Agriculture Reference
In-Depth Information
to attempt to separate them, except perhaps in the case of the most incipient soils, some
man-made and heavily man-influenced soil systems.
The initiation of simple energy flow and basic material cycling pathways occurs at
the earliest stages of ecosystem development and soil formation and these trophically-
based systems continue to develop in degree and complexity over time (Odum, 1983).
Of major importance in ecosystem and soil development and maintenance are those
species that directly or indirectly control the availability of resources to other species,
the ecosystem engineers (Jones
et al.,
1997). Through their non-trophic activities, these
organisms physically modify, maintain or create new habitats for other organisms.
One effect of such organisms is to create greater habitat diversity which may in turn
lead to increased species diversity.
Physical ecological engineers may be divided into two groups. The first comprises
the autogenic physical engineers that modify their environments through endogenous
processes such as tree growth that alter the structure of the engineer which then remains
as part of the environment. Allogenic engineers alter their environments by transforming
living or non-living materials from one physical state to another, as occurs in the creation
of large voids in the soil through root decay (Jones
et al.,
1997). Trees also act as
geochemical engineers in that deep roots take up soluble silica released through litter
decomposition processes in the surface horizons, thereby ensuring the stability of clay
minerals (Lucas
et al.,
1993).
A number of organisms may also be designated ecosystem chemical engineers for
the roles they play in mediating changes to inorganic and organic substrates through
secretory and excretory activities. This group includes the micro-organisms because of
their roles in such diverse processes as rock weathering, organic matter breakdown,
nutrient cycling, nitrogen fixation and the acquisition of nutrients through mycorrhizal
associations. Other organisms including higher plants and animals also play substantial
roles in this respect. Many examples of both major types of engineers appear throughout
this topic.
Close interactions between the biota and the other soil forming factors are also clearly
apparent. Climate, parent materials and topography control the continually-changing
chemical and physical environment within which the biota acts. In turn, the biota acts to
progressively modify topography, the parent materials and the developing soil.
2.5
Topography
The importance of topography lies in its control of site hydrological regimes and it
therefore exercises an important influence on soil development and maintenance.
In addition, through its influence on the distribution of erosional products, topography
modifies the effects of such pedogenetic processes as creep and frost heave thereby
affecting the landscape distributions of such parent materials as colluvium. The lateral
and downhill transport processes that occur at and below the soil surface in all
landscapes are largely controlled by topography and lead to the formation of the suites
of spatially-related soils known as toposequences or catenas, discussed later in this chapter.
