Soil Science Weeks 8-9

What are the 2 Mechanisms of Gas Exchange between atmosphere and soil?

Gas Convection: The gas molecules of interest are carried into (or out of) the soil by bulk air flow in response to a pressure difference between the atmosphere and the soil. (10% of gas exchange) Gas Diffusion: Random molecular motion of gas molecules along a concentration gradient results in gas flow into (or out of) the soil.(90%)

To grow more microbes, and to better degrade C substrates, an appreciable amount of ___ is needed as well as C.

N

Three major gases of soil air and their %

N2: 78% Atmosphere; 78% Soil Air O2: 21% Atmosphere; ranges from 0 to 20% Soil Air CO2: 0.04% Atmosphere; ranges from 0.35 to 5% Soil Air

Classes of Soil Redox States:

Oxic, Eh > 0.4 volts. Suboxic, 0.1 < Eh < 0.4 volts. Anoxic, Eh < 0.1 volts.

Carbon pools

reservoirs of carbon in soil; present as SOM or biomass

Processes that create air pressure differences between the atmosphere

-Temperature differences between air and soil 0.5% -Barometric pressure changes in the atmosphere 1.0% -Wind 0.1% -Rainfall/Drainage 7 to 9%

How are CO2 and O2 levels dynamic? Are they associated?

-vary through the seasons and dependent on soil texture -yes, larger CO2 levels correspond with smaller O2 levels - because metabolic activity in soil produces CO2 and consumes O2

Organic matter within the surface horizons of an arable soil has C:N ratios ranging from

-8 to 15, averaging about 12 -Humus has small too

How does Mechanical soil disturbance reduces SOC content?

-A macro-aggregate contains physically protected POM encrusted by fine soil particles and small microaggregates -Mechanical disturbance disrupts the macro-aggregate and exposes the protected POM fragments to microbial attack. -This results in a decrease of SOC -The soil also looses aggregation, becoming a dispersed collection of micro-aggregates and primary soil particles.

Explain Soil Oxidation - Reduction (Redox) Potential

-As the reaction proceeds to the right, Fe2+ becomes oxidized and looses an electron to yield Fe3+ . -If the reaction proceeds to the left, Fe3+ is reduced by receiving an electron to form Fe2+ -The Redox Potential (Eh) is a measure of the aeration status of a soil and dictates whether redox reactions are proceeding to the right or to the left

What happens to obligate aerobes such as plant roots, & fungi when oxygen is limited in soil?

-Cannot perform aerobic respiration. -These organisms switch to fermentation which is a much less efficient energy production pathway. -This reduced energy production inhibits water & nutrient uptake by roots and generates ethanol & lactic acid. -Fungi that are compromised under anaerobic conditions are no longer able to perform their role in decomposing complex organic polymers resulting in the accumulation of POM as found in peat bogs

How does Frequent tillage or other soil disturbance affect SOC?

-Depletes organic carbon near the soil surface.

Gas Diffusion in Soil(driven by and requires)

-Driven by differences in individual O2 or CO2 concentration between the soil and the atmosphere. --A continuous process provided a concentration difference exists. -Requires an adequate air-filled pore space

The general dynamics of carbon substrates addition to the soil

-Following a lag period, much of the organic carbon of the original tissue is labile and rapidly becomes microbial biomass and CO2 -Quantity of microbial biomass increases during the period of active decomposition -With time, the labile carbon becomes depleted, microbial populations decline and their remains become new humus -Existing humus is decomposed due to the presence of fresh substrate - the priming effect.

Aeration

-Gas exchange between the soil and the atmosphere. -Responsible for maintaining adequate O2 levels for root and microbe respiration -well aerated soil displays rapid gas exchange with the atmosphere

How is SOC affected by landscape?

-Greater SOC accumulation in poorly drained soils due to low oxygen levels that reduced decomposition rates -Upland locations generally have well drained soils & bottomland locations generally have poorly drained soils.

structural soil organic carbon pool

-Litter components that are more resistant to decomposition such as particulate leaf fragments containing cellulose and lignin polymers

Why does the process of moving carbon into the soil aid in decomposition.

-Litter layer is frequently dry and occasionally hot or cold and not the most favorable environment for the microflora. -Within the soil it is more commonly moist with modulated temperatures that favor microflora activity. -Delivering litter into the soil commonly speeds decomposition by the microflora, the principal decomposers

C:N < 25

-Microbial activity "blooms" because of the readily available N in the residues -Nitrate levels in soil increase without a lag period

C:N > 25

-Microbial activity increases in response to C substrate addition -Nitrate within soil supplements the microbial demand till C:N ratio of SOM from substrate declines

metabolic soil organic carbon pool,

-More soluble leaf components that are leached from the litter, experiences a sharp increase with leaf drop but subsequently declines rapidly due to the labile and short half-life nature of these organic substrates -Rapidly decomposed and their carbon flows to the active soil organic carbon pool or is taken up by the heterotrophs

What happens to POM fragments with more intimate contact with the soil?

