Soil Aeration

1. Soil Aeration

2. Ventilation of soil allowing gases to be exchanged with atmosphere • Gas is exchanged by: • Mass flow: air forced in by wind or pressure • Diffusion: gas moves back and forth from soil to atmosphere acc. to pressure

3. Redox potential • Tendency of a substance to accept or donate electrons • Oxidation-reduction potential • a way to characterize aeration • Eh

4. Oxidation • Loss of electrons • Fe+2 Fe+3 e- -26 -25 +28 +28 Fe+2 Fe+3

5. Reduction • Gain of electrons • Fe+3 Fe+2 e- -26 -25 +28 +28 Fe+2 Fe+3

6. Iron Fe+2(ferrous) Fe+3 (ferric) Nitrogen N+3 in NH+4(ammonium) N+5 in NO3- (nitrate) Manganese Mn+2 (manganous) Mn+4 (manganic) Oxidized/Reduced forms of…

7. Sulfur S-2 (sulfide) SO4-2(sulfate) Carbon CH4(methane) CO2 R O R O

9. Oxidation reaction electrons that could potentially be transferred to others 2FeO + 2H2O 2FeOOH + 2H+ + 2 e- Fe+2Fe+3 H+ ions formed

10. Redox potential • Tendency of a substance to accept or donate electrons • Measured in volts or millivolts • Depends on pH and presence of electron acceptors (oxidizing agents) • Used to quantify the degree of reduction in a wetland soil

11. Oxidizing agent • Substance accepts electrons easily • Oxygen is very strong electron acceptor

12. Reducing agent • Substance donates electrons easily

13. Aerobic Respiration • Oxygen is electron acceptor for organic carbon, to release energy. • As oxygen oxidizes carbon, oxygen in turn is reduced (H2O) O2 + C6H12O6 CO2 + H2O Electron acceptor Electron donor

14. To determine Eh (See graph) • Insert electrode in soil solution: • free dissolved oxygen present : Eh stays same • oxygen disappears, reduction (electron gain) takes place and probe measures degree of reduction ( mv) • As organic substances are oxidized (in respiration) Eh drops as sequence of reductions (electron gains) takes place:

16. Graph (handout) shows: • sequence of reductions that take place when well aerated soil becomes saturated with water • Once oxygen is gone, the only active microorganisms are those that can use substances other than oxygen as electron acceptors (anaerobic) • Eh drops • Shows Eh levels at which these reactions take place • Poorly aerated soil contain partially oxidized products: • Ethylene gas, methane, alcohols, organic acids

17. organic substrate oxidized (decomposed) by various electron acceptors: • O2 • NO3- • Mn+4 • Fe+3 • SO4-2 • rates of decomposition are most rapid in presence of oxygen

18. Aeration affects microbial breakdown: • Poor aeration slows decay • Anaerobic organisms • Poorly aerated soils may contain toxic, not oxidized products of decomposition: alcohols, organic acids • Organic matter accumulates • Allows Histosol development

19. Some conclusions about aeration: • Forms/mobility • Roots • Decomposition

20. Some conclusions about aeration: 1. Forms and Mobility Soil aeration determines which forms of chemicals are present and how mobile they are • Redox colors in Poorly and Well-Aerated Soil • Nutrient elements

21. 1. Forms and Mobility: A) Poorly aerated soils • reduced forms of iron and manganese Fe+2, Mn+2 • Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors (redox depletions) • Reduced manganese : hard black concretions

23. 1. Forms and Mobility B) Well-aerated soils: • Oxidized forms of iron and manganese Fe+3 Mn+4 • Fe precipitates as Fe+3 in aerobic zones or during dry periods • Reddish brown to orange (redox concentrations)

24. Plate 26  Redox concentrations (red) and depletions (gray) in a Btg horizon from an Aquic Paleudalf.

30. Plate 16  A soil catena or toposequence in central Zimbabwe. Redder colors indicate better internal drainage. Inset: B-horizon clods from each soil in the catena.

31. Plate 21  Effect of poor drainage on soil color. Gray colors and red redox concentrations in the B horizons of a Plinthaquic Paleudalf.

32. Manganese concretions

33. 1. Forms and Mobility C. Nutrient Elements • Plants can use oxidized forms of nitrogen and sulfur • Reduced iron, manganese • Soluble/”good” in alkaline soils • More soluble in acid soils; can reach toxic levels

34. Some conclusions about aeration: 2. Root respiration • Good aeration promotes root respiration • Poor aeration: water-filled pores block oxygen diffusion into soil to replace what is used up in respiration

35. Some conclusions about aeration: 3. Decomposition In aerated soils, aerobic organisms rapidly oxidize organic material and decomposition is rapid In poor aeration, anaerobic decomposers take over and decomposition is slower

36. Hydric Soils

37. Wetland criteria • Hydrology • Hydric soils • Hydrophytic plants

38. Hydric soil • soil that is saturated, flooded, or ponded long enough during the growing seasonto develop anaerobic conditions in the upper part. • Oxygen is removed from groundwater by respiration of microbes, roots, soil fauna • Biological zero = 5°C

39. Why is “during growing season” important part of definition? • If wet period is during COLD time of year (too cold for microbial growth and plant root respiration), might not have anaerobic conditions. • It is anaerobic conditions that cause a soil to be hydric, not just saturation!!!

40. Hydric soils support growth and regeneration of hydrophytic plants.

41. Hydric Soils and Taxonomy • Histosols • (all Histosols except Folists) • (all Histels except Folistels) • Aquic suborders and subgroups • Definition of aquic soil moisture regime: “reducing regime in soil virtually free of dissolved oxygen because it is saturated. Some soils are saturated at times while dissolved oxygen is present, either because the water is moving or the environment is unfavorable for microorganisms; such a regime is NOT considered aquic”. Organic soils made up mostly of forest litter’ not saturated

42. Aquic Conditions: • Periodic or continuous saturation • Redoximorphic features • Verify by measuring saturation or reduction

44. Exception to Aquic conditions: • Artificial drainage • Removal of free water from soils with aquic conditions • Artificially drained soils are included with aquic soils • Because soil Taxonomy is based on soil GENESIS and minimizes human disturbance • Pertains to Hydric soils also

45. Artificially wet soils are considered hydric • Artificially “dry” (drained) soils are considered hydric

46. Types of saturation • endosaturation: all soil layers sat’d to 2 m depth • Episaturation: sat’d layers in upper 2 m (perched) • Anthric saturation: controlled flooding (rice, cranberries)

48. Hydric soil indicators: • Color • Chroma 1or 2 or gley (Fe++2 grey or green) • May have redox concentrations or concretions • Sulfidic materials (odor of rotten eggs) • Sulfate reduction

50. Plate 30  Dark (black) humic accumulation and gray humus depletion spots in the A horizon are indicators of a hydric soil. Water table is 30 cm below the soil surface.