The Soil Biota

http://www.nature.com/scitable/knowledge/library/the-soil-biota-84078125

By: Ann-Marie Fortuna (Dept. of Crop & Soil Sciences, Washington State University) © 2012 Nature Education
Citation: Fortuna, A. (2012) The Soil Biota. Nature Education Knowledge 3(10):1
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A ‘biological universe’ exists in a gram of soil. Find out how the soil biota within this tiny universe transform energy, create and modify their habitat, influence soil health, and aid in the regulation of greenhouse gases.

In his famous poem, The Auguries of Innocence, the poet William Blake wrote:

“To see a world in a grain of sand,

And a heaven in a wild flower,

Hold infinity in the palm of your hand,

And eternity in an hour.”

In a similar vein, one might see the ‘biological universe’ in a single gram of fertile soil, approximately a teaspoon in size, containing all the domains (Bacteria, Archaea and Eukarya) and elements of life! The majority of life on Earth is dependent upon six critical elements: hydrogen (H), carbon (C), nitrogen (N), phosphorus (P), oxygen (O), and sulfur (S) that pass through, and are transformed by, soil organisms (the soil biota). The process of biogeochemical cycling is defined as the transformation and cycling of elements between non-living (abiotic) and living (biotic) matter across land, air, and water interfaces (Madsen 2008). Biogeochemical processes are dependent upon the biota in the soil or pedosphere, the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere (rock), atmosphere (air), hydrosphere (water), and biosphere (living matter). This article addresses the role of soil biota in the pedosphere using ecological principles that link soil organisms and plants to biogeochemical processes occurring within the soil in natural and managed ecosystems.

Aggregates: Model of a Pedosphere

Soil texture (fineness or coarseness) affects plant rooting, soil structure and organic matter content. Soil texture and structure determine the pore-size distribution, soil water holding capacity and the amount of water to air-filled pore space in soil aggregates that provide habitat for soil organisms. Aggregates can be broadly classified into macroaggregates (>250 µm) and microaggregates (20-250 µm) (Six et al. 2004). An aggregate is a naturally formed assemblage of sand, silt, clay, organic matter, root hairs, microorganisms and their “glue” like secretions mucilages, extracellular polysaccharides, and hyphae (filaments) of fungi as well as the resulting pores. Soil aggregates often contain fine roots that grow into soil pores (Figure 1) associating aggregates with the rhizosphere “the zone of soil under the influence of plant roots” (Sylvia et al. 2005). Persistent binding agents like organic matter and metals stabilize microaggregates. The temporary binding agents (polysaccharides and hyphae) produced by soil organisms aid in the formation of macroaggregates contained within the more stable microaggregates. These macroaggregates function as “ecosystems or arenas of activity” (see: Arenas of Activity in the Pedosphere of a Forest) (Beare et al. 1997, Coleman et al. 2004). Thus, an aggregate is a unit of soil structure that could be considered as a very small-scale model of a pedosphere. One can visualize all the interactions of gases, water, organisms and organic and inorganic constituents at the “microscale” hence the “glimpse of the universe” in a gram of soil (Figure 1).

A soil aggregate or ped is a naturally formed assemblage of sand, silt, clay, organic matter, root hairs, microorganisms and their secretions, and resulting pores.

Figure 1: A soil aggregate or ped is a naturally formed assemblage of sand, silt, clay, organic matter, root hairs, microorganisms and their secretions, and resulting pores.
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