Petrichor (n.)
What is Petrichor?
You know I love weird words. Well, today’s is Petrichor. It’s the pleasant, earthy smell you notice after rain.
Petrichor is the scent produced when rain falls on dry soil. The word is constructed from Ancient Greek πέτρα ‘rock’, or πέτρος ‘stone’, and ἰχώρ, the ethereal fluid that is the blood of the gods in Greek mythology. The human nose is very sensitive to geosmin – a chemical (C12H22O) released by dead microbes (commonly Streptomyces bacteria) – and we are able to detect it at very low concentrations. Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival. Ok, so now I want to know more about geosmin! 
So what is Geosmin?
Well, though our noses get dismissed as amateur hour compared to some animals like dogs, there is one compound where we do really well; we can smell geosmin, which causes that earthy smell, at a level of just 5 parts per trillion. That’s right, trillion.
We talk about the scent of newly fallen rain but it’s not the rain itself that smells, of course.
It’s the interaction between rain and the soil that releases volatile compounds into the air.
Best known of these compounds is geosmin, meaning ‘earth odour’. Soil microorganisms make it, then release it into the soil when they die. So its dead aminals in the soil, right? Right. But it’s not until it rains that geosmin is aerosolised and duly wafts up our noses. 
Our bodies have evolved to be highly sensitive to geosmin and as I say we can pick up just a few molecules a trillion. Why would that be? That’s easy. Because where there’s geosmin, there’s water.
We can live without food for days. But we can live without water for much shorter periods. So think of early humans searching for a drink in a parched landscape and you understand why having a nose for geosmin was crcuial for our survival as a species.
On a more frivolous note, if your perfume smells earthy, chances are that synthetic geosmin is one of its ingredients.
Rain on a dry garden brings on many more changes, some more obvious than others. If conditions are right, seeds can germinate (those dreaded weed seeds too) and the grass grows some more before slowing down or stopping over winter.
But perhaps less well known is how rain affects carbon in your garden.
What do we need to know about rain and carbon?
How carbon is held and released from the soil (carbon cycling) is one of Dr Yolima Carrillo’s research topics at the Hawkesbury Institute for the Environment at the University of Western Sydney.
She says that the drying and re-wetting of the soil, for instance with autumn rains after a dry summer, influences this carbon cycling, which in turn affects the soil’s fertility and its ability to hold water and nutrients.
A dump of rain after a dry spell kickstarts the breakdown of soil microorganisms that didn’t survive the summer, Carrillo says. That releases a sudden pulse of carbon dioxide into the atmosphere within minutes or hours of the downpour.
Then there are the soil micro-organisms that survived the dry spell but are ‘shocked’ by the sudden rain. These die, again releasing more carbon dioxide into the air.
Finally, she says water mobilises carbon and nutrients in the soil, making it available in places where it wasn’t before.
“For example, if you have dead plant material on the surface of your garden, like fallen leaves, when it’s dry, the carbon has nowhere to go. But when it rains, carbon and nutrients flow down through the soil, making them available to micro-organisms that did survive the dry conditions,” she says. Again, as these survivor microbes get to work, more carbon dioxide is released into the air.
Why does this matter? Well, Carillo says that thinking about carbon in your garden is important year round.
“It’s important to know that people’s gardens have an impact on the carbon cycle,” she says, recommending gardeners boost the organic content of their soils by adding compost, or shredding and spreading healthy plant clippings or prunings around the backyard.
Adding carbon this way also improves the soil’s ability to hold moisture, she says, meaning less watering is necessary.
Another tip is to avoid unnecessary digging in the garden, she says, a practice borrowed from no-till farming. Over-working the soil, she says, not only disturbs its structure but affects its ability to hold onto its carbon.
“How people garden impacts on carbon in the atmosphere, an important driver of climate change,” she says. “I don’t think we’re aware that soil is such an important bank of carbon. We all think about trees and forests. Soil is a little less sexy.”
So now you know so much about Petrichor and carbon and geosmin. Put down that garden spade and just stand out there and take a big sniff.
I hope you enjoyed this little blog. jenie xx
