Nitrogen fixation maybe? Lightning breaks N2 so it bonds with O2 into NO2. Rain then brings it to the soil to be taken up by plants. But lightning does this at

Nitrogen fixation maybe? Lightning breaks N2 so it bonds with O2 into NO2. Rain then brings it to the soil to be taken up by plants. But lightning does this at a much smaller scale than microorganisms do, so maybe not too important.