-POM fragments become smaller & increasingly accessible to enzymatic attack from soil microflora. -Within the soil, organisms consume each other either as active predators or passively as larger organisms scour the earth for food sources. -Then, as carbon substrates are diminished, or soil conditions become unfavorable, many soil organisms simply die. Giving their bodies back to the SOM pool

Explain the Soil Food Web related to carbon

-Primary consumers gain their C from all forms of organic matter -Primary consumers are consumed=some of the carbon incorporated into higher level consumer and some becoming lifeless remains -C in lifeless remains returns to the organic matter pool in an altered form to re-enter the food web *basic splitting of carbon occurs at all trophic levels within the food web Respiration -Respiration of food web members release a significant quantity of carbon in the form of CO2 (or CH4 in wet soils) that returns to the atmosphere; most (60 to 80%) of C in litter returns to the atmosphere via respiration -Small portion becomes humus, resistant to decomposition such that it can accumulate

How is SOC affected by silt & clay percentage?

-SOC content increases with silt & clay percentage -Coarse textured soil (low silt & clay) limit plant productivity reducing carbon substrate input. -Finer textured soil (high silt & clay) limit oxygen needed for active decomposition, are better able to physically sequester carbon & form clay-humus complexes that protect SOC from degradation

Active Pool

-Size associated with biological activity in the soil -Longer half-life of 0.5 to 15 years. -Carbon can be enzymatically converted directly into passive carbon or the active pool organic substrates; can also be taken up by the heterotrophs through the same fashion as with the structural & metabolic pools. -also contribute to the soil physical status (via aggregation)

Compare the half lives of structural and metabolic carbon

-The metabolic carbon has a very short half-life of 0.1 to 0.5 years, because these organic substrates are quite easily decomposed or highly labile -Structural carbon has a longer half-life of 0.4 to 4 years, being comparably more difficult to decompose

Passive Pool

-Very stable, or recalcitrant, with very low turnover rates. Generally thought of as humus -Size influences the chemical reactivity of the soil (via nutrient retention) -Very long half-life of 500 to 5000 years. -when it does turn over, taken up by the heterotrophs(commonly occurs when a large store of labile organic substrates is added to the soil) -also contribute to the soil physical status (via aggregation)

What ways do organisms living in the soil decompose litter that leads to humus accumulation? *releases Dissolved Organic Matter (DOM), which is leached into the soil, and Particulate Organic Matter (POM) as litter fragments that settle into the soil.

-Windblown microflora release enzymes to decompose carbon substrate -Shredder meso- and macrofauna chew through litter *This fragmentation of the litter exposes fresh surfaces to the microflora for colonization and Fauna passively consume the microflora -Litter directly pulled into the soil by anecic earthworms(food for microflora)

Explain the Carbon Cycle at the Plant and Soil Scale

-carbon substrates (plant litter, dead root etc.)when in contact with soil, are consumed by soil organisms that respire CO2 (or in wet soils methane, CH4) -uneaten fragments or remains of dead soil organism become SOM -So the carbon in soil cycles between the living & the dead -during this process components of the SOM pool undergo transformations from one form to another leading to humus

SOC Levels of -Forest & undisturbed grassland soils -desert soil

-large SOC levels at the surface, but the decline with depth is more gradual in grassland soils. -Limited plant productivity as in a desert soil clearly leads to lower SOC levels

Other oxidative reactions oxygen in soil also participates in

-oxidize inorganic nitrogen (when as ammonium, NH4 + ) & sulfur (when as sulfide, S2-), converting these nutrient elements into forms that are readily taken up by plants -Used to oxidize iron and manganese which is a process in the weathering of rock minerals and soil formation

Explain the oxygen cycle in terms of soil

-oxygen gas (O2 ) in the atmosphere enters the soil -it oxidizes organic carbon substrates (CH2O) during aerobic respiration, forming CO2 -CO2 returns to the atmosphere to participate in carbon fixation by vegetation that releases oxygen gas back to the atmosphere -The carbon compounds return to the soil where they (eventually & mostly) become oxidized

Surface deposited substrates decompose_________ than incorporated substrates. Why?

-slower -more extreme drying and wider temperature variations that occur at the surface do not favor the organic matter degrading organisms -Surface dwelling insects that shred coarse litter and earthworms that pull litter into the soil must first work on surface deposited substrates -Thick litter layers in some forest soils result from the composition of these substrates and/or soil conditions that do not favor shredding insects or earthworms(Indigestible needle litter, acidic pH soils, sandy soils, and periodic flooding)

CO2 and O2 levels in the soil are controlled by what 2 factors?

