NRES 201 Lecture 15 (Fall 2014): Soil Organic Matter11NRES 201Soil Organic Matter2Today’s lecture topics- What is soil organic matter (SOM)?- Functions of soil organic matter- Soil organic matter management- Decomposition in soils- Factors affecting soil content and distribution of organic matter- The global carbon cycleSoil Organic MatterNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter23What is soil organic matter (SOM)?Defined as- The organic componentof soil, consisting of: - Living organisms(biomass)- Organismal remains(residues)- Decay products(humus)Estimated from soil organic C (SOC)- SOM = SOC 1.8- Excludes inorganic C in carbonatesSource: http://fyi.uwex.edu4Fractions of soil organic matterSource: http://www.nrcs.usda.govNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter35Nutritional functions- Energy and C source forheterotrophic microbes- Supplies N, P, and S toplants and microbes- Increases nutrient-holding capacity- Binds (chelates)micronutrients- Contributes buffercapacityFunctions of soil organic matterSource: http://www.zingbokashi.co.nz6Physical functions- Improves soil structure,aggregation, and macroporosity- Critical for temperate soils- Enhances water infiltrationand aeration- Increases water-holding capacity- Important for sandy soils- Reduces erosion and compaction- Promotes root growth- Decreases evaporation- Reduces crusting- Important for clayey soilsSource: http://soilquality.orgNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter47Biological functions- Essential for all heterotrophs, including:- Most microbes- Macro- and microfauna(earthworms, nematodes, etc.)Ecological functions- SOM is the Earth’s major surface C reservoir050010001500200025003000Vegetation Atmosphere SurfaceoceanSoilC Storage, metric tons  10955076092024008And many more functions- Far too many to cover hereSource: Brady and Weil (2010)For more details, see Fig. 11.10 on page 377 of the textbookNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter59Need to balance opposing factors- SOM decreases when residuedecomposition > residue input- SOM increases when residueinput > residue decompositionResidue inputs increased by- High crop yields with:- Improved varieties- High planting rates- Optimal fertilizer use- Adequate cropproduction practicesSoil organic matter managementTwin-row corn (high population)Source: http://www.hpj.com10Decomposition decreased by- Reduced tillage- Less decomposableresidues having:- A higher C/N ratio- Or a higher lignin content- Avoiding excessive N or Pfertilization- Reduced microbial activity under:- Lower soil temperature- A higher soil moisture content- Increased soil acidity- Fine-textured soilSource: http://www.thecropsite.comNo-till cornNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter611Composition of plant residuesDecomposition in soilsSource: Brady and Weil (2008)Typical composition12Organic constituents and their ease of decompositionMost decomposable- Sugars, starches, simple proteins- Crude protein- Cellulose- Fats, waxes- Lignin- Black carbon, charcoal, etc.Least decomposableNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter713Sequence of residue decomposition- Bacteria and fungi multiply rapidly- They generate energy by oxidizing organic C toCO2, and utilize C and N to build biomass- Earthworms, nematodes, and protozoa feed on the biomass- Excess N is excreted as ammonium (NH4+)- Or remains in the biomass if carbonaceous residues are still undergoing decomposition- The NH4+is then converted to nitrate (NO3-), which becomes available for plant uptake14Dynamics of decomposition- For residues with a high C/N ratio (corn stalks)- Decomposition is prolonged- N is tied up (immobilized)- Plant N availability is depressedSource: Brady and Weil (2008)NRES 201 Lecture 15 (Fall 2014): Soil Organic Matter815- For residues with a low C/N ratio (alfalfa tops)- Decomposition is rapid- N is liberated (mineralized)- Plant N availability increasesSource: Brady and Weil (2008)16Rate of decomposition- Faster with a low C/N ratio- Slower with a high C/N ratioSource: Brady and Weil (2008)NRES 201 Lecture 15 (Fall 2014): Soil Organic Matter917Pedologic factors- Temperature and moisture regime- Type of vegetationFactors affecting soil content and distribution of organic matterSource: Brady and Weil (2008)18- Soil texture- More organic matterin fine-textured soilsbecause:• A higher watercontent reducesaeration• Clay protectsorganic matterSource: Brady and Weil (2008)NRES 201 Lecture 15 (Fall 2014): Soil Organic Matter1019Management practices- Tillage and drainage promote the loss of soilorganic matter- By stimulating microbial decompositionSource: Brady and Weil (2008)20- Synthetic N fertilizers also promote organic matter depletion- As documented for the Morrow Plots Continuous cornCorn-soybeanCorn-oats-hayThe Morrow Plots: America’s oldest experiment fieldNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter1121• N fertilization increased residue C inputs but not SOC storage In five decades of continuous cornSource: Khan et al. (2007)-51535557595115135Tons of C per AcreResidue C inputSoil C storage (0-18 in.)Unfertilized NPK†Morrow Plots(1955-2005)Continuous corn†189 lb N/acre per yr.UnfertilizedNPK22• N fertilization increased residue C inputs but not SOC storage In four decades of a corn-soybean rotation Source: Khan et al. (2007)-51535557595115135Tons of C per AcreResidue C inputSoil C storage (0-18 in.)Unfertilized NPK†Morrow Plots(1955-2005)Corn-soybean(Corn-oats until 1967)†100 lb N/acre per yr.NRES 201 Lecture 15 (Fall 2014): Soil Organic Matter1223- The same trend can be found:• At the world’s oldest experiment station inRothamsted, EnglandSources: Glendining and Powlson (1990), Jenkinson (1991)Broadbalk continuous wheat‡††FYM, farmyard manure.‡144 kg N ha-1yr-1.24- The same trend can be found:• At Sanborn Field in Columbia, Missouri(second oldest experiment field in the USA)0.00.40.81.21.6Unfertilized NPKSoil Organic C (%)19141938Source: Albrecht (1938)Continuous wheat††38 lb N/acre per yrNRES 201 Lecture 15 (Fall 2014): Soil Organic Matter1325Continuous cornNPKCorn-oats-hayNPKMorrow Plots(August, 2006)Poor management- The conclusion is obvious:• Synthetic N fertilization does not replacecrop rotationGood management26The global carbon cycleC cycles among- Vegetation- Animals andmicrobes- Soil organicmatter- Carbonates in:- Soils- Rocks- Water- CO2in:-Air-