Reason for rotting smell after first rain of the season?
When rain displaces pore gasses in the soil, bubbles of soil gas are released into the atmosphere. It's impossible to be precise about the composition because it is so variable from place to place. It varies according to soil type, organic content, degree of rotting, temperature, and the microbiological cross section (itself hugely variable). Most soil gas is odourless, such as nitrogen, carbon dioxide, methane, etc. Others, like geosmin, are very smelly. In addition  to 'petrichor' there are terpenes, aldehydes, alcohols and esters, to name but a few of the natural volatile organic compounds.
Of course, one cannot rule out the foul smell or rotting animals in some places.