Here are some potential causes of our odoriferous joy:
- ozone from a thunderstorm,
- petrichor (click here to read multitudinous gargantuan technical words), and
- geosmin, which is produced by the Streptomyces bacteria, a genus within the group actinomycetes!
Besides that wonderful smell, there are other reasons why actinomycetes are awesome. I imagine you've heard that bacteria such as rhizobium help legumes obtain nitrogen from the air. Well, the actinomycetes genus Frankia helps non-legumes fix nitrogen from the air. This means that they can help plants colonize infertile and newly forming soils. Also, many actinomycetes can digest what others can't. Those hard to decompose substrates such as chitin and cellulose are prime picking for these guys. That is great news for compost with sawdust and branches!
Want more actinomycetes? They can tolerate arid and salt-affected soils. They work best in soils with a pH between 6.0 - 7.5, but are more sensitive to acidic soils. Don't worry too much about acidity though, since fungi can take over the decay where actinomycetes leave off (info also from reference #1).
Also, this great article says that daily turning of compost, while good for some purposes, can inhibit growth of actinomycetes. Although actinomycetes can either be mesophilic* or thermophilic*, they are mostly known to work in the later, cooler stages of compost creation. If you see something that looks like gray spider webs in the outer edges of your compost, it just might be actinomycetes. Please remember to say, "thank you!"
Please enjoy your "rain smell" responsibly.
Mesophilic: microbes that function at temperatures above 113 °F
Thermophilic: microbes that function at temperatures between 50-113 °F (10-45 °C) (ref)