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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.