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"Data from Neidhardt et al. (1996).

'Dry weight calculated on basis of 70% cell water content. cReliable estimates are lacking.

"Data from Neidhardt et al. (1996).

'Dry weight calculated on basis of 70% cell water content. cReliable estimates are lacking.

of potentially mineralizable N. Soil fungi often concentrate their cytoplasm and active metabolic constituents at their growing tips. In soil, they have been shown to have only one-tenth as much DNA per unit weight as bacteria and generally have higher C:N ratios.

Bacteria have C:N ratios that range from 3.5:1 to 7:1, while fungi range from 10:1 to 15:1. The average total microbial biomass has a C:N ratio of 4:1 to 8:1. The C:N ratio depends on the broad groups of bacteria and fungi present. The microbial C:N ratio as determined by physiological methods can be a crude measure of micro-bial diversity. Bacteria often dominate highly disturbed systems such as agricultural fields. Other ecosystems where bacteria flourish are wet meadows, marshes, and wet tropical forests. Fungi tend to dominate in soils that are well aerated and less disturbed, such as grasslands, forests, and reduced-tillage agricultural systems.

Microbial growth reflects the dominant groups of organisms present in a soil. The importance of microbial products for forming and maintaining SOM has been shown through the addition of glucose to soil, which produces microbial products that are ultimately more stable than some plant-derived compounds. The biochemical composition of microbial cells varies significantly among groups of microbes (such as fungi, G+ bacteria, G- bacteria, and actinomycetes). Microbial products such as phenols, amino sugars, and fungal melanins may contribute significantly to maintaining SOM. Information on the composition of the microbial community can be used to improve the process-based C cycling models described in Chap. 16 and provide more insight into humification processes.

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