Unfortunately, most things that prevent Nature from running it's course (like preventing wood in the ground from rotting) aren't really very eco-friendly themselves.
About the most rot resistant plywood you're likely to find is called "PWF" plywood, and it's meant for Permanent Wood Foundations on houses. That is, instead of the house having concrete basement walls, it has wooden basement walls made