NRES 201 Lecture 15 Fall 2014 Soil Organic Matter NRES 201 Soil Organic Matter 1 Soil Organic Matter Today 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 cycle 2 1 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter What is soil organic matter SOM Defined as The organic component of soil consisting of Living organisms biomass Organismal remains residues Decay products humus Source http fyi uwex edu Estimated from soil organic C SOC SOM SOC 1 8 Excludes inorganic C in carbonates 3 Fractions of soil organic matter Source http www nrcs usda gov 4 2 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Functions of soil organic matter Nutritional functions Energy and C source for heterotrophic microbes Supplies N P and S to plants and microbes Increases nutrientholding capacity Binds chelates micronutrients Contributes buffer capacity Source http www zingbokashi co nz 5 Physical functions Improves soil structure aggregation and macroporosity Critical for temperate soils Enhances water infiltration and aeration Increases water holding capacity Important for sandy soils Reduces erosion and compaction Promotes root growth Decreases evaporation Reduces crusting Important for clayey soils Source http soilquality org 6 3 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Biological functions Essential for all heterotrophs including Most microbes Macro and microfauna earthworms nematodes etc C Storage metric tons 109 Ecological functions SOM is the Earth s major surface C reservoir 3000 2400 2500 2000 1500 1000 500 550 760 920 0 Vegetation Atmosphere Surface ocean Soil 7 And many more functions Far too many to cover here For more details see Fig 11 10 on page 377 of the textbook Source Brady and Weil 2010 8 4 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Soil organic matter management Need to balance opposing factors SOM decreases when residue decomposition residue input SOM increases when residue input residue decomposition Residue inputs increased by High crop yields with Improved varieties High planting rates Optimal fertilizer use Adequate crop production practices Twin row corn high population Source http www hpj com 9 Decomposition decreased by No till corn Reduced tillage Less decomposable residues having A higher C N ratio Or a higher lignin content Avoiding excessive N or P Source http www thecropsite com fertilization Reduced microbial activity under Lower soil temperature A higher soil moisture content Increased soil acidity Fine textured soil 10 5 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Decomposition in soils Composition of plant residues Typical composition Source Brady and Weil 2008 11 Organic constituents and their ease of decomposition Most decomposable Sugars starches simple proteins Crude protein Cellulose Fats waxes Lignin Black carbon charcoal etc Least decomposable 12 6 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Sequence of residue decomposition Bacteria and fungi multiply rapidly They generate energy by oxidizing organic C to CO2 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 uptake 13 Dynamics of decomposition For residues with a high C N ratio corn stalks Decomposition is prolonged N is tied up immobilized Plant N availability is depressed Source Brady and Weil 2008 14 7 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter For residues with a low C N ratio alfalfa tops Decomposition is rapid N is liberated mineralized Plant N availability increases Source Brady and Weil 2008 15 Rate of decomposition Faster with a low C N ratio Slower with a high C N ratio Source Brady and Weil 2008 16 8 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Factors affecting soil content and distribution of organic matter Pedologic factors Temperature and moisture regime Type of vegetation Source Brady and Weil 2008 17 Soil texture More organic matter in fine textured soils because A higher water content reduces aeration Clay protects organic matter Source Brady and Weil 2008 18 9 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Management practices Tillage and drainage promote the loss of soil organic matter By stimulating microbial decomposition Source Brady and Weil 2008 19 Synthetic N fertilizers also promote organic matter depletion As documented for the Morrow Plots The Morrow Plots America s oldest experiment field Continuous corn Corn soybean Corn oats hay 20 10 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter N fertilization increased residue C inputs but not SOC storage In five decades of continuous corn Tons of C per Acre 135 115 Morrow Plots 1955 2005 Continuous corn Unfertilized 95 75 Residue C input Soil C storage 0 18 in NPK 55 35 15 5 Unfertilized NPK 189 lb N acre per yr Source Khan et al 2007 21 N fertilization increased residue C inputs but not SOC storage In four decades of a corn soybean rotation Tons of C per Acre 135 115 95 75 Morrow Plots 1955 2005 Corn soybean Corn oats until 1967 Residue C input Soil C storage 0 18 in 55 35 15 5 Unfertilized NPK 100 lb N acre per yr Source Khan et al 2007 22 11 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter The same trend can be found At the world s oldest experiment station in Rothamsted England Broadbalk continuous wheat FYM farmyard manure 144 kg N ha 1 yr 1 Sources Glendining and Powlson 1990 Jenkinson 1991 23 The same trend can be found At Sanborn Field in Columbia Missouri second oldest experiment field in the USA Soil Organic C 1 6 Continuous wheat 1914 1938 1 2 0 8 0 4 0 0 NPK Unfertilized 38 lb N acre per yr Source Albrecht 1938 24 12 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter The conclusion is obvious Synthetic N fertilization does not replace crop rotation Morrow Plots August 2006 Poor management Continuous corn NPK Good management Corn oats hay NPK 25 The global carbon cycle C cycles among Vegetation Animals and microbes Soil organic matter Carbonates in Soils Rocks Water CO2 in Air Water Source Brady and Weil 2008 26 13 NRES 201 Lecture 15 Fall 2014 Soil Organic Matter Atmospheric CO2 concentrations Elevated since the Industrial Revolution From 280 to 390 ppm This increase attributed