Not the right sub, but entirely the right person to be asking this question to.
Like with most soil things, the answer is a mix beteeen "it’s complicated’ and “we don’t know”. This may sound like a cop out, but soil science didn’t really take off until the 1940s (thanks Jenny), so it’s a very new science. On top of that you have multiple disciplines (biology, mineralogy, chemistry, physics, and pedology itself) that tie into soil. Soil itself also has substantial spatial heterogeneity.
Short answer: I don’t know.
**Long answer:**I don’t know, but I suspect it might be more labile in soil.
Here is why:
First thing to consider is the chemical and biological resistance of the plastic. Generally plastics are pretty stable to degradation, but plasticizers are more biologically active and can have all kinds of nasty effects.
Stability will ultimately be determined by feedstock (type of plastic) climatic factors, soil chemistry (primarily pH), organic matter content (high OM -> possibly broader soil Ecosystem -> more chance of some weird soil microbe being able to break things down), and particle size (smaller particles break down faster).
Given that soils have more microbial activity (which can generate weak humic acids too - see podzolification), particle size of the plastics are sand sized or less, and that soils are generally in the pH 5-7 range, it’s likely that they are more mobile and easily degraded versus inert sand. Whether this gets tied back up into soil organic matter is a whole other can of worms.
So yeah. Idk. I think so though.
If you’re comparing micro plastic to just sand in terms of stability - sand is far more stable.