Rhizosphere: the patch of soil a root shapes for itself
In short
The Rhizosphere is the narrow zone of soil immediately around a living root. It differs from bulk soil because the plant releases carbon compounds there and so feeds a denser and different microbial community. The same earth, a few centimetres further out, behaves differently.
Why things happen there that otherwise do not
The difference is measurable. In the hemp field trial, four of seven only tentatively identified PFAS compounds fell by more than 20 % in the planted plot, while they did not in the unplanted control. The authors suspect microbial breakdown of precursor compounds in the root zone, but with only two replicates they could not test it statistically. The finding therefore stands as an indication rather than proof.
The picture is clearer for dioxins. At a contaminated site in northern Italy, tall fescue reduced soil dioxin and furan levels most among seven species tested, by 11 to 24 % over 18 months, and did so by stimulating microbial breakdown rather than by taking the substances up.
How this differs from removal with the harvest
Both routes get called plant-based clean-up and do opposite things. With phytoextraction the substance leaves the site in the harvest and has to be disposed of afterwards. In the rhizosphere it is dismantled and never leaves the site at all. For PFAS that distinction decides the case, since plants and their associated bacteria generally leave the bond between carbon and fluorine intact, so complete breakdown is not to be expected there.
Link to cannabis
Hemp roots deep and fast, which enlarges the root zone. What happens to contaminants there has barely been studied for this species, and the only indication comes from a trial with two replicates. What measurably passes into the plant is covered in the article on contaminant uptake in hemp.