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

Key Points

Chemical Factors

macro nutrients (base extractable organic C), extractable P, nitrate & ammonium, ph & EC, ect

- Toxins poisonous substance produced by living cells or organisms that is active at very low concentrations.

- Osmotic pressure is the hydrostatic pressure produced by a solution in a space divided by a semi-permeable membrane due to a differential in the concentrations of solute.

- pH is important because microorganisms and plants respond markedly to chemicals in their environment. Most soil microorganisms and plants prefer a near-neutral pH range of 6 to 7 because the availability of most soil nutrients is best in this pH range.

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 (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 o 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.) flooded soils link (notes) • 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

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

Mutualism (symbiosis) Commensalism Synergism Amensalism Predation Competition (antagonism) Parasitism

Population 1	Population 2	Example
+	+
+	0
+	+ or 0
-	+ or 0
-	+
-	+
-	+

- Bioavailability - Bioavailability is the amount of a contaminant that is absorbed from soil.

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

References

Edited by students of Kate Scow