Lecture 6: Soil Aeration and Temperature and Wetlands

Question 1

Soil aeration (ventilation) definition

Answer 1

the amount/ concentration of oxygen that is available in soil

Question 2

How does porosity affect soil aeration/ventilation?

Answer 2

Gasses move through macropores like water
Compaction can block gas exchange pathways

Question 3

How does water content affect soil aeration/ventilation?

Answer 3

Water and air compete for pore space in soils. The presence of water can block gas exchange pathways and reduces the amount of air that can be stored in the soil
Oxygen becomes limiting when 80-90% of the pore space is filled with water and in saturated/waterlogged soils

Question 4

How does plant and microbial activity of soils affect soil aeration/ventilation?

Answer 4

The more plants and micro bacteria growing on the soil and the more active they are, the more they are consuming oxygen and other gasses
Some plants are adapted to life in waterlogged soils

Question 5

How are gasses exchanged between soils and the atmosphere through mass flow?

Answer 5

When soil gas is pushed into the atmosphere by large changes in atmospheric pressure

eg. Arctic soils that are covered in an O layer and a layer of moss. the moss has a higher pore volume and the gasses coming out of the soil can be trapped in this layer so then there is an atmospheric pressure and the gasses get ventilated out

Question 6

How are gasses exchanged between soils and the atmosphere through diffusion?

Answer 6

Gasses move along pressure (concentration) gradients
Oxygen diffuses into soils and carbon dioxide will move out of soil and into the air

Question 7

Waterlogged soil

Answer 7

Much lower redox potential
thick layer of accumulation of organic matter
grey

Question 8

Hydrophyte

Answer 8

plants that are adapted to life in waterlogged soils

Question 9

Aerenchyma

Answer 9

hollow tubes that plants have that facilitate the movement of oxygen into the rooting zone through the plant. can vent soil gasses like CO2 into the atmosphere to reduce the toxicity of certain gasses in the soil

Question 10

Ferric/ferrous Fe

Answer 10

Ferric iron is 3+ meaning it lost an electron and was oxidized
Ferrous iron is 2+ meaning it gained an electron and was reduced

Question 11

Denitrification

Answer 11

-In many waterlogged soils, nitrate NO3- is reduced to nitrite NO2 and then further to N2 gas
-called leaky pipeline bc creates other gasses like NO or N2O
-major pathway to remove nitrogen from soils as gasses, which is bad because nitrogen is a major plant nutrient

Question 12

How does the redox potential of a soil control the availability of nutrients, the toxicity of elements, and emissions of greenhouse gases?

Answer 12

Describes how much oxygen is available
-Determined the presence and availability of nutrients and toxins
-flooding of soils/ adding organic matter can (im-)mobilize toxic elements

Question 13

Approximate redox potential (mV) of aerobic and anoxic soils (Figure 5 pg 179)

Answer 13

Oxygen is the main electron acceptor and in anoxic soils there is less available oxygen.
-Anoxic soils have a redox potential of +4 and above volts (sulfate and CO2 are most important electron acceptors)
-Only negative redox potentials describe incredibly anaerobic soils

Question 14

Most important oxidation state rules

Answer 14

-the oxidation state of an individual atom is 0
-the total oxidation state of all atoms in a neutral species is 0 and in an ion is equal to the ion charge
-hydrogen usually +1 in compounds
-oxygen usually -2 in compounds

Question 15

Fe3+ usually

Answer 15

precipitates and is a poorly soluble mineral
can co precipitate organic matter so that stabilize organic matter in soils

Question 16

When you reduce Fe(III) to Fe(II)

Answer 16

the carbon that was attached to the iron becomes available and get decomposed into a greenhouse gas

Question 17

When O2 is absent, microorganisms seek alternate e- acceptors. In order of decreasing energy gain, they are:

Answer 17

NO3-, Mn4+, Fe3+, SO4 2-, CO2

Question 18

Only oxic soils emit

Answer 18

N2O and CO2

Question 19

Only anoxic soils emit

Answer 19

methane

Question 20

Carbon- normal form in well-oxidized soils and reduced form found in waterlogged soils

Answer 20

CO2 and C6H12O6

CH4, C2H4, CH3CH2OH

Question 21

Nitrogen- normal form in well-oxidized soils and reduced form found in waterlogged soils

Answer 21

NO3-

N2 and NH4+

Question 22

Sulfur- normal form in well-oxidized soils and reduced form found in waterlogged soils

Answer 22

SO4 2-

H2S and S 2-

Question 23

Iron- normal form in well-oxidized soils and reduced form found in waterlogged soils

Answer 23

Fe 3+ [Fe(III) oxides]

Fe 2+ [Fe(II) oxides]

Question 24

Manganese- normal form in well-oxidized soils and reduced form found in waterlogged soils

Answer 24

Mn 4+ [Mn(IV) oxides]

Mn2+ [Mn(II) oxides]

Question 25

Ecosystem services of wetlands

Answer 25

-flood control
-cool and stabilize regional climate bc they have high rates of evapotranspiration
-stores a lot of carbon
-coastal wetlands protect from erosion and flooding and are important nurseries for fish

Question 26

Northern wetlands

Answer 26

Poor ventilation + low redox -> accumulation of organic matter
lots of carbon

Question 27

Wetland chemistry

Answer 27

-low oxygen
-low redox (low Eh numbers, low like 0.3 volts)