NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature NRES 201 Soil Aeration Temperature 1 Soil Aeration Temperature Today s lecture topics Why is this topic important Soil aeration Basic concepts Composition of soil air Oxidation reduction redox potential What happens when a soil is waterlogged Factors affecting soil aeration and Eh 2 1 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Why is this topic important Aeration ventilates the soil Supplies O2 for respiration C6H12O6 6O2 6CO2 6H2O By plant roots And by microorganisms Removes CO2 and other toxic gases Soil temperature affects Plant and microbial growth Soil drying by evaporation Soil aeration 3 Soil aeration Basic concepts Relationship to pore space 5 Macropores 45 Micropores Source Troeh and Thompson 2005 4 2 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Gas exchange processes Mass flow Movement with a medium Promoted by Soil moisture changes Wind Changes in barometric Source http waterencyclopedia com pressure Of minor importance for soil aeration mainly impacting Surface horizons Shallow soils Soils with extensive macroporosity 5 Diffusion Movement of an individual gas in response to its own partial pressure gradient Net effect for soil Gain of O2 Loss of CO2 The major process by which soil aeration occurs Source Brady and Weil 2008 6 3 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Oxygen supply Can limit subsoil rooting especially in wet soils High soil moisture reduces Air filled porosity O2 diffusion Cropped to cotton 10 000 slower through water than air Plant growth severely limited at 20 air filled porosity When there is 10 O2 in soil air Source Brady and Weil 2008 7 Composition of soil air Generalizations Compared to ambient air soil air has About the same level of N2 Less O2 More CO2 O2 and CO2 changes are greatest for wet soils wet soil Source Buckman and Brady 1969 8 4 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Other gases Water vapor Soil air is at 100 relative humidity Except at or very near the soil surface Waterlogging produces Methane CH4 Hydrogen sulfide H2S Ethylene C2H4 1 L L is toxic to plant roots 9 Oxidation reduction redox potential Definitions Original meaning for reactions involving O2 Oxidation addition of oxygen example C O2 CO2 Reduction removal of oxygen example 2H2O electrical energy H2 O2 Current meaning Oxidation loss of electrons Reduction gain of electrons 10 5 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Oxidizing and reducing properties Different substances vary widely in their affinity for electrons Oxidizing agents take electrons Reducing agents give electrons The electron affinity Source http www quora com of different substances can be compared in a relative sense Using an electrochemical cell 11 Source http hyperphysics phy astr gsu edu Anode oxidation Zn Zn2 2eEh 0 76 V Cathode reduction Cu2 2e Cu Eh 0 15 V Redox reduction potential versus a standard H electrode H2 H e 12 6 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Importance to soil Mineral solubility Microbial activities Composition of the soil atmosphere Aerobic conditions Plenty of O2 High Eh Organic matter decomposes Anaerobic conditions Absence of O2 Low Eh Organic matter accumulates 13 What happens when a soil is waterlogged Initial changes Dissolved O2 DO decreases Due to respiration by aerobic microbes that use organic C as an energy source Organic C supply determines the rate of decrease DO not replaced by slow O2 diffusion in water Source Brady and Weil 2008 14 7 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Eh decreases From 0 4 0 7 V for a well aerated soil To about 0 3 V within the first couple days after saturation And as low as 0 3 V After the first week of saturation Or more rapidly with a high organic C content Source Brady and Weil 2008 15 Anaerobic microbes utilize terminal electron acceptors other than O2 Such as Fe in soil minerals Fe OH 3 e 3H Fe2 3H2O Other reduction reactions can involve N Mn S or C The sequence of these reactions depends on Eh pH Source Brady and Weil 2008 16 8 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature pH increases Due to Fe reduction Fe OH 3 e 3H Fe2 3H2O Known as the self liming effect Source Brady and Weil 2008 17 Factors affecting soil aeration and Eh Soil drainage Mainly determined by macropore volume which depends on Texture Bulk density Aggregate stability Organic matter content Biopore formation Source http agsci psu edu 18 9 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Respiration rates Microbial activities largely determine soil O2 and CO2 levels Roots and soil animals also have an effect These activities Increase with soil New York Spodosol in a hardwood forest temperature And require organic C as an energy source So there is a stimulating effect of organic C inputs such as Plant residues Source Brady and Weil 2008 Or manure 19 Profile depth Compared to the surface soil subsoils are usually Lower in O2 Higher in CO2 These differences are increased by Higher soil moisture Higher temperature Presence of organic substrates Brazilian Oxisol in a rain forest Source Brady and Weil 2008 20 10 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Tillage The short term effect is to increase aeration as the surface soil is loosened and dried The long term effect is lower macroporosity Loss of organic matter reduces aggregate stability Moldboard plowing forms a plow pan Source Brady and Weil 2008 Aggregation Micropores impede aeration in clayey or compacted soils Because of slow drainage And limited O2 diffusion Aeration is poorer Within a microporous aggregate Than at the aggregate surface 21 O2 concentrations for a wet aggregate from an Iowa Mollisol Muscatine Source Brady and Weil 2008 22 11 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Vegetation Upland plants decrease soil aeration Because of root respiration Wetland plants can increase soil aeration Because aerenchyma tissues transport O2 Source Brady and Weil 2008 23 Vegetation Upland plants decrease soil aeration Because of root respiration Wetland plants can increase soil aeration Because aerenchyma tissues transport O2 Transpiration can improve soil aeration By lowering the water table Source Brady and Weil 2008 24 12 NRES 201 Lectures 22 24 Fall 2014 Soil Aeration Temperature Season of the year A major factor for many temperate soils Limited aeration in early