Geoscience Reference
In-Depth Information
As plants grow, their fi ne roots penetrate through the soil by pushing and tearing
it into many small fragments separated by a network of pores fi lled by roots and root
hairs. When the plants expire and their network of roots starts to decay, they
bequeath an inheritance of a crumbled soil. A certain portion of the root organic
matter is transformed into humus that cements coagulated fl occules and small frag-
ments together. The humins and humic acids serving as the glue are resistant against
the action of water, especially against their dissolution in water. For a long time in
the past, it was assumed that the content of all humifi ed soil organic matter had this
positive effect: the more of it in soil, the more advanced is the existence of aggrega-
tion. During those times it was taken for granted that humins played the decisive
role regarding the quality of soil aggregates. When we speak about quality of aggre-
gation, we must fi rst accept the necessity of virtually all components of soil being
transformed into aggregates of long-lasting stability and then base our judgment on
their capacity to retain plant-available nutrients, including water.
A special group of molecules detected during 1996 has an action on the quality
of aggregates that is several times stronger than that of humins. These molecules got
the name glomalin since they have been produced by symbiotic fungi
Glomales
(order
Glomerales
according to the International Botanical Taxonomy). Glomalin
consists of special types of proteins, and because of their unique structures, they are
called glomalin-related soil proteins. Since this term is too long and complicated for
many nonspecialized readers, soil researchers use the abbreviation GRSP. Proteins
are polymers built from alpha amino acids in the form of a chain. All alpha amino
acids consist of alpha carbon, i.e., the carbon that is attached at the fi rst (alpha) posi-
tion, to which an amino group (H
3
+
N) is bonded together with carboxyl group
(COOH) and side chain which is very variable in chemical structure. The chain is
three-dimensional like a wide ribbon twisted into a spiral with a variety of bonds.
These variations result in plurality of properties.
Fungi
Glomales
have occurred in mycorrhizal symbiosis with plants when the
green plants entered into terrestrial conditions in Silurian and Devonian about 400
million years BP. The name of the symbiosis is derived from the Greek
mykos
,
meaning fungus or mold, and
rhiza
, meaning root. These fungi exist in about 80 %
of all terrestrial vascular plants and in some mosses. They are branching like a tree
on the microscopic scale, and since the Latin
arbuscula
denotes also very small
arbos
(tree), we defi ne the symbiosis between those fungi and plants as arbuscular
symbiosis. It is known that fungi penetrate into fi ne hair roots of plants without
damaging their cells. The long fi ne outside fi bers of
Glomales
fungi bring water and
plant nutrients into roots, especially phosphorus and trace elements from the soil.
Because the external fungi fi bers occupy a much bigger space of soil than the roots,
they pump much more water and elements into plant roots than the root system is
able to achieve. The plant acknowledges this benefi cial gesture by supplying photo-
synthetically produced organic compounds to that portion of fungi fi bers inside its
roots. With the fungi receiving mainly glucose and similar metabolic compounds,
the plants and the
Glomales
both profi t from this symbiosis. The surface of the
external fungi fi bers is reinforced and fortifi ed by molecules of glomalin that are
produced in the body of fungi from organic compounds delivered by the plant. After
