Fundamentals of Soil Science Soil Organic Matter

Reading Assignment: Brady and Weil, Chapter 11 Reading Assignment: Brady and Weil, Chapter 11 Soil Organic Matter Soil Organic Matter – 3 lectures Why Worry about SOM? Why Worry about SOM? Creating SOM Creating SOM SOM’s Influence on Soil Properties and Plants SOM’s Influence on Soil Properties and Plants – Lab for Chapter 10 and Chapter 11 – Blended learning materials available on web site and DVD

Learning Objectives Lecture 4 – Lecture 4 – −Define and explain the importance of soil organic matter −Describe the carbon (C) cycle, including pools and fluxes −Explain basic principles of organic decomposition including chemical formulas Lecture 5 – Lecture 5 – −Describe the factors the control litter decay −List characteristics of low quality litter −Describe the importance of C:N ratio −Distinguish between factors that control decomposition of litter vs. decomposition of soil organic matter

Learning Objectives (cont.) Lecture 6 – Lecture 6 – – List values of organic matter in soil: physical, chemical, biological – Relate the names and activities of three SOM pools – Discuss changes in various pools of organic matter as a result of changes in land use and management – Describe the interaction between SOM pools over time – Explain Conundrum – selecting materials to attain balance in particular ecosystem – Articulate value of managing organic matter in soil

Lecture 4 Topics Recall Previous Learning Recall Previous Learning Definition and Importance of Soil Organic Matter Definition and Importance of Soil Organic Matter Global Carbon Cycle Global Carbon Cycle Balance and Imbalance Balance and Imbalance

Lab Description of lab and objectives of lab relating to Chapter 10 and Chapter 11 Description of lab and objectives of lab relating to Chapter 10 and Chapter 11

Lecture 5 Topics Decomposition in Soil Decomposition in Soil By-products of decomposition By-products of decomposition Humus and its sources Humus and its sources Mechanisms for SOM Stabilization Mechanisms for SOM Stabilization Carbon to Nitrogen Ratio Carbon to Nitrogen Ratio Nitrogen Fixation Nitrogen Fixation

Lecture 6 Topics Influence of SOM: Physical, Chemical, Biological Influence of SOM: Physical, Chemical, Biological Pools of Organic Carbon Pools of Organic Carbon Managing SOM Managing SOM Greenhouse Gases Greenhouse Gases

Lecture 4 Why Worry about SOM?

Learning Objectives Lecture 4 – Lecture 4 – −Define and explain the importance of soil organic matter −Describe the carbon (C) cycle, including terrestrial pools and fluxes −Explain basic principles of organic decomposition

Lecture 4 – Topics Recall Previous Learning Recall Previous Learning Definition and Importance of Soil Organic Matter Definition and Importance of Soil Organic Matter Global Carbon Cycle Global Carbon Cycle Balance and Imbalance Balance and Imbalance

What is Soil? Air Air Water Water Minerals Minerals Organic Matter Organic Matter – What is soil organic matter? Living biomass (plant tissues, animal tissues and microorganisms) Living biomass (plant tissues, animal tissues and microorganisms) Dead roots and dead plant residues or litter Dead roots and dead plant residues or litter Mixture or organic substances no longer identifiable as tissues Mixture or organic substances no longer identifiable as tissues

Soil Function and Properties The percentage of soil organic matter (SOM) in most soils is small. The percentage of soil organic matter (SOM) in most soils is small. SOM exerts a dominant influence on soil physical, chemical and biological properties: SOM exerts a dominant influence on soil physical, chemical and biological properties: −Soil cation exchange capacity −Water-holding capacity −Formation and stabilization of soil aggregates −Storehouse of plant nutrients, especially nitrogen (next section) −Soil supplies energy and body-building constituents for microorganisms

Carbon (C) in soil Soil organic matter is a complex and varied mixture of organic substances Soil organic matter is a complex and varied mixture of organic substances All organic substances contain C All organic substances contain C Lignin glucose sugars cutin cellulose protein

Global Carbon Cycle Imbalance Pg/yr enters the atmosphere while only 215 Pg/yr of carbon is removed. Carbon dioxide levels in the atmosphere are rising. SOM plays large role in global carbon balance. Small changes in rates of stabilization/destabilization are important for global budgets ,400 Soil 40,000 Oceans and Lakes 75,000,000 Carbonate Rocks 5,000 Fossil Fuel Sediment

Carbon inputs – outputs = storage PlantsLitterSoil organic matter

Soil C Soil carbon represents a larger global pool than living plant and atmospheric pools combined. Soil carbon represents a larger global pool than living plant and atmospheric pools combined. – Carbon comprises about ½ the mass of soil organic matter – The world’s soils contain 3x as much carbon as the worlds vegetation.

Global C in upper 100 cm Wetlands (Histosols) cover 2% of global land cover, but contain approx. 30% of soil Carbon OrganicInorganicTotal Soil OrderGlobal area, 10 3 km 2 PgTotal % Entisols21, Inceptisols12, Histosols1, Andisols Gelisols11, Vertisols3, Aridisols15, Mollisols9, Spodosols3, Alfisols12, Ultisols11, Oxisols9, Misc.land18, Total130, , Wetlands cover xx percent of global land cover but contain 30% of global C Andisols cover xx percent of global land cover, but contain xx% of global C

Stages of decomposition: Stages of decomposition: 1)Leaching by water removes simple soluble compounds and ions 2)Fragmentation (physical break down) creates surface area 3)Chemical alteration – Microbially mediated by- products Decomposition Breakdown of large organic molecules into simpler components

Decomposition Process Microbially mediated reactions: Microbially mediated reactions: 1)Enzymatic oxidation of organic matter to CO 2, H 2 O, energy, and biomass 2)Mineralization of proteins into their inorganic constituents: e.g., NH 4, NO 3, P and SO 4 3)Formation of compounds that resist further decay Decomposition is an oxidation process Decomposition is an oxidation process Input entire eqn 11.1 In an aerobic or anaerobic environment

Composition of Plant Material Plant residues are the primary source of soil organic matter. Plant residues are the primary source of soil organic matter. Plants = 75% Water, 25% Dry Matter Elemental analysis of dry matter? Structural components of dry matter? Carbohydrates Lignins and polyphenols Proteins

Plant Decomposition 1.Sugars, starches, simple proteins 2.Crude proteins 3.Hemicellulose 4.Cellulose 5.Fats and waxes 6.Lignins and phenolic compounds Rapid Decomposition Very slow decomposition

Detecting organic decomposition Soil Respiration rate (level of CO 2 emission from the soil) Soil Respiration rate (level of CO 2 emission from the soil) What happens when falling leaves replenish litter layer? What happens when falling leaves replenish litter layer? R-strategists – opportunistic species R-strategists – opportunistic species K-strategists – slow and steady decomposers K-strategists – slow and steady decomposers Priming – decay of older organic matter as a result of addition of fresh inputs Priming – decay of older organic matter as a result of addition of fresh inputs

Detecting organic decomposition r-strategists (opportunists), microorganisms who were dormant awaken and overtake k-strategists (slow and steady decomposers) During r-strategists reign: respiration high, microbial biomass high r-strategists go dormant or die when food is gone, k- strategists eat them and help promote mineralization Some carbon from decomposition process is converted to soil humus