Environment of Earth

September 23, 2009

Plant cover and soil

Filed under: plants,Soil — gargpk @ 4:32 pm
Tags: ,

Soils are formed by mixture of weathered rock material with the organic matter derived from decomposition of mostly plant litter. The role of plant cover in pedogenesis and determination of the soil type of an area is clear from the fact that all zonal soil types correspond to specific types of plant covers.

Tundra soils

Rock tundra is associated with isolated patches of lichens and mosses with occasional higher plants. Tundra moor is associated with peat-forming mosses mainly Polytrichum sp., lichens like Cladonia sp. Or Cetraria sp., grasses like Carex, Eriophorum, herbs like Potentilla, Ranunculus, Gentiana, Saxiraga, Dryas octapetala, shrubs and trees like Betula nana, Salix herbacea, S. reticulata, S. arctica and heathers like Empetrum nigrum, Cassiope tetragonal.

Podsols

Iron-humus podsol develops under heathland with Ericaceous dominants while iron-podsol develops under coniferous forests. Podsolization is strongly influenced by the type of vegetation. Certain species hasten podsolization e.g. Ericaceous heath Calluna vulgaris and Erica cinerea that usually occupy cleared forests on acid brown soil. Some conifers and Fagus sylvatica and Quercus sp. In Britain and Populus trichocarpa in Alaska are strong acidifiers. Sphagnum sp., Eriophorum sp., and Molineae caerulea form blanket peat, forming peat podsol or peaty-gley.

Brown forest soils

Very high productivity of broad-leaved summer forests plays considerable part in the pedogenesis of brown forest soils by maintaining quite high activity of soil microorganisms and sol fauna, particularly earthworms.

In North Europe, clearing of broad-leaved forests present on acid brown soils that had developed on siliceous parent material resulted in the establishment of Ericaceous heath lands. This hastened podsolization in those areas. These podsols are being maintained today by burning and felling of trees along with maintenance of heath-land. In absence of such interference, Calluna-Erica heath-land is easily replaced first by bracken (Pteridium aquilinum) and then by conifers. These plants bring back the podsol soil to acid brown soil.

In the absence of normal forest, the brown forest soils can be maintained under grass cover because much organic matter is returned to the soil by extensive root system. However, inorganic fertilizers are needed even then. If land is under crop cultivation, both inorganic and organic fertilizers are needed to maintain the soil.

Red & brown soils of arid subtropics

The soils in arid subtropics formerly had luxuriant vegetation of Quercus ilex and Pinus halpensis. However, overgrazing in the areas of brown soils on limestone in Europe resulted in sparse vegetation of low trees causing conversion of brown soils (Terra fusca) to red soils (Terra rosa). Brown soils in many areas still have comparatively better sclerophyllous cover. Afforestation on red soils protects them from summer solar insolation and changes them to brown soils again.

Other red and brown soils

While red brown soils develop under subtropical dry forests, red-yellow soils develop under subtropical forests and red laterite soils under tropical forsts. Brown and grey soils are developed under deciduous forests, grey-brown soils under semi-desert or desert scrub forests and chestnut brown soils under steppe.

The plant cover and the cycling of nutrients are intricately interlinked with each other in the ecosystem. Nutrient cycling and biomass normally reach equilibrium under climax conditions. External influences, particularly destruction of plant cover may break this cycling and disrupt this equilibrium causing deterioration of the environment. For example, in North Europe, destruction of deciduous forest cover by human activities during Late Stone Age and Bronze Age caused loss of nutrients from the upper layers of soil. The deteriorated soil caused establishment of heath-lands on them. These heath-lands are presently maintaining and are being maintained by reduced nutrient cycling from the upper layers of soils only. In the tropical areas, laterite soils (latosols) have very deep crusts of weathering and are greatly leached of nutrients. However, rain forests of ancient geological ages on these soils provide rich vegetation of great biomass in which most of the ecosystem nutrients are locked up. Despite poor nutrient availability in the soils in these tropical rain forests, such forests are presently being maintained in these areas only due to highly efficient nutrient cycling from the upper layers of the soil. The huge amount of plant organic matter from the vegetation falling on the soil decomposes and provides nutrients to the plants again.

In the soil under a forest cover, the absorption of several nutrients from the deeper layers initially reduces their availability but the return of these nutrients with fall of litter again increases and maintains their availability in the upper layers of the soil. This effect is particularly marked for Mg and Ca. The efficiency of nutrient cycling is greatest in rain forests followed by deciduous forests, coniferous forests and grasslands in that order. Coniferous forests return 50-100 kg/ha/yr of ash elements while deciduous forests may return 200-270 kg/ha/yr. The return of Ca in the rain forests is 200-300 kg/ha/yr while in deciduous forests is only 150 kg/ha/yr.

The soil structure is also greatly affected by the plant cover because roots of plants have a direct influence in maintaining the rhizosphere bacteria whose capsular slimes and gums stabilize the soil crumbs. Rhizosphere zone in the soil provides nearly ideal conditions for both aggregate formation and aggregate stabilization by incorporation of bacterially synthesized macromolecules. In the grassland cover, rapid aggregate promotion is certainly due to rapid and prolific root production of these plants.

The plant cover also influences soil fauna and consequently, the soil structure. In the forest mull soils, the plant cover provides litter that promotes and maintains rich earthworm population in the soil. In these soils, earthworms create pore space through voided casts that are stabilized initially by fungal growth and later by cementation with bacterially produced polysaccharide macromolecules.

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