NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature11NRES 201Soil Aeration & Temperature2Today’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 EhSoil Aeration & TemperatureNRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature23Why is this topic important?Aeration ventilates the soil- Supplies O2for respirationC6H12O6+ 6O2 6CO2+ 6H2O- By plant roots- And by microorganisms- Removes CO2and other toxic gasesSoil temperature affects- Plant and microbial growth- Soil drying by evaporation- Soil aeration4Soil aeration:Basic conceptsRelationship to pore space(45%)(5%)(Macropores)(Micropores)Source: Troeh and Thompson (2005)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature35Gas exchange processes- Mass flow- Movement with a medium- Promoted by:• Soil moisture changes•Wind• Changes in barometricpressure- Of minor importance forsoil aeration, mainly impacting:• Surface horizons• Shallow soils• Soils with extensive macroporositySource: http://waterencyclopedia.com6- 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 majorprocess bywhich soilaerationoccursSource: Brady and Weil (2008)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature47Oxygen supply- Can limit subsoil rooting, especially in wet soils- High soil moisture reduces:- Air-filled porosity-O2diffusion• 10,000 slowerthrough waterthan air- Plant growth severelylimited at 20%air-filled porosity- When there is 10%O2in soil airSource: Brady and Weil (2008)Cropped to cotton8Generalizations- Compared to ambient air, soil air has:- About the same level of N2- Less O2-More CO2- O2and CO2changes are greatest for wet soilsComposition of soil airSource: Buckman and Brady (1969)wet soilNRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature59Other 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 roots10Definitions- Original meaning for reactions involving O2- Oxidation: addition of oxygenexample: C + O2 CO2- Reduction: removal of oxygenexample: 2H2O + electrical energy  H2+ O2- Current meaning- Oxidation: loss of electrons- Reduction: gain of electronsOxidation-reduction (redox) potentialNRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature611Oxidizing and reducing properties- Different substancesvary widely in theiraffinity for electrons- Oxidizing agentstake electrons- Reducing agentsgive electrons- The electron affinityof different substancescan be compared in arelative sense- Using an electrochemical cellSource: http://www.quora.com12Anode (oxidation):Zn  Zn2++ 2e-Eh= –0.76 VCathode (reduction):Cu2++ 2e- CuEh= 0.15 VSource: http://hyperphysics.phy-astr.gsu.eduRedox (reduction) potential(versus a standard H electrode: ½H2 H++ e-)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature713Importance 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 accumulates14What happens when a soil is waterlogged?Initial changes- Dissolved O2(DO) decreases- Due to respiration by aerobic microbes thatuse organic C as an energy source- Organic C supply determines the rate ofdecrease - DO not replaced byslow O2diffusion inwater Source: Brady and Weil (2008)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature815- Ehdecreases- From 0.4-0.7 V for a well-aerated soil- To about 0.3 V within the first couple daysafter saturation- And as low as –0.3 V• After the first weekof saturation• Or more rapidlywith a high organicC contentSource: Brady and Weil (2008)16- Anaerobic microbes utilize terminal electronacceptors other than O2- Such as Fe in soil mineralsFe(OH)3+ e-+ 3H+ Fe2++ 3H2O- Other reductionreactions can involveN, Mn, S, or C- The sequence of thesereactions depends on:• Eh•pHSource: Brady and Weil (2008)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature917- pH increases- Due to Fe reductionFe(OH)3+ e-+ 3H+ Fe2++ 3H2O- Known as the “self-liming” effectSource: Brady and Weil (2008)18Factors affecting soil aeration and EhSoil drainage- Mainly determined bymacropore volume,which depends on:- Texture- Bulk density- Aggregate stability- Organic matter content- Biopore formationSource: http://agsci.psu.eduNRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature1019Respiration rates- Microbial activities largely determine soil O2andCO2 levels- Roots and soil animals also have an effect- These activities:- Increase with soiltemperature- And require organic Cas an energy source- So there is a stimulatingeffect of organic C inputssuch as:• Plant residues• Or manureSource: Brady and Weil (2008)New York Spodosol in a hardwood forest20Profile depth- Compared to thesurface soil, subsoilsare usually:- Lower in O2- Higher in CO2- These differences areincreased by:- Higher soil moisture- Higher temperature- Presence of organicsubstratesSource: Brady and Weil (2008)Brazilian Oxisol in a rain forestNRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature1121Tillage- 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 aggregatestability- Moldboard plowing forms a plow panSource: Brady and Weil (2008)22Aggregation- Micropores impedeaeration in clayey or compacted soils- Because of slowdrainage- And limited O2diffusion- Aeration is poorer- Within a microporousaggregate- Than at the aggregatesurfaceSource: Brady and Weil (2008)O2concentrations for a wet aggregate from an Iowa Mollisol (Muscatine)NRES 201 Lectures 22-24 (Fall 2014): Soil Aeration & Temperature1223Vegetation- Upland plants decrease soil aeration- Because of root respiration- Wetland plants canincrease soil aeration- Because aerenchymatissues transport O2Source: Brady and Weil (2008)24Vegetation- Upland plants decrease soil aeration- Because of root respiration- Wetland plants can increase soil