Please have a look at this link PDF attachment.

https://www.researchgate.net/figure/223674572_fig2_Fig-2-Distribution-of-soil-organic-carbon-SOC-and-total-nitrogen-TN-concentrations

I think it is not very different from respective values in moderate climate soils.

As average 10 grams of carbon (SOC - soil organic carbon) per 1 gram of N (TN - total nitrogen), but it may vary in greater range.

I base this informaton on short looking at

Santos et al.2015. Manual de descricao dos solos e coleta do solo no campo, 7a edicao (4 examples of profile descriptions, C:N ratio between 11 and 16 to 1) and

Cochrane et al. 1982. Land in tropical America, Vol. 3. There are many profile descriptions including results of analyses (here the variations of C:N ratio in A1 horizon are much greater, but generally and most frequently about 10:1).

Best regards

Please have a look at this link PDF attachment.

https://www.researchgate.net/figure/223674572_fig2_Fig-2-Distribution-of-soil-organic-carbon-SOC-and-total-nitrogen-TN-concentrations

Dear Agevi,

Mean C:N ratio of the soils ranges from 9.9 for arid Yermosols to 25.8 for Histosols. For details, I suggest you to read the Landmark Paper No. 3 entitled ''total carbon and nitrogen in the soils of the world'' by N.H. Batjes published in European Journal of Soil Science 65(1):10-21 in January 2014.

Hope you find this input interesting and useful. 

Look the Links.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4338072/

www.tandfonline.com/doi/pdf/10.4155/cmt.13.77

www.ccsenet.org/journal/index.php/jsd/article/.../29014/18554

https://su.diva-portal.org/smash/get/diva2.../FULLTEXT01.pdf

C:N ratio of a stable soil with wel decomposed organic matter normally 10:1. This ratio may be wider with freshly added organic materials like rice straw, sugarcane trash etc. C:N ratio of tropical soils are normally less than temperate soil due to rapid decomposition of OM in the tropic.

I worked in Arid Soils (Aridisol). The C:N ratio here varies between 10:1 to 13.5:1. Under well decomposed condition you may find between 10 and 12:1.

Dear Humphrey -  Some limited data are available. Please refer to the attached pdf.