Introductory Soils Lab 9 Biological Activity NRES 201 Page 1 of 9 LABORATORY 9. BIOLOGICAL ACTIVITY IN SOILS Soils contain a vast array of microorganisms, including bacteria, actinomycetes, fungi, algae, and protozoa. Bacteria typically outnumber the other four groups combined, with populations of at least 108 per gram of dry soil. Their total live weight has been estimated to be of the order of 300 lb/acre, somewhat less than the estimate for fungal biomass (480 lb/acre). Most soil bacteria are aerobic heterotrophs, meaning that they require atmospheric oxygen (O2) and utilize organic substrates as a source of energy and carbon. Their activities bring about the decomposition of plant, animal, and microbial residues, an essential function that sustains plant growth, and thus life in general, by recycling carbon and mineral nutrients. 9.1 DECOMPOSITION OF ORGANIC MATERIALS IN SOILS. Plant residues are complex in chemical composition, consisting largely of proteins, cellulose, hemicellulose, and lignin. As depicted in Figure 9-1, decomposition occurs in stages, beginning with proteins, which are easily degraded. Depending on the microbes involved and their growth rate, from 10 to 70% of the carbon undergoing decomposition will be utilized for biomass synthesis, the remainder being liberated as CO2. In the second stage, this biomass becomes a substrate for other microbes as cellulose and hemicellulose are decomposed, with further loss of CO2. As the latter constituents disappear, lignin and other resistant materials come under microbial attack, accompanied by an ongoing loss of CO2 during biomass decomposition. At the end of the first year, at least two-thirds of the residue carbon will have been lost as CO2, and perhaps more depending on weather conditions and management practices. Because of the universal need for nitrogen as well as carbon by all living systems, there is a close coupling between these elements during microbial decomposition of organic residues. This coupling is clearly apparent from the fact that nitrogen availability is a prerequisite for carbon utilization, while carbon availability controls the heterotrophic demand for nitrogen. Both of these principles will be explored in the biological activity lab, with emphasis on three key N-cycle processes: mineralization, immobilization, and nitrification. 9.2 MINERALIZATION. Mineralization is the conversion of an organic to an inorganic, or mineral, form. The term is often employed in referring to microbial CO2 production from organic carbon, and in this context can be used interchangeably with decomposition. In the case of nitrogen, organic amino compounds such as proteins, amino acids, and amino sugars serve as the substrate, and inorganic ammonium (NH4+) is generated as a waste product.Introductory Soils Lab 9 Biological Activity NRES 201 Page 2 of 9 Figure 9-1 Stages of microbial activity when organic residues are decomposed in soil. The residue disappears as increasingly resistant constituents are utilized, with synthesis of microbial biomass (B) and partial loss of carbon as CO2. Mineralization plays a key role in supplying inorganic nitrogen for use by plants and microbes, since soil nitrogen occurs largely in organic forms that are otherwise unavailable. The process is promoted by the input of fresh residues, adequate but not excessive soil moisture, warm temperatures (40-60°C is optimal), good aeration, and the absence of soil acidity. Wetting and drying cycles also have a positive effect, as does tillage. Drying kills many soil microbes, but survivors are always present to utilize the dead biomass when the soil is rewetted, leading to a flush of mineralization. Tillage improves aeration and exposes fresh organic surfaces to microbial attack. 9.3 IMMOBILIZATION. The conversion of inorganic to organic nitrogen is an integral aspect of microbial growth. This process, known as immobilization, is carried out by the same heterotrophic organisms that are responsible for mineralization, but has the opposite effect of reducing nitrogen availability to plants. Either NH4+ or nitrate (NO3–) can be immobilized;Introductory Soils Lab 9 Biological Activity NRES 201 Page 3 of 9 Table 9-1 C/N ratios for various organic substances. Substance C/N Substance C/N Soil microorganisms 8:1 Corn stover 60:1 Soil organic matter 10:1 Wheat, oat, or barley straw 80:1 Sweet clover (young) 12:1 Timothy 80:1 Barnyard manure (rotted) 20:1 Asphalt 94:1 Sewage sludge 28:1 Hardwood sawdust 400:1 Green ryegrass 36:1 Spruce sawdust 600:1 however, there is such a strong preference for NH4+ that only in its complete absence is NO3– utilized. Both mineralization and immobilization occur more or less continuously, although not necessarily at the same rate. Inorganic nitrogen concentrations will increase if mineralization exceeds immobilization, and will decrease if immobilization exceeds mineralization. The type of residue undergoing decomposition determines which process is more rapid. Most plant residues contain about 40% carbon by weight, but there is considerable variability in nitrogen content, and thus in the ratio of carbon to nitrogen (C/N). Table 9-1 demonstrates how widely C/N ratios vary for different plant residues and other organic substances, and shows that the ratio is always wider than for soil microbes or humus (organic matter). The tie-up of nitrogen becomes a concern with materials having a C/N ratio > 30:1, particularly if another crop is to be grown soon after the residues are turned under. Fertilization will be necessary unless the soil’s capacity for mineralization is sufficient to satisfy microbial as well as plant nitrogen needs. 9.4 NITRIFICATION. As residues are consumed by decomposition, a diminishing supply of organic carbon begins to limit heterotrophic activities, so there is less immobilization of NH4+ generated by mineralization. The result is an increase in the utilization of NH4+ for NO3– production by nitrification. The latter process actually occurs in two steps involving chemoautotrophic bacteria that oxidize inorganic nitrogen as a source of energy while utilizing CO2 (in the form of dissolved HCO3–) as a carbon source. In the first step, NH4+ is oxidized to nitrite (NO2–) by the genus, Nitrosomonas. In the second, NO2– is further oxidized to NO3– by a different group of bacteria classified as Nitrobacter. Owing to the