ORGANIC MATTER DECOMPOSITION CHAPTER 7

SOIL There are three major properties of soil. Physical – soil structure and texture Chemical – chemical components; pH, nutrients Biological – micro and macro fauna/flora Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process

Contain five major groups of microorganisms Bacteria Actinomycetes Fungi Algae Protozoa

All these microorganisms participate in the various activities that take place in the soil. Among the activities are; Decomposition of organic matter Nutrient Cycling Nutrients transport/flow Protection

ESSENTIAL PLANT NUTRIENTS There are at least 16 essential chemical elements for plant growth Plant must have these nutrients to performance the various physiological functions C, H and oxygen (O), (from air & water) N, phosphorus (P), potassium (K), Ca, Mg, sulfur (S), iron (Fe), manganese (Mn), Zn, copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl) (from soil) Sodium (Na), silicon (Si), and nickel (Ni) Cobalt (Co) (required by certain plants)

SOURCES OF PLANT NUTRIENTS IN THE SOIL 1) weathering of soil minerals 2) decomposition of plant residues, animal remains, and soil microorganisms 3) application of fertilizers and liming materials, 4) application of manures, composts, biosolids (sewage sludge) and other organic amendments

5) N-fixation by legumes 6) ground rock powders or dusts including greensand, basalt, and rock phosphate 7) inorganic industrial byproducts 8) atmospheric deposition, such as N and S from acid rain or N-fixation by lightning discharges, 9) deposition of nutrient-rich sediment from erosion and flooding

Basic Plant Nutrient Cycle The basic nutrient cycle highlights the central role of soil organic matter and microorganmisms. Cycling of many plant nutrients, especially N, P, S, and micronutrients, closely follows the Carbon Cycle. Plant residues and manure from animals that are fed forage, grain, and other plant-derived foods are returned to the soil.

This organic matter pool of carbon compounds becomes food for bacteria, fungi, and other decomposers. As organic matter is broken down to simpler compounds, plant nutrients are released in available forms for root uptake and the cycle begins again. Plant-available nutrients such as K, Ca, Mg, P, and trace metal micronutrients are also released when soil minerals dissolve.

DECOMPOSITION OF ORGANIC MATTER Definitions: breakdown of dead plant and animal material and release of inorganic nutrients Decomposition is a biological process that includes the physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules (Juma, 1998).

SOURCE OF ORGANIC MATTER Plant remains Animal tissues and excretory products Cells of microorganisms However, plant is the main contributor to organic matter

ORGANIC CONSTITUENTS OF PLANTS Cellulose, most abundant 15 to 60% of dry weight Hemicelluloses, 10 to 30% Lignin, 5 to 30% Water soluble fraction include simple sugar, amino acids, and aliphatic acids, 5 to 30% of tisue weight Ether and alcohol-soluble constituents; fats, oils, waxes, resins and a number of pigments Proteins

WHY MICROORGANISMS DECOMPOSE ORGANIC MATTER Supplying energy for growth Supplying carbon for new cell synthesis The cells of most microorganisms commonly contain approximately 50% carbon. This is derived mainly from the substrates.

WHY DO WE CARE ABOUT DECOMPOSITION? Decomposition is important in releasing nutrients tied up in dead organic matter and return it back to the soil.

WHO ARE THE DECOMPOSERS? A. Soil fauna (e.g., earthworms, arthropods): physical fragmentation (comminution) increases surface area, distributes organic matter within soil profile, doesn’t alter litter chemistry B. Soil microorganisms: heterotrophic bacteria and fungi derive energy, carbon, and nutrients from dead organic matter; in the process they release CO2 through respiration; RESPONSIBLE FOR BULK OF DECOMPOSITION!!

DECOMPOSITION PROCESSES There are three main processes 1. Assimilation = conversion of substrate materials into protoplasmic materials. Eg. Organic matter carbon to microbial carbon. Protein to microbial protein. 2. Mineralization = conversion of organic substance to inorganic form. Eg. Protein from the organic matter will be converted to inorganic nitrogen in the soil. 3. Immobilization = conversion of inorganic form into organic form. Eg. Inorganic nitrogen NH4 from the soil converted into microbial protein.

FACTORS AFFECTING RATE OF DECOMPOSITION Environmental Factors Temperature Microbial activity responds exponentially to increased temperature until enzymes denature, etc. Moisture Microbial activity has optimum moisture Low moisture = dessication, slow diffusion High moisture = low O2 availability; no lignin degradation pH Most microbes exhibit optimum acitivty near pH 7. Fungi most active in acid soil and bacteria in moderate soil pH.

Substrate Quality: Carbon Different carbon compounds are decomposed at different rates. Cellulose faster Lignin slower decomposition as compared to cellulose. C:N of the organic matter determine the rate: high slower, this is due to insufficient of N for microorganisms to assimilate carbon; low faster, nitrogen is sufficient for rapid assimilation of carbon.

The C:N ratio is the most commonly used in soils because N is the most limiting element. Knowing the impact of litter decomposition on available N is therefore very important. Microbes can easily out-compete plants for available N

C:N OF SOME ORGANIC MATTER C:N ratio legumes 13 – 25:1 manure 20 - 30:1 straw 80:1 sawdust 400 – 600:1 microorganisms 5 – 10:1

In general, for every gram carbon used for microbial biomass, another 2 gram will be respired as CO2. A microbe with a C:N ratio of 8:1 would require organic matter with a C:N ratio of 24:1. Because there is a suite of microorganism and organic matter quality, generally we can predict whether mineralization or immobilization will take place base on the C:N ration range 1. C:N > 30 = Nett N immobilization 2. C:N > 20 but < 30 N immobilization = N mineralization 3. C:N < 20 = Nett N mineralizarion