Soil Air and Temperature

Chapter 7

I Soil Aeration

O2 for aerobic respiration

C6H12O6 + 6O2 6CO2 + 6H2O

CO2 must go

The above reaction can be split into a oxidation ½ reaction anda reduction ½ reaction. This concept is important to understandingsoil aeration as shown a few slides later.

Gas exchange mostly by diffusion

O2 CO2 H2O

O2 CO2 H2O

Above ground

Below ground

Thus, soil air is typically lowerin oxygen but higher in carbondioxide and water vapor thanthe above ground atmosphere.This is due to soil respirationand slow gas exchange.

Some mass flow due to changes in pressure, too.

What’s the effect of water on aeration?

It occludes soil pores, slowing the already slow process of diffusion (and massflow) through soil pores. Diffusion of gas in water is orders of magnitude slowerthan in air.

Aeration indirectly measured by

Redox potential

Tendency of substance to gain or lose e-s

This is measured using a platinum electrode in conjunction with a referenceelectrode. The platinum electrode develops a potential that depends on thechemical environment in the soil, either tending to be oxidizing or reducing, i.e.,relatively high or low potential. Biological activity in the soil largely controlsthis chemical environment.

Eh = Eo + (RT/nF)ln([oxidized]/[reduced])

V or mV

Redox potential

Fe2+ Fe3+ + e-

This is the Nernst equation. It can be derived from free energy relations and says that the potential depends on the standard potential (reactants andproducts in their standard states) and the relative concentrations of oxidizedand reduced forms of a redox couple, like iron (below).

Clearly, if [oxidize] > [reduced] the Eh > Eo

and conversely. So, if larger values for Eh(which is measured by the Pt electrode)indicate an oxidizing environment and lowervalues, a reducing environment. Oxidizingcan be equated with well-aerated and the opposite.

C6H12O6 + 6H2O 6CO2 + 24H+ + 24e

6O2 + 24H+ + 24e 12H2O______________________________C6H12O6 + 6O2 6CO2 + 6H2O

Half reactionsAs far as microbial respiration goes, there arebugs that can couple the oxidation ½ reactionwith a different reduction ½ reaction, one notinvolving oxygen as the terminal e- acceptorand get along just fine. This is their natural wayof respirating.

Supply of O2 interrupted by heavy rain

Respiration in soil depletes O2

Eh = Eo + (RT/nF)ln([oxidized]/[reduced])

2H2O O2 + 4H+ + 4e

This the ½ reaction for oxidation ofoxygen in water (-2 state goes to 0state). More O2, higher Eh (see Nernst equation, below). Make sense?

If O2 supply poor, aerobic respiration slows

and

Anaerobic begins

Certain microorganisms use other elementsas terminal e acceptors

C6H12O6 + 6H2O 6CO2 + 24H+ + 24e

12Fe2O3 + 72H+ + 24e 24Fe2+ + 36H2O____________________________________________

C6H12O6 + 12Fe2O3 + 48H+ 6CO2 + 24Fe2+ + 30H2O

This is Fe3+ reduction, the Fe3+ being in the solid form.

High Eh Low Eh

Oxidized Reduced

NO3- N2

MnO2 Mn2+

Fe2O3 Fe2+

SO42- S2-

CO2 CH4

When oxygen is depletedby aerobic respiration, theaerobes shut down. But thereare microbes that can useoxidized N in NO3

- (nitrate) asThe terminal e- acceptor in their respiration, reducing it toN2 (or another reduced form).However, the supply of nitrate isfinite, these guys use it up andthen shut down. During theiractivity the chemical environmentbecomes more anaerobic andthe Eh decreases. Similarly,there are bugs that can useMn4+ and Fe3+ in their respiration.They become active once theEh becomes sufficiently low forthese ½ reactions to occur.

The C reducers are active whenthe Eh is really low.

