It is, technically, but with caveats. It has to be buried deeply enough that any gasses created by decomposition won't be released or escape on their own.
More worrisome is that, done at a scale that would matter to the atmosphere, it would have to be buried in containers that are impermeable to water forever, lest all of our underground aquifers become contaminated by bacteria and heavy metals that are naturally occurring in plants.
More worrisome is that, done at a scale that would matter to the atmosphere, it would have to be buried in containers that are impermeable to water forever, lest all of our underground aquifers become contaminated by bacteria and heavy metals that are naturally occurring in plants.