Soil Organic Matter Section C Soil Fertility and Plant Nutrition.
Post on 14-Dec-2015
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Soil Organic Matter Section C Soil Fertility and Plant Nutrition Slide 2 Review - Soil organisms Bacteria Most numerous, smallest Aerobic and anaerobic Actinomycetes Share characteristics of bacteria and fungi Active in degradation of resistant compounds Fungi Aerobic only, filamentous Active in degradation of resistant compounds Slide 3 Major Soil Organisms Bacteria 10 8 /gram Actinomycetes 10 7 /gram Fungi 10 6 /gram Slide 4 Soil Microorganisms Can be classified according to C and energy sources and their oxygen requirement: photoautotrophs Energy from sunlight & C from CO 2 Some bacteria and algae only chemoautotrophs Energy from oxidizing inorganic material, C from CO 2 Some bacteria only chemoheterotrophs Energy and C from oxidation of organic materials Most bacteria, all fungi and actinomycetes Slide 5 Soil Microorganisms Oxygen requirement aerobic Require free O 2 for respiration All fungi and actinomycetes, most bacteria anaerobic Must use alternative electron acceptors instead of O 2 NO 3 -, SO 4 2-, Fe 3+, CO 2 Some bacteria are anaerobic facultative Can be aerobic or anaerobic. Some bacteria Slide 6 Decomposition of Plant Residues (Under aerobic conditions) Plant Residues CO 2 NH 4 +, SO 4 2-, etc. (inorganic waste) Humus (organic waste) + Dead Microorganisms More microbial biomass Slide 7 Soil Organic Matter Soil organic matter: all organic matter in the soil, including humus, microbial biomass, and plant and animal residues in various stages of decomposition. Composed of a wide range of organic materials, from highly decomposable to resistant to decomposition. Slide 8 Roles of Soil Organic Matter Microbial substrate Nutrient reserve (esp. N, P, S) CEC Water-Holding capacity Soil structure Slide 9 Humus The stable portion of soil organic matter that results from microbial degradation of residues. Dark colored About 58% C, 5% N Complex chemical structure, aromatic plus aliphatic functional groups Difficult to break down because of structure high CEC Slide 10 Humus The major organic waste by-product of OM degradation. The percentage of a residue that will become humus is approx. proportional to its lignin content. Slide 11 Lignin Slide 12 Humus Carbon Hydrogen Oxygen Nitrogen Slide 13 Decomposition of Organic Matter Organic materials are decomposed by heterotrophic microorganisms. The organic matter is a source of _______, __________, and _____________ to these organisms. carbon energynutrients Slide 14 Humus and Nutrients Humus contains about 58% C, 5%N, 0.6% P, and 0.6% S How much humus in soils? How much OM does this represent? An Aridisol with 0.5% SOM in the top 30 cm will contain 3000 m 3 /ha x 1500 kg/m 3 x 0.005 = 22,500 kg/ha (top 30 cm) A Mollisol with 5.0% SOM in the top 30 cm will contain 3000 m 3 /ha x 1500 kg/m 3 x 0.05 = 225,000 kg/ha (top 30 cm) An Aridisol might contain 0.5% SOM by weight, a Mollisol 3-5% by weight Slide 15 Decomposition of Humus The rate of decomposition of humus is most strongly affected by soil moisture and temperature ( 5%/yr). Humus is chemically complex and has a C:N ratio of about 11:1 High soil temperatures, abundant (but not excessive) moisture encourages rapid humus breakdown In soils where OM content is not decreasing, synthesis of new humus approximately equals decomposition of old humus. Slide 16 Decomposition (Mineralization) of Humus Releases N as NH 4 +, available for plants If 2.5% of the N in SOM is mineralized each year, how much N would be released for plant uptake? Aridisol (from previous example) 22,500 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (% min) = 28 kg N/ha Mollisol (from previous example) 225,000 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (% min) = 280 kg N/ha Slide 17 Decomposition of Plant Residues (Under aerobic conditions) Plant Residues CO 2 NH 4 +, SO 4 2-, etc. (inorganic waste) Humus (organic waste) + Dead Microorganisms More microbial biomass Slide 18 What Happens to Residues? Chemically simple residues Chemically complex residues Slide 19 Decomposition of Plant Material The rate of decomposition of plant residues is governed mostly by: Chemical makeup of the residue C:N ratio Available soil N Temperature, moisture, oxygen, and other environmental conditions that affect microbial growth Slide 20 Chemical Composition of Plant Residues Sugars Complex proteinsHemicelluloseCellulose Lignin Simple proteinsWaxes Starchs Increasing chemical complexity Increasing rate of decomposition Slide 21 C:N Ratio Why is the C:N ratio important? Microorganisms need C and N in fixed ratios, because C and N are used to synthesize proteins, nucleic acids, etc. Bacterial cell C:N is 5:1 to 8:1. Since about 50% of the C in an organic material is converted to CO 2, they need roughly a C:N of 10:1 to 16:1 in the residue they consume. Fungi need a C:N of about 40:1 in their diet Slide 22 decomposition C:N Ratio 50 g C 20 g as CO 2 20 g as biomass Microbial biomass has an average C:N of 10:1, therefore how much N is needed to balance the new biomass C? 10 g as waste 2 g Therefore, if the residue containing 50 g of C contains < 2 g of N (C:N>25:1), it will have insufficient N for microbial needs. What about >2 g N (C:N