If Aeration

High water content Good or bad?Mostly microporesDeep in soilHigh temperature These conditions all favor development of anaerobic conditions. Clearly,high water content does because it impedes gas exchange with the aboveground atmosphere. Small pores slow drainage, keeping a soil wet once itbecomes so. Obviously, aeration is poorer deeper in the soil than near thesurface. The effect of temperature is to increase biological activity. Thus,if the soil is wet, what oxygen that is there is more rapidly depleted when thesoil is warm than cool, bringing on anaerobic conditions faster and quickerprogression through the series of terminal e- acceptors, thus, more intenselyreducing conditions.

Poor aeration

Anaerobic metabolism

Less energyToxic end products

Slow nutrient and water uptake

Affects nutrient availability

C2H5OH

With poor aeration, things goanaerobic and this kind of metabolism in the soil isadverse to plants for severalreasons. Here are some.

Nutrient availability

NO3- N2

Fe3+ Fe2+

As for the matter of nutrient availability, consumption of nitrate isundesirable because it is a major source of N for plant uptake. Onthe other hand, reduction of iron or manganese, both essential micronutrients, may in some cases be a bad thing even though neitherare depleted. This is because both elements in their reduced formsare more soluble, potentially excessively soluble and available for plantuptake, causing toxicity. Too much of a good thing, you know.

Management of aeration

Land drainageTolerant plants

Let wetlands be

You can drain land (provided it’s legal) oryou can grow a variety that is perhapsmore tolerant of wet soil conditions.

We have recognized for quite some timenow that wetlands are not wastelands butareas that are critical to the functioningof the overall environment.

Wetlands

Soils wet sufficiently long when the soiltemperature is warm so that anaerobicconditions occur

Delineated on the basis of

Wetland hydrologyHydrophytic plantsHydric soils

A B

Which is wetter?

Indicators of hydric soils

Organic matter accumulation

Iron oxidation-reductionGleyMottledIron depletion

More organic matter where wet due toslower decomposition where less aerated.

Recall that the gley condition indicates presence of chemically reduced iron, Fe2+.

Mottling that indicates concentrations of oxidized iron, Fe3+, in a matrix that is other-wise depleted in iron suggests alternating anaerobic and aerobic conditions. When thesoil is wet, the more soluble Fe2+ is producedand partially washed out of the profile andwhen dry, it oxidizes back to Fe3+, pre-cipitating in concentrated zones.

This condition supposes ironwas initially present but re-duced, solubilized and washedout.

Chroma 1 or 2, i.e., gray, ormaybe bluish.

II Soil Temperature

Affects rates of biological, chemical andphysical processes

Plants and microbes

Factors affecting temperature

Solar energySpecific heatWater evaporationThermal conductivity

Radiation absorbed depends on

Soil color

Vegetative cover

Slope and aspect

Slope and aspect

Affect solar energy per unit area

Q = C T

Amount heat needed to increase soiltemperature

cal g-1 oC-1

Ctotal = Csoil solids + Cwater

Specific heat

Neglect air because its specificheat is very small. Since waterhas a large specific heat (~ 1 cal g-1oC-1),increasing water content substantiallyincreases specific heat.

Clearly, a soil when wet warms more slowly than when dry.

Heat of vaporization of water

Hvap = 540 cal/g

Cooling effect

It takes heat to evaporate water andthis heat comes from the mediumwhere water evaporates, tending to cool it.

Thermal conductivity KT

Heat from hot cold

Does water increase KT ?

This is a proportionality factoranalogous to hydraulic conduc-tivity,

Q = KT (THot – TCold) / L

Where q is heat flux, T is temperature and L is distance.

The greater the contact of soilsolids, the greater the KT. Sincewater increases thermal contactincreasing water content increasesthermal conductivity. Air is a poorconductor.

Subsurface high lags behind surface high

More daily fluctuation at surface

This phenomenon is due to non-instantaneous heat flow, i.e., it takestime for heat to be conducted from relatively hot to cold.

The same thingis seen on an annualbasis –less variationdeeper in the soil andout-of-phase fluctu-ations.

Soil Temperature Management

Control soil water content

Use surface cover

Air

Upper soil no mulch

Upper soil with mulch

So, a mulch keeps the soil cooler insummer and warmer in winter.

You can adjust water content or usea mulch.