1) The rates of O2 consumption and CO2 production. 2) The rate of gas exchange with the atmosphere. *Correspondingly, O2 consumption, CO2 production and gas exchange are influenced by soil wetness & temperature.

4 Factors Influencing the Flow of Carbon in Soil (C Substrate decomposition depends on...)

1. Its biochemical form. 2.Its placement - either on the surface or within the soil. 3. Availability of nitrogen. 4. A suitable environment for food web members.

Ranking of biochemical forms of carbon substrates( fastest to slowest:)

1.Sugars, starches & simple proteins 2.Hemicellulose 3.Cellulose 4.Fats, waxes, & etc. 5.Lignins & phenolic compounds -the more easily decomposed biochemicals are consumed first leaving the more recalcitrant materials behind

Decomposition begins to slow when soil oxygen concentrations drop below____. Low oxygen levels are attributed to what?

10%; poor gas exchange from inadequate air filled porosity - wet soils *applies to aerobic microbes -similar to that seen with plant root respiration & growth

More CO2 molecules in soil pore than atmosphere results what?

A net migration of CO2 out of soil. *For O2 it is the opposite

Black layer

A soil aeration problem seen in turfgrass soils where inadequate air-filled pore space leads to poor gas exchange and a depletion of oxygen in the soil. These low O2 levels favor the growth of anaerobic bacteria that produce hydrogen sulfide creating the black layer

wet wilt

A stress where the soil within the rooting zone contains inadequate air-filled pore space, then O2 supply to the root is limited and the plant cannot take up water to meet the atmospheric demand

What is Respiration that requires oxygen and the greatest sink for oxygen (O2) in soil

Aerobic Respiration

Structural and metabolic carbon flow into what with increased decay

Biomass, Active & Passive pools

Simple Aerobic Respiration equation

Carbon Substrates + O2 CO2 + H2O + Energy -This reaction, where O2 is the terminal electron acceptor, mostly uses sugars in roots but for microbes a wider variety of carbon substrates can be utilized

Humus

Carbon-based product of multiple decomposition steps that is darkly colored, polycyclic, resistant to further decay, and is often associated with a fertile soil -60 - 80% of total SOM

Why is there more C is leaving the soil (62 Pg/yr) than is entering (60 Pg/yr)?

Cultivation

O2 Consumption Rates Rapidly_______Soil Oxygen.Thus, the importance of ________ processes

Deplete; gas exchange

What does cultivation with intensive soil disturbance do to SOC pools over time?

Depletes -freshly deposited plant residue & the active C pools are primarily affected. This reduces soil biological activity & has physical status implications due to the loss of aggregation

Metabolic Carbon

Essentially components of the cytoplasm

Structural carbon

Has cell walls and those more complex cellulose and lignin type compounds

What remains as carbon cycles from living to dead

Humus

Warmer temperatures and vegetation______CO2 production. O2 Consumption?

Increases; Same pattern, but much less than co2 production

If C substrates having C:N ratios greater than about 25 are incorporated into soil, what happens to the mineral nitrogen (mostly as nitrate) that would otherwise be available to the plant?

It is consumed by microbes and is "immobilized" for a period of time

Decomposition is optimum in wet or dry soils?

Moist soils (between field capacity & wilting point) and slows as the soil dries -Fungi and actinobacteria are more tolerant to drying than bacteria *microbial activity can continue even after a plant wilts *as soil dries, some shredder organisms migrate to deeper soil depths

Aerobic Respiration for soil food web members -carbon substrates in aerobic respiration mostly comes from decomposition of organic polymers with most of this decomposition mediated by the heterotrophic microflora

Organic Polymers with N, P & S bonded to C -----extracellular enzymes--> Organic Monomers with N, P & S bonded to C -Then these monomers (<600 Daltons) are taken up by the microflora: Leading to growth and respiration *Or if there are not enough nutrient ions from monomers, the microflora with scavenge the soil for more

What is another way a stable form of carbon in soil occurs?

Physical Sequestration of carbon -Microbial fragments become encrusted with clay, protecting otherwise labile material from enzymatic attack -Pores of the encrustations are too tiny to provide access by the microflora and their enzymes so carbon can be chemically or physically sequestered in soil and this carbon does not return to the atmosphere, but instead accumulates.

potent greenhouse gasses (e.g. CH4) and toxic gases to plants (e.g. C2H4) are formed under what conditions?

Reducing conditions found in waterlogged soils

Within the soil what is the largest pool and the largest component of it?

