@ Ajeet, there is no good correlation between the organic carbon % and available N content in the most of the soils. Therefore, hardly any reliable factor or formula is developed on it for conversion.
Availability indices of a nutrient element can predict the possibility of the response of a crop to the application of the nutrient through fertilizer. There hardly exists any index for available nitrogen in the soil. Therefore, it is not valid to convert organic carbon into an availability index for nitrogen in the spoil.