Not experince, but the suffer dioxide will most likely turn into sufuric acid on contact with water. Weather it will be enough to to harm your roots is the question.
Either cover the roots and burn, and or do a flood right after and check the ph of the res. If your res doesn't drop in ph it was probly a non issue.
Covering your res may be a way to get your RH down a bit if that is part of your PM issue.