I don't think it is acceptable to calculate N from CN ratio because this ratio differs based on the organic matter source, in addition I dout that this method will be accepted for publishing.

The C percentage in soil organic matter is not a constant. In some soils there is 50% C, while in others it may be higher. This depends on the relative contributions of alkanes/alkenes (cutan, subera; high-C), lignins, and carbohydrates (low C). The C/N ratio is even more variable. In litters it may be above 50, while in some tropical soils it is as low as 6 or 7.

If you are doing a meta-analysis, you should group your soil samples and find an acceptable C:N:P:S for each group. If you just want to get the C, N , P  and S concentrations of your soil samples through an easy way and not by doing the analytical work, you should look for the ratios in the papers with particular reference to the soils you are examining now. Anyway, I agree with Heba that data generated this way is not reliable and acceptable for publication.

Nothing for it but to do measurements.  There are instruments that will simultaneously measure C, N and S after  high temperature combustion.  If you have large numbers (thousands) of similar soils then you might consider doing the chemical analysis on say 1000 and using the data for 500 to calibrate an IR technique then testing the quality of the prediction on the remaining 500. 

Interesting discussion . Find below an interesting work , that may help you in finding some solution to your question...

[Stoichiometric characteristics of plant and soil C, N and P in different forest types in depressions between karst hills, southwest China].

Ying Yong Sheng Tai Xue Bao. 2014 Apr;25(4):947-54.

Abstract : The stoichiometric properties of plant carbon (C), nitrogen (N) and phosphorus (P) and their relationships with soil were studied in six dominant plant communities in three forest types, i.e., plantation forest, secondary forest and primary forest in depressions between karst hills, southwest China. The C, N and P contents of both plant and soil had significant differences among the different forest types. Soil C and N contents were the highest in the secondary forest and the lowest in the plantation forest. Soil P content was the highest in the plantation forest and the lowest in the primary forest. Plant C and P contents were in the order of plantation forest > primary forest > secondary forest, and plant N content was the highest in the plantation forest and the lowest in the primary forest. Soil N:P,C:P and plant C:P ratios were significantly higher in the primary forest than in the other two forest types. There were no significant difference for the soil C:N ratio among the three forest types. Plant N:P ratio was the highest in the secondary forest and the lowest in the plantation forest. Plant C:N ratio was in the order of primary forest > plantation forest > secondary forest. There were significantly positive linear correlations between N and P contents, C:N and C:P ratios, C:P and N:P ratios of arbor leaves in the different forest types, and significant negative linear correlations between plant C:N and N:P ratios, and between soil C:N and N:P ratios. There were no significant correlations between plant and soil C, N, P contents and C:P ratio, suggesting that the supply of C, N and P from soil had little influence on plant C, N and P contents. source ;PMID: 25011284

Deep Soil C, N, and P Stocks and Stoichiometry in Response to Land Use Patterns in the Loess Hilly Region of China by Changzhen Li, Luhong Zhao, Pingsheng Sun, Fazhu Zhao, Di Kang, Gaihe Yang , Xinhui Han , Yongzhong Feng, Guangxin Ren, Published: July 14, 2016https://doi.org/10.1371/journal.pone.0159075

Abstract : In the Loess Hilly Region of China, the widespread conversion of cropland to forestland and grassland has resulted in great increased in organic carbon (C), nitrogen (N) and phosphorus (P) stocks in the shallow soil layers. However, knowledge regarding changes in C, N, and P in deep soil is still limited. To elucidate the responses of deep soil C, N, and P stocks and stoichiometry in response to changes in land use, the soil from a 0–200 cm soil profile was collected from the following three typical land use patterns in the heartland of the region: forestland, grassland, and cropland. Compared with cropland, forestland and grassland had improved soil organic carbon (SOC) and total nitrogen (TN) contents and stocks at most soil depths but decreased total phosphorus (TP) contents and stocks. At soil depths of 0–200 cm in the forestland and grassland, the cumulative SOC stocks were improved by 34.97% and 7.61%, respectively, and the TN stocks were improved by 54.54% and 12.47%, respectively. The forestland had higher SOC, TN and TP contents and stocks compared to the grassland in almost all soil layers. The soil depths of 100–200 cm contained the highest percentages of SOC, TN and TP stocks (47.80%–49.93%, 46.08%–50.05% and 49.09%–52.98%, respectively). Additionally, the forestland and grassland showed enhanced soil C:P, N:P and C:N:P ratios, and the forestland had higher C:P, N:P and C:N:P ratios compared to the grassland. Furthermore, the SOC and TN stocks had significant impacts on the soil C:N, C:P and N:P ratios. It was concluded that afforestation was the best choice for soil nutrient restoration of degraded land, and deep soil provided an extremely important resource for evaluating soil C, N and P pools and cycling.  

Mr. Hanadi  Ennab, I think,  that this article will be useful for you:

Kirkby, C. et al. Stable soil organic matter: a comparison of C: N: P: S ratios in Australian and other world soils. Geoderma. 163, 197–208 (2011).

Kirkby, C. A. et al. Carbon-nutrient stoichiometry to increase soil carbon sequestration. Soil Biol Biochem. 60, 77–86 (2013).

Barbhuiya AR, Arunachalam A, Pandey HN, Arunachalam K, Khan ML, et al. (2004) Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest. European Journal of Soil Biology 40: 113–121.

Sharma P, Rai SC, Sharma R, Sharma E (2004) Effects of land-use change on soil microbial C, N and P in a Himalayan watershed. Pedobiologia 48: 83–92.

Rui, Y. et al. Microbial respiration, but not biomass, responded linearly to increasing light fraction organic matter input: Consequences for carbon sequestration. Sci. Rep. 6, 35496; doi: 10.1038/srep35496 (2016)

Sincerely, Zenon

I think you should measure it and there is no stable ratio for it. And depend on many things for example N to P ration depend on the uptake of the living organisms from them and also on the organic content of the soil.

Regards

Dear Hanadi Ennab,

Lal (2008) has suggested that C:N:P:S ratio in the humus is 10,000:833:200:143.

Ref: Lal R. (2008) Promise and limitations of soils to minimize climate change. J SOIL WATER CONSERV, 63, 113–118.