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Abstract "Heft 15"


Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich


Heft 15


Franz Lamparski

Der Einfluß der Regenwurmart Lumbricus badensis
auf Waldböden im Südschwarzwald


Freiburg im Breisgau 1985

ISSN 0344-2691


Summary:

In the southern part of the Black Forest (SW-Germany) conditions favor a rapid breakdown of the litter. The humic-form is a mull, also where conditions normally tend towards raw humus formation (1000 msl.wet and cold climate, deficiency of bases, needle litter).
Such occurences of mull are usually connected with a high population density of the earthworm Lumbricus badensis. This spezies is endemic to the Black Forest, requires atlantic climate conditions and apparently shows a high tolerancy to low pH-values since it is able to raise the pH-value within the bounds of its living-space.
Other biogeographical interesting Lunbricus-species in the investigation site are: Lumbricus polyphemus, Lumbricus moli-boeus and Lumbricus friendi.
The atlantic species L.friendi must be considered as the direct ancestor of L.badensis. In the foothills of Weitenau and on the southern border of the Black Forest both species are found together in the same biotope but without bastardi-zation. The difference in size seems to be a sufficient isolating mechanism: In the investigations area L.friendi achei-ves a fresh weight of 4 g, L.badensis ranges from 25 g to 40 g fresh weight.
The distribution of L.badensis must be seen in connection with the area of its ancestor: L.friendi. This area encompasses Spain, France, Ireland, Switzerland and a small part of SW-Germany. The area of L.badensis is directly connected to the area of L.friendi and even with its relatively small distribution L,badensis shows a requirement for a humid climate. This requirement is particularly evident in the eastern, northeastern till northwestern bounderies:
The borders of the area in the west and south are effective barriers. Here L. badensis touches with a high population density soils wich are rich in limestone or clay. Such soils are generally avoided by L.badensis. Towards southeast and northeast the population decreases slowly and becomes limited to patches with humid soil-conditions. Here there is a true limit, marked by increasingly continental conditions.
As the area is geographically small, it is justified to seek to interpret the area limits as caused by a single factor. A comparision with Reichelt's dryness index shows that the area frontier is extended parallel with one isoline of the dryness index in most regions.
The speciation of L.badensis must    have started after the glaciation period. After the glaciation period came a classic condition for speciation: On the northwestern limit of L. friendi's area the Rhine river could have geographically separated a founder population. The establishment of this founder population may have started in the Praeboreal/Boreal. During the Atlanticum the founder population was probably separated for a long time from L.friendi, its sister-species. In this period the soil formation indicates a dry climate condition, not just the Rhine but the whole Rhine valley could have worked as a barrier.
L.badensis is an anecic earthworm, he use the deep mineral soil as well as the soil surface as living space. The soil surface is used especially for feeding, thus this earthworm show a close relationship to the epigeic types. The juveniles of L.badensis live in fact epigeically. Nevertheless it is justified to classify the adult earthworms as inhabitants of mineral soil. Actually the older worms build the humic wall of their tubes with elements of the soil surface, with a strongly humic mineral soil. In other words, they enlarge their living-space i.e. the humus layer near the soil surface, by boring deeper throughout development of the individual .
The influence on the soil by these earthworms is mostly acheived by the burrow construction. The burrow of L.badensis reaches a depth of 2.5 m. Its diameter is a uniform 14 - 16 mm over its whole length, and allows the animals a U-like turn anywhere in the whole length of the burrow. Although the tube ascends by bends up to the soil surface, the general axis of the burrow is at a right angle to the surface. The distance of these bends is proportional to the bodysize of the worms, thus when the animal growth larger it must dig a new tube. Numerous cocoon-chambers are made at a depth between 150 -40 cm. Near the soil surface the burrow branches into 5-7 outlet tubes. They penetrate a burrow surface, which is a loose mixture of earthworm excrements and litter. The outlet tube allows the earthworm contact with the soil surface and the exterior. But most important, it enhances the pre-decomposition of litter by microorganisms. Such a pre-breakdown is necessary for most animals which feed on high cellulose diet. For L.badensis the burrow surface serves as an "external rumen", in which the activity of microorganisms is promoted and therefore also the litter breakdown. This litter breakdown benefits from the warm day temperature and the good aeration on the soil surface. But the loose compost heap produced by the atlantic type L.badensis with exkrements and litter is sensitive to desiccation. So the construction of the burrow surface can explain the strict reaction of L. badensis to the more continental or dryer conditions of the northern and eastern part of its area.
The stimulation of microorganismic activity and the feeding of the earthworms causes an immediat litter breakdown on the burrow surface. The reason for this are moderate temperatures, a humid microclimate, an increase in pH-values and an increase in nitrogen supply, due to the presence of mucus and excrements on the burrow surface. Due to a strong mixture a substance— exchange by diffusion is possible between organic matter and mineral soil.
The coming together of single burrow surfaces with a diameter up to 30 cm creates the humus form mull in an area. In this areas the population density reaches between 5 and 10 adults/m2 Sporadic L.badensis burrows are found on spruce stands where the humus form is moder. Here a comparision was possible between a litter breakdown which creates moder and a litter breakdown influenced by earthworms: without earthworms only the cellwalls of a needle are pre-decomposed. The cuticula and the central channel of the spruce needles remains intact. This becames apparent on observation under polarized light. The residuals of this incomplete breakdown have the tendency for accumulation on the top of mineral soil as F-and H-layers. The complete pre-decomposition of a needle by microorganisms is caused by the earthworminfluence and on the same time the needles are mixed with the mineral soil. This means, that a great part of the energy content in the needles is to utilize by detritophagous and there is the possibility of an energy transfer in the mineral soil. Such energy transfer is the basis for the aggregation and porosity of soils with the humus form mull.
Changing a raw humus to a mull the activity of L.badensis influences the freshly fallen needles on the surface. These needles are put into a "mull-enviroment" through the mixing with earthworm excrements and are therefore quickly decomposed. The supply of the F- and H-layers is then cut off. Later this layers are also mixed with mineral soil forming an A-horizon with a wide C/N ratio.
The earthworm activity in the soil creates a profil with a small Ah-horizon, a thick AhBv-horizon, with patches enriched with humus ernbeded in a matrix with less humus. In such patches rootgrowth and earthworm activity is concentrated. In the deeper mineral soil a loose soil structure and the burrow tubes with its dark humic wall shows evidence of "earthworm activity.
Soil transport by L.badensis in such a soil is mainly concentrated in the first 6-10 cm. From 10 - 50 cm transport of soil is acheived more readily by moles then earthworms. Soil deeper than 50 cm can only be transported by the earthworm to the soil surface. These transports are relatively small, despite this elements from a depth till 2.5 m can be introduced into the nutrient cycles.



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