 |
| Titel |
Linking plants, fungi and soil mechanics |
| VerfasserIn |
Anil Yildiz, Frank Graf |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250150051
|
| Publikation (Nr.) |
 EGU/EGU2017-14475.pdf |
|
|
|
|
|
| Zusammenfassung |
| Plants provide important functions in respect soil strength and are increasingly considered for
slope stabilisation within eco-engineering methods, particularly to prevent superficial soil
failure. The protective functions include hydrological regulation through interception and
evapo-transpiration as well as mechanical stabilisation through root reinforcement and, to a
certain extent, chemical stabilisation through sticky metabolites. The ever-growing
application of plants in slope stabilisation demanded more precise information of the
vegetation effects and, concomitant, led the models for quantifying the reinforcement shoot
up like mushrooms. However, so far, the framework and interrelationships for both the
role of plants and the quantification concepts have not been thoroughly analysed
and comprehensively considered, respectively, often resulting in unsatisfactory
results.
Although it seems obvious and is implicitly presupposed that the plant specific functions
related to slope stability require growth and development, this is anything but given,
particularly under the often hostile conditions dominating on bare and steep slopes. There, the
superficial soil layer is often characterised by a lack of fines and missing medium-sized and
fine pores due to an unstable soil matrix, predominantly formed by coarse grains. Low water
retention capacity and substantial leaching of nutrients are the adverse consequences. Given
this general set-up, sustainable plant growth and, particularly, root development is virtually
unachievable.
At exactly this point mycorrhizal fungi, the symbiotic partners of almost all plants used in
eco-engineering, come into play. Though, they are probably well-known within the
eco-engineering community, mycorrhizal fungi lead a humble existence. This is in spite of
the fact that they supply their hosts with water and nutrients, improving the plant’s ability to
master otherwise unbridgeable environmental conditions. However, in order to support their
plant partners, the fungi themselves need to have access to water and nutrients. For this
purpose, a resilient soil matrix consisting of stable micro- and macro-aggregates is an
indispensable prerequisite. Luckily, the fungi are among the pioneers in assembling stable
aggregates. The fungal hyphae intensively penetrate the unstructured soil body, enmeshing
small organic and inorganic soil particles and form and cement them to micro- and
macro-aggregates.
On the one hand, growing hyphae are able to align primary particles and, on the other
hand, exert pressure on surrounding particles and compounds forcing them together, such as
clay and organic matter. Under physiological (or neutral) pH values, the fungal mycelia have
a net negative charge. It is suggested that negatively charged fungal polysaccharides are
bound to negatively charged clay minerals by bridges of polyvalent cations which
have been proven to be stronger than some direct bonds between clay and organic
matter.
The formation of aggregates up to a size of 2 mm is associated with hyphal length of
fungi. With regard to the assemblage of aggregates >2 mm both fungal mycelia and roots are
involved. Indirectly, the mycorrhizal fungi affect the aggregate establishment through their
host plants, particularly by accelerating the development of their root network and by serving
as a distribution vector for associated micro-organisms, mainly bacteria and archaea,
additionally contributing to cementation. Therefore, root-reinforcement as addressed for
quantification of vegetation effects on slope stability almost ever is a combined contribution
of fungal mycelia and root networks. With soil aggregates as the "bricks" for building a stable
soil matrix and pore structure, root-reinforcement strongly depends on aggregate strength
controlling potential, efficiency, and sustainability of growth and development of the
protective vegetation.
From a geotechnical point of view, aggregation of fines may be such pronounced
that characteristics of coarse-grained soils are adopted, often mirrored by higher
values of the shear strength parameters, particularly the angle of internal friction
Φ’. Consequently, neither the positive relationship between the strength of soil
aggregates and slope stability is astonishing nor is the positive correlation between root
characteristics – architecture represented by 3D-complexity, specific length and its density –
and factor of safety calculations related to superficial soil failure. As far as the
latter is concerned, however, so far almost exclusively the common shear strength
parameters have been considered, namely angle of internal friction Φ’ and root
cohesion c’. However, similarly to the way fungi were ignored in biological slope
stabilisation, the soil mechanically relevant parameter dilatancy (Ψ) was not in the
concepts and modelling approaches for quantifying root-reinforcement. Nevertheless,
dilatancy (Ψ) is an important mechanism and a contributing factor to the shearing
behaviour of root-permeated soil that definitively cannot be ignored. Such evidence
is soundly based on the fact that specific root characteristics combined with the
maximum dilatancy angle (Ψmax) can explain the most variation in peak shear strength
parameters.
Therefore, a combined approach including soil, fungi, and roots under consideration
of dilatancy is a promising way towards better understanding and more reliably
quantifying the shear strength of root-permeated soil. Since sound quantification of
biological stabilisation effects is the key for both sustainable slope stabilisation and wide
acceptance of eco-engineering measures within the scope of risk and hazard prevention. |
|
|
|
|
|
|