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Even though a high proportion of both fungi and bacteria are decomposers in the soil, they degrade plant residues differently and have different roles in the recycling of nutrients. This is partly due to their different choice of habitats within the soil and the different types of organic matter they consume.
Fungi are generally much more efficient at assimilating and storing nutrients than bacteria. One reason for this higher carbon storage by fungi lies in the chemical composition of their cell walls. They are composed of polymers of chitin and melanin, making them very resistant to degradation. Bacterial membranes, in comparison, are phospholipids, which are energy-rich. They degrade easily and quickly and function as a food source for a wide range of microorganisms.
The different proportions of carbon and nitrogen (i.e. different C:N ratios) of bacteria and fungi might also play a role in the mineralisation and immobilisation processes of nutrients in the soil. Due to their structure and C:N ratio between 7:1 and 25:1, fungi need a greater amount of carbon to grow and reproduce and will therefore 'collect' the required amount of carbon available for this from the soil organic matter. Bacteria, however, have a lower C:N ratio (between 5:1 and 7:1) and a higher nitrogen requirement and take more nitrogen from the soil for their own requirements.
The microbial community in soil is made up largely of bacteria and fungi. The fungal-to-bacterial ratio is a measure of what proportion of the microbial community is bacteria compared to the proportion of the microbial community that is fungi.
In general, the greater the fertility of a landscape, the greater the soil microbial biomass. The fungal population will increase at a greater rate than that of bacteria, leading to a higher fungal-to-bacterial ratio. Fungi dominate most of the arable agricultural soils in the temperate climate. Fungal-to-bacterial ratios range from approximately 1.0 to 2.3 but much higher ratios and soils dominated by bacteria are also well documented. Grassland soils are also often dominated by fungi.
Fungi and bacteria differ in their responses to changes in agricultural management practices. Fungi are usually more sensitive to these changes. The fungal-to-bacterial ratio is therefore a good indicator of environmental changes in the soil. For example, when plant residues are applied as mulch fungi prosper because their hyphae are able to grow into the litter layer. Tillage, however, destroys large amounts of the fungal hyphae. Incorporation of plant residues into the soil also favours the bacterial population because the contact surface between the substrate and bacteria is increased. This response further depends on the soil type.
In addition, the dominance of either fungi or bacteria also depends on the quality of the plant residue. Important aspects of plant residue quality are the substrate structure, C:N ratio and cellulose content. Fungi are the predominant cellulose decomposers, even though one group of bacteria, the Actinomyces also contribute significantly to its decomposition. Cellulose has a high carbon content and a corresponding high C:N ratio, making it the ideal food source for fungi.
Bacteria, which have a smaller C:N ratio than fungi, need food rich in nitrogen (e.g. green manure, legume residues). A fertiliser rich in nitrogen therefore favours the bacterial community in a soil whereas a substrate with a relatively wide C:N ratio enables growth of the fungal population.