Please check the following link and PDF attachment.

http://www.cifor.org/library/934/impact-of-logging-and-forest-fires-on-soil-erosion-in-tropical-humid-forest-in-kalimantan/

Dear Tommaso,

Forest fires have a direct impact of reducing vegetation cover, and in many cases create a pre-condition for the increase of soil erosion rates during high rainfall. Such an increase in soil erosion rate may be higher than the normal threshold rate from a sustainable forest land productivity viewpoint.

Reference: Sundarmadji, T (2001) Impact of logging and forest fires on soil erosion in tropical humid forest in Kalimantan. Pages 35-44 In Kobayashi, S., Turnbull, J.W., Toma, T., Mori, T., Majid, N.M.N.A. (eds.) Rehabilitation of degraded tropical forest ecosystems: workshop proceedings, 2-4 November 1999, Bogor, Indonesia. Published by CIFOR, Bogor, Indonesia in 2001. ISBN: 979-8764-70-6.

I hope you find this input useful.

Dlip Kumar's is quite logical. However, as other colleagues suggested, this is a very good topic of research.

Such instances of forest fires destroy vegetation and it has positive correlation with soil erosion rates during high rainfall. Such an increase in soil erosion rate may be higher than the normal threshold rate from a sustainable forest land productivity viewpoint.

Vegetation has an important role to reduce erosion specialy in locality with high slops, such as the mediterranean forests. So, if this vegetation is destroyed by fires and the frequence of fires is important in the same region, we have a strong erosion observed after each rainfall, specialy after summer rainfall.   

it for sure is site specific as already explained.

Yes Seema, it is highly site specific but need some data to support this view. Therefore, good research is required on this topic.

It is quite obvious, forest fire may increase soil erosion in due course.

The topic has been endlessly studied and whether or not erosion will occur will depend on a number of factors: fire behaviour, fire severity, vegetation type, terrain slope, soil texture, postfire rainfall, ...

You can start by reading some reviews of the literature on post-fire soil erosion:

Bento-Gonçalves, A., Vieira, A., Úbeda, X., & Martin, D. (2012). Fire and soils: key concepts and recent advances. Geoderma, 191, 3-13.

Moody, J. A., Shakesby, R. A., Robichaud, P. R., Cannon, S. H., & Martin, D. A. (2013). Current research issues related to post-wildfire runoff and erosion processes. Earth-Science Reviews, 122, 10-37.

Shakesby, R. A. (2011). Post-wildfire soil erosion in the Mediterranean: review and future research directions. Earth-Science Reviews, 105, 71-100.

Vieira, D. C. S., Fernández, C., Vega, J. A., & Keizer, J. J. (2015). Does soil burn severity affect the post-fire runoff and interrill erosion response? A review based on meta-analysis of field rainfall simulation data. Journal of Hydrology, 523, 452-464.

Williams, C. J., Pierson, F. B., Robichaud, P. R., & Boll, J. (2014). Hydrologic and erosion responses to wildfire along the rangeland–xeric forest continuum in the western US: a review and model of hydrologic vulnerability. International Journal of Wildland Fire, 23, 155-172.

In general, soil without vegetation are often eroded easily. If a fire is able to remove the vegetation of that site that obviously accelerate the erosion process but also depend on post fire rainfall and strong wind as well. Also, fire intensity and behavior need to counted. Intensive fire can remove whole vegetation and increase erosion. Surface fire may enhance erosion than crown fire. Also, fire in grassland or shrub land can be more eroded than forest fire. Hence, to make a correlation you must consider all these factors mentioned by Paula Fernandes too.

According to our experience the use of fire as a cultural tract as well as unintentional fires are predisposing to the occurrence of erosion. Certainly soil damage levels are dependent on other factors but the fact is that soil exposure to rainfall by eliminating protective vegetation is an important factor. On the other hand as the ashes are hydrophobic they reduce the infiltration of water in the soil and compete to increase the superficial flow that is precursor of erosive processes of the soil.