of the Benthals.
Often fossil traces of bioturbation can be found and interpreted in sedimentary rocks . Sedimentary rocks with traces of strong fossil bioturbation indicate an oxygen-rich, life-friendly environment, while finely layered sediments without traces of bioturbation provide evidence of a life-hostile, oxygen-poor or oxygen-free environment. Under certain conditions, relics of bioturbation in the form of trace fossils ( Ichnofossils ), for example residential buildings, feeding structures or traces of life such as crawl tracks, rest tracks, grazing tracks, etc., are preserved in rocks.
- RG Bromley (1999): Trace Fossils: Biology, Taphonomy, and Applications. - Springer, Berlin / Heidelberg, 347 pp. ISBN 978-3-540-62944-3 .
- Herbert Kuntze, Günter Roeschmann and Georg Schwerdtfeger: Soil science . 5., rework. and exp. UTB, Stuttgart 1994, ISBN 3-8252-8076-4 .
- Fritz Scheffer and Paul Schachtschabel : Textbook of soil science . 15., rework. and exp. Edition. Spectrum Akademischer Verlag, Heidelberg and Berlin 2002, ISBN 3-8274-1324-9 .
- Maurice Tucker: Introduction to Sediment Petrology . Enke-Verlag, Stuttgart 1985. pp. 38-40. ISBN 3-432-94781-X