The microbial C:P and N:P ratios were both negatively associated with all components of the soil microbial communities. However, the fungi to bacteria ratio only regulated the microbial C:P ratio. a wide C:N ratio may immobilize and sequester nitrogen in the tissues of the microbial decomposers leaving little available to satisfy plant needs. Generally, microbes have a cellular C:N ratio of approximately 10:1. Organic matter with C:N ratios significantly higher than 10:1 will require further decomposition, resulting in nitrogen immobilization until microbial needs are satisfied. Thus the C:N ratio becomes a critical criterion when considering the addition of organic fertilizers and amendments. This flexibility is partly induced by shifts in microbial community structure and variations in environmental conditions.It is defined as the ratio of the weight of organic carbon to the weight of total nitrogen in a soil or organic matter. It is the relationship between organic matter and nitrogen content of soils or plants.The wide C:N ratio leads to slow decomposition rate, nutrient immobilization may occur, carbon and energy wastage in large quantities. Activity of microorganisms restricted total amount of N is limited.In Narrow C:N ratio, Carbon and energy starvation occur. Plant residues decompose quickly and release nitrates readily. Amount of CO2 released/unit of carbon decomposed is less as more of it is metabolized and converted into microbial tissues. When the residue with high C/N ratio is added to soils, there will be intense competition among the microorganism for available N. The C/ N ratio in residues helps determine their rate of decay and the rate at which N is made available to plants. Ex : Speed of decomposition becomes slow with more/wide C/N ratio residue or low N percentages. On the contrary low/narrow C/N ratio or high N percentages speeds the decomposition rate.

Article Linking microbial C:N:P stoichiometry to microbial community...

The C:N ratio of bacteria, fungi and actinomycetes range from 5:1 to  10:1 and for cultivated soils it ranges from 8:1 to 15:1. The C:N ratio is lower in the sub-soils than the surface soils. The C:N ratio for a particular soil is one of the characteristic equilibrium values. These values are important for two reasons:

1. When wider C:N ratio organic residues are added, there is a competition among microorganisms and plants for available soil N.

2. The maintenance of SOC i.e. SOM is dependent on the level of soil N.

If large amounts of fresh organic residues with wider C:N ratio (>30:1) are incorporated in the soil, the heterotrophic microorganisms become active and increase their population, with production of more CO2 through their respiration. Under these conditions, the nitrate nitrogen levels in soil decrease as the microorganisms utilize the native soil nitrogen.Nitrogen is immobilized at the initial stages and is not available to plants. Hence, basal application of N through inorganic fertilizers is made to meet the N requirements of crops.As the decomposition proceeds, the C:N ratio of organic matter decreases with loss of C and conservation of N at a given point of time.

Somewhere , it is a kind of biological equilibrium between microbial C/N and organic C/N ratio..but what exactly guides the final  microbial C/N ratio. 

I would simply say , that means microbes (  Functional microbial diversity as well ) are more active in  mixed culture than mono-culture ...

This is a good article that deals on the linkage of stoichiometry in soil, microbial biomass, and enzyme:

Sinsabaugh, R.L., Hill, B.H. and Shah, J.J.F., 2009. Ecoenzymatic stoichiometry of microbial organic nutrient acquisition in soil and sediment. Nature, 462(7274), pp.795-798.

Does the higher microbial Cmic/Nmic ratio in mono-culture imply a more fungi-dominated community whereas the lower value in mixed-culture indicate a bacterial-dominated?

It may be, as fungi contributed more biomass in soil than bacteria. Microbes play a dual role in the soil. Firstly, they act as agents for the degradation of plant residues with concomitant release of nutrients and CO2 and secondly serve as a labile pole of nutrients. The size of microbial biomass carbon may account for 2% of the total soil C and major contributed by fungi.

Interesting response Dr Tsegai and Dr Tarafdar. can we predict the dominance of fungal population over bacterial population with the help of Cmic/Nmic ratio..??

Yes Dr. Srivastava you can as carbon to nitrogen ratios averaged around 5.5 for the bacteria and 8.3 for the fungi and average C contents (mg per dry weight of cells) were 430 for the bacteria and 373 for the fungi.