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Soil Respiration

Unit: Soil Science

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Objectives

Define: soil respiration and soil microbes
Explain the role of soil respiration in determining soil health
Diagram the role of soil respiration in the cycle of life on earth
List and explain inherent factors that affect soil respiration
List and describe soil respiration management processes
Interpret management impacts on soil respiration and soil organic matter
Measure soil respiration and interpret data

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Soil Respiration

Soil respiration is a measure of the carbon dioxide released from the soil by microbes decomposing soil organic matter and from the respiration of plant roots.

Soil respiration indicates soil health (soil organic matter content, soil organic matter decomposition and the level of microbial activity).

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Soil Respiration

Respiration rate can be based on the amount of soil organic matter present.

Soil organic matter is a food source for microbes and when microbes are present and working, respiration is higher.
When soil organic matter is absent or low, there is less decomposing activity.
Soil microbes are responsible for soil respiration and many important soil processes. (bacteria, fungi, protozoa, algae)
One heaping tablespoon of soil can contain over 9 billion microbes.

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Soil Respiration

Plants

Food + Oxygen

Soil

Water + Carbon Dioxide

Sun

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Definitions

Ammonification: occurs in the nitrogen cycle when soil organisms decompose organic-nitrogen converting it to ammonia.

Available Water Holding Capacity: Soil moisture available for crop growth; also defined as the difference between field capacity and wilting point, typically shown in inches/foot.

Nitrification: Occurs in the nitrogen cycle when soil organisms convert ammonia and ammonium into nitrite and next to nitrate-nitrogen which is available to plants.

Bulk Density: Weight of dry soil per unit of volume, more compacted soil with less pore space will have a higher bulk density.

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Definitions

Denitrification: Conversion and loss of nitrate nitrogen as nitrogen gases when soil becomes saturated with water.

Respiration: Carbon dioxide release from soil from several sources (decomposition of organic matter by soil microbes and respiration from roots).

Soil Porosity: Percent of total soil volume made up of pore space.

Soil Water Filled Pore Space: Percent of pore space filled with water.

Soil Water Content, Gravimetric: Weight of soil water per unit of dry soil weight.

Volumetric Water Content: Amount (weight or volume) of water in soil core by volume.

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Factors Affecting Bulk Density: Climate

Climate

Cannot be changed
Affects temperature, moisture and indirectly affects biological activity

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Factors Affecting Bulk Density: Biological Activity

Biological activity

Varies with the seasons and times of day

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Factors Affecting Bulk Density: Soil Moisture

Soil moisture

As moisture increases, respiration rates increase until the pores are overly saturated, resulting in lower oxygen content and lower soil organism respiration
60% pore space saturation (field capacity) is ideal for respiration
Dry soils have low respiration rates because of less support for biological activities

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Factors Affecting Bulk Density: Soil Organic Matter

Amount and quality of soil organic matter

Affects microbe activity

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Factors Affecting Bulk Density: Soil Texture

Soil texture

Clay – soil organic matter is “protected” from decomposition
Sand – too little organic matter
Medium texture (silt and loam) – favorable for soil respiration

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Managing Soil Respiration

Soil Respiration Management Practices
Leave crop residues on the soil surface.

Residues with low C:N ratios decompose faster than those with high C:N ratios
High residue crops + added Nitrogen = higher decomposition rates and accrual of organic matter

Use no-till practices.

Tilling practices decrease decomposition and accrual of organic matter
Minimize equipment use in fields
Minimize farm equipment use in general when soils are wet
Use designated locations for equipment traffic

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Managing Soil Respiration

Use cover crops.

Roots provide respiration

Add organic matter.

Nourishes microbes

Irrigate dry soil.

Added water boosts the quality of the environment for microbes and therefore boosts soil respiration

Drain wet soil.

Too much water prevents microbes from being productive

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Management Practice	Application	Short Term Impacts	Long Term Impacts
Solid manure or organic material application	Provide additional carbon and nitrogen source for microbes to breakdown and increase biomass production.	Increased respiration when manure begins to breakdown and increased biomass production.	Positive impact on soil structure, fertility and soil organic matter content.
High residue crops or cover crops used in rotation with high C:N ratio	High C:N ratio crops and added nitrogen increase decomposition and accrual of soil organic matter.	High C:N ratio crop residue tie up nitrogen temporarily in order to break down residue, increased soil moisture, decreased erosion.	Positive impact on long term soil quality, fertility and soil organic matter content.
Tillage such as annual disking, plowing, etc.	Stirs the soil providing a temporary increase in oxygen for microbes to break down carbon sources.	Provides a flush of nitrogen, other nutrients and carbon dioxide release immediately after tillage. Increases erosion rates, decomposition rate of residue, and other carbon sources.	Declines in soil organic matter, soil quality, soil fertility.
Crop residue management	Leave residue on the surface increasing ground cover to protect the soil.	Increased crop residue cover can tie up nitrogen temporarily in order to break down residue, increased soil moisture, decreased erosion and cooler soil temperatures.	Positive impact on long term soil quality, fertility and soil organic matter content.
Nitrogen fertilizer or manure application	Provides nitrogen (energy) source for microbes to break down high C:N ratio residue quicker.	Temporary increase in respiration due to increased rate of breakdown of organic materials.	When managed correctly has an overall positive impact on soil organic matter and soil quality by increasing production levels and residue amounts.
Vehicle or farm equipment traffic	Compacts soil decreasing pore space, water movement, oxygen for microbes and nitrogen loss from denitrification.	Decreases respiration, yields, water infiltration and increases runoff.	Production declines, increased soil erosion and runoff, decreased soil quality, compacted soils and reduced microbial activity.