Soil Environment

Introduction

Microbial activity basically means the generation of microbial. It could be affected by the soil environmental and physical factors.This page explores the soil environmental factors regulating microbial activity.

Warm-up with this quote by Shuhei Ono from the Carnegie Institution's Geophysical Laboratory:

“We think that there were microbes in the oceans, before the oxygenated atmosphere, which would have used methane for energy... Oxygen first appeared on the surface of the Earth when microbes developed the capacity to split water molecules to produce O2 using the Sun's energy. This is a bit advanced biochemistry, but we think this biological revolution emerged sometime before 2.7 billion years ago," he continued.

Reuters (2006, March 22). Climate Change And The Rise Of Atmospheric Oxygen. ScienceDaily. Retrieved March 4, 2008 from http://www.sciencedaily.com/releases/2006/03/060322140017.htm

Chemical Factors

pH

pH changes in soils is due to both biological processes in the soil that consume or release H+ through oxidation/reduction reactions and fermentation, and/or from rainfall that can leach bases (book). The majority of soil microbes thrive in neutral pH (6-7) due to the high availability of most nutrients in this pH range, but there are examples of microbes (especially fungi) that can tolerate pH of 1 to 13 (book). Alterations in pH can render essential microbe enzymes inactive and/or denature proteins within the cells and prevent microbial activity from occurring (notes). pH changes can also effect microbes in their access to metals and organics that react differently under varied pH régimes (notes).

Oxygen

Oxygen levels dictate the oxidation-reduction reactions that occur and largely what microbial processes occur. In the presence of oxygen, O2 will be used for aerobic respiration, but when concentrations are low other electron acceptors are used (such as sulfur, iron, etc.) (notes). This phenomenon can be seen in flooded soils. Some microbial enzymes require O2-, so the level of O2 can regulate the enzymatic activity. Some products of O2 reactions are toxic (such as superoxide radical O2-) and without the proper enzymes to inactivate these toxins microbes are susceptible to harm

Cation Exchange Capacity (CEC)

Charged soil particles allows for charged soil microorganisms (due to charged organic molecules) to be attracted or repelled from soil particles. The full understanding of how this mechanism works is not understood yet, but postulated mechanisms include: ion exchange attractions, weak attractive forces, coordination bonding, and hydrogen-bonding

Physical Factors

Soil type: texture, density/porosity, moisture, depth, color, age, ect.

-Soil Texture: Sand (0.05-2.0 mm) Silt (0.002-0.05mm) Clay ( < 0.002mm) Soils that are coarse textured are less likely to have a well-defined structure and therefore fewer structured pore space than s soil high in clay content.

- Soil pores play a major role in water and air movement. Also, soil microorganisms reside in pores. Pore space is largely determined by size and arrangement of aggregates and affects the movement of water, air, and organisms in soil.

- Pore Size Distribution: Macropores ( >75um) Mesopores ( 30-70um) Micropores ( 5-30um) Ultramicropores (0.1-5um) Crytopores ( <0.1um)

- Soil Structure: Aggregation of primary soil particles is a critical determinant of soil structure.Structure is strongly affected by climate, biological activity, density and continuity of surface cover, and soil management practices. Soils that are coarse textured are less likely to have a well-defined structure and therefore fewer structured pore space than s soil high in clay content. Ecological relationships among soil organisms are influenced by soil structure.

- Soil water is essential for soil microorganisms. Without some water, there is no microbial activity.

- Soil aeration measures how well a soil is oxygenated. Ideally, a well-aerated soil would have sufficient oxygen for the respiration of plant roots and the function of most aerobic microorganisms.

- Soil temperature greatly influences the rates of biological, physical, and chemical processes in the soil. Within a limited range, the rates of chemical reactions and biological processes double for every 10 degree increase.

-aggregates

Biological Factors

growth rates (plant measurements/microbial measurements), assimilation, ect.

Major Groups: viruses, bacteria, archaea, fungi, slime mold, protozoa.

Organism Interactions

There are several types of interactions between organisms. The table below outlines the positive and negative aspects that each organism receives as a result of the interaction.

- Interaction Type	- Population 1	- Population 2	- Example
Mutualism (symbiosis)	+	+	Rhizobia/legumes, mycorrhizal fungi/most plants, fungi/green algae, fungi/cyanobacteria
Commensalism	+	0	Aerobic oxygen consumption creates an anaerobic environment to support anaerobes
Synergism	+	+ or 0	Flooded soils: fermenters provide carbon and hydrogen gas for methanogens and sulfate reducers, which in turn reduce toxic effects of acetate and hydrogen gas to fermenters
Amensalism	-	+ or 0	Allelopathic chemical production
Predation	-	+	Protozoa, nematodes, and slime molds are all major soil predators that consume bacteria
Competition (antagonism)	-	+	Growth rates that allow for utilization of a resource before a competitor
Parasitism	-	+	Viruses

Bioavailability

Bioavailability is the amount of a contaminant that is absorbed from soil. The EPA lists this definition of bioavailability:

“The term bioavailability designates the state of that fraction of a chemical that is available for uptake and/or transformation by living organisms. Although associated primarily with ecotoxicology, and usually in reference to organic and metallic pollutants, the term bioavailability is also relevant to native organic material. Thus, the “problem” of bioavailability, has existed for microorganisms far longer than has the presence of xenobiotic chemicals in the environment. Sorption, insolubility, and related processes are largely responsible for controlling bioavailability of many pollutants to microorganisms in soils and sediments” (EPA 2003).

- Rhizodeposition, mycorrhiza, plant growth-promoting rhizobacteria (PGPR), ect.

Current Research

Text Book JSTOR: Florida Entomologist Vol.75, No. 4, p.539,1991

Edited by students of Kate Scow