First of all, no question should be considered as weird or dumb, in my view. It is an interesting question but very complex as well. I think about it and get back to you later. 

Given that the connection in higher species is glutamate/glutamine, I'd guess it would be something similar, aminoacids going through transamination reactions to determine if the nitrogen will be used for synthesis of aminoacids, nucleic acids or alkaloids or removed and the carbon skeleton used for energy or sugar production.

Hey Thomás:

For your question, I think you asked the connections of N and C from two broad perspectives, fundamental conceptual understanding in terms of its biogeochemical cycles and practical application to farming. I found it is challenging to answer it well from both sides and that is beyond my expertise. In stead of stretching myself too much, I just list couple of things I think might be relevant to this topic for you to think about it, or other experts who want to contribute to the discussion.

1 In photosynthesis, CO2 uptake or the rate of photosynthesis is related to leaf nitrogen content. As far as how nitrogen affect photosynthesis and crop yield, you can refer to the second reference here. 

Evans, J. R. 1989. Photosynthesis and nitrogen relationships in leaves of C3 plants. Oecologia 78: 9–19

 Photosynthesis, Grain Yield, and Nitrogen Utilization in Rice and Wheat. DOI: https://doi.org/10.1104/pp.110.165076

2 In organic decomposition process, the rate of mineralization relates to  C/N ratio of litter. For soil organic carbon sequestration and how this related to farming practice, you can find research paper written by Rattan Lal who is an world leading expert in this field. 

3 In wetland soil, the denitrification rate to a large extent depends on availability of dissolved organic carbon in addition to anaerobic condition. 

Other resources, you can look at USDA NRCS website, 590 is the code for nutrient management as a conservation practice. 

Regards

Qiang

Thank you for your answer Tomàs and Qiang.

If I may rephrase my question a little bit, I should tell that I am interested in this relation through C/N ratios during mineralization and humification processes.

Let's start with a simple question : Why does organic matter with a low C/N degrades faster than an OM with a high C/N ?

Dear Thomas,

your rephrased question concerns energy balance and mass balance that shapes ecological interactions.

Very briefly: You can understand C:N ratio as illustration of ratios of organic molecules. C may represent C-rich sugars - source of energy - and N may represent N-rich biomass - source of compounds used to build and maintenance cells and tissues. In this context particular C:N ratio may be a treshold for limitation of the decomposition process: high C:N means limitation by scarcity of N-rich molecules but particularly low C:N ratio would mean limitation by scarcity of energy. 

In other words: organic matter should be nutritionally balanced for decomposing organisms. If it is not, it degrades slower, because the development and growth of decomposers is limited.

This book contains answers for your questions concerning elemental ratios:

http://press.princeton.edu/titles/7434.html

Kind regards,

Michał Filipiak

http://press.princeton.edu/titles/7434.html

In the soil the process of decomposition is very much a function of the soil nitrogen available for decomposing microbes to oxidize the carbonaceous materials. Strawy or woody material does not decompose readily. The high C to N ratio is a measure of having a limited immediate ability to recycle carbonaceous plant residues. For the composting carbon cycle to work effectively without large delay the C to N ratio must be less than 30. A rich top soil will have a C to N ratio of about 10. It also is depend on water.

There are 5 to 15 tons of carbonaceous material produced for each human on the earth each year. A hugu resource which is many times not used for maximum benefits. Water is the most limting resource on our planet for plants, animals, and microbes. 

In our soils there is an optimum ratio of C N and other elements. In photosynthesis the oxidized carbon of carbon dioxide is reduced and the gaseous carbon is linked up with water being hydrated into glucose of sugar which is than transformed as the planetary energy source. In soil the ability to hold and use water is largely a function of the soil organic matter content. Soil organic matter a funcation of decomposition by the microbial mechanism in the soil.  Decomposition can be seen as reverse process of photosynthesis where the reduced carbonaceous material is returned to oxidized carbon and free water. 

The ability of microbes especially bacterial requires a high content of Nitrogen compared to plants. 

The amounts of either Nitrogen in the biosphere is not limited since legumes and other plants team up with bacteria to be able to access the 80% of gaseous Nitrogen in the air to be able to convert it into ammonium utilizable for plants, animals and decomposers. 

When soils are depleted in soil organic matter there ability to hold and supply water the principal limiting ingredient for life on earth is severly compromised. A cultivated soil of less than 1% soil organic matter and capture and recycless that 30 units of water for 100 units of dry soil. In a virgin state of over 5% soil organic the same 100 units of dry soil can capture and recycle over 200 unites of water. 

The soil can seen as a Carbon based battery which is at optimium function and capacity when soil organic matter is charged and is depleted when it is lost and soil carbon battery is discharged. Nitrogen is particularly important for the soil organic matter formation through decomposition. When compost cycle is optimized the stabilized organc content can greatly promote the water cycle of principal importance for our planetary survival. 

Soluble nitrogen fertilizers have played a stimulatory effect on short term yield of many crops but do have a serious effect on acidifying soils and many times do not contribute to optimized soil organic mantenance or accrural. 

Our ability to reverse climate change will uttimately depend on our ability not only to reduced greenhouse gas emmission but also on the ability to optimize the soil organic matter levels which are severely depleted throughout our agricultural zones on the earth. 

Hey Thomas:

Thanks for clarifying your question and make it more specific. Both Michal and Paul's response are great. I am just adding a little on their insight.

Michal pinpoints the C/N ratio relates to Stoichiometry and nutrient balance, which is exactly the right direction to understand this question. Decomposer (or microbes, either fungi or bacterial) tends to have very high nitrogen and phosphate contents which means they need high concentration of those elements. This is why low C/N ratio (high N proportionally) being picked up first by microbes. Simply, they need it and they like it. 

If you want to gain an more comprehensive understanding, here is a good paper.  “Patterns in decomposition rates among photosynthetic organisms: the importance of detritus C: N :P content. “written by Susana Enríquez and other researcher in 1993. DOI: 10.1007/BF00566960

Paul put a great efforts on explaining the factors affecting soil organic matter process. In addition to soil water content, temperature is also important for mineralization. Both mineralization and immobilization affect the fate of N in soil. 

Again, this is a complex process, and I would suggest you look at from a system view and try to understand the process. For example, what are the sources of soil organic matter, how these are being processed in soil? How long might it take for different substrates? etc. 

Regards

Qiang

Thank you for all these answers and sources gentlemen. I will try to investigate to topics further to get a better idea of the system.

I think I found some interesting elements in this paper :