SOM; Humus

Soil O2 concentration Diminishes with

Soil Depth

Human intervention in the carbon cycle through cultivation, forestry practices and other land uses is depleting what?

Soil Organic Matter

Aerobic Respiration equation for plant roots, where the sugars come from photosynthesis in the leaves

Sugars from Leaves + Mineral Nutrients from Soil uptake by root cells ---->Growth ---->Respiration

Plant substrates, and principally structural carbon, often have a higher C:N ratio than that of the microbes, why is this bad?

The higher the C:N ratio of the substrate, the greater the need for an external source of N to degrade it; and consequently the slower it will degrade -Ex. Sawdust has a much higher ratio than green plants

Soil Properties that influence Gas Diffusion

The magnitude of air-filled porosity: A minimum of 10% is required for adequate gas diffusion and soil aeration. Total porosity: Important because gas cannot diffuse through solids. Soil Wetness: Gas diffusivity is 10,000 times greater in air than in water. *principally the air-filled porosity value that controls gas diffusion; but the airfilled porosity value is also influenced by the by the total porosity and the wetness

How are the oxygen and carbon cycles linked?

Through the process of Aerobic Respiration of plants, animals and aerobic microflora, as organic carbon substrates are oxidized and CO2 is produced.

Soil microbes contain between 5 to 10 times more ______ than ________. Thus, the________of microbes range from 5 to 10.

carbon; nitrogen; C:N ratio

Within the soil, the formation of humus is referred to as

chemical sequestration of carbon (ie. it's stable and protected)

Regarding temperature, Decomposition is slow in ______soils and optimum in very ________soils.

cold; warm -Decomposition rates increase 2-fold for each 10° C increase in temperature between 0 and 40° C. -microbial population in total is more tolerant to temperature extremes as compared to plants

As dissolved oxygen is consumed by soil respiration, the Eh value

declines -The soil is said to go from an oxidizing to reducing condition. -Substances such as iron (Fe) in soil minerals are reduced by becoming acceptors of electrons from soil metabolism

freshly deposited litter is considered to be what type of carbon?

either structural or metabolic

Decomposition is inhibited by

extremes of soil pH (below 4.5 and above 9 -there are some "extremeophile" organisms in the soil that can maintain their activity above & below this range

As mean annual temperature decreases, Soil Organic Carbon________. As mean annual precipitation increases, Soil Organic Carbon ________.

increases, increases

even with a small C:N ratio, humus remains recalcitrant because of what?

its complex, poly-cyclic structural form

How much C is in Soil?

more (2400 Pg) than in vegetation (550 Pg) & atmosphere (750 Pg) combined

Why is atmospheric C stock increasing with time on the global scale?

more flow is entering the atmosphere than leaving the atmosphere

What happens if a soil contains inadequate air-filled pores space for an extended period of time?

oxygen cannot diffuse into the soil and CO2 cannot diffuse out -Not a problem in winter, but if soils are biologically active, the resulting O2 depletion and CO2 accumulation in the soil can negatively impact plants -Min 10% air-filled pores needed for adequate gas diffusion & aeration

Aside from soil temperature, substrate decomposition generally operates optimally at soil conditions optimal for

plant growth *Due to the wide diversity of organisms in the soil, however, decomposition can continue under extreme conditions

O2 concentrations are _______ moving from the exterior to the interior of an aggregate(localized pattern)

reduced

Legume substrates have a _______C:N ratio and degrade ______

small; quickly

In most soils, SOC is greatest at the_________& amounts decline with

soil surface; depth

½-life in carbon soil cycle

time for ½ of the carbon substrate in a pool to cycle into another pool or become CO2.

How do facultative or obligate anaerobe bacteria perform anaerobic respiration?

using electron acceptors such as nitrate (NO3 - ), ferric iron (Fe3+), sulfate (SO4 2-) & carbon dioxide (CO2)

Magnitude of Organic Carbon in Soil Direct Effects:

• Increases nutrient, and in some cases available water retention. • Stabilizes soil aggregates leading to improved soil structure and increased macroporosity. • Increases the diversity and activity of soil organisms.

Magnitude of Organic Carbon in Soil Subsequent Environmental Effects:

• Lessens need for frequent management inputs such as fertilizer helping to mitigate waste & pollution. • Reduces storm water runoff and increases groundwater recharge. • Increases carbon permanence which plays a role in global climate change. • Sustainably increases biomass production.

Excess soil wetness can inhibit oxygen entry into soils, resulting in oxygen depletion and anaerobic soil conditions, which can cause....

• direct plant injury, • reduced organic matter decomposition, • an interruption of nutrient cycling, • release of the greenhouse gasses methane & nitrous oxide

SOC spatial variation is due to differences in

•vegetative biomes •climate •soil texture •landscape position •land use of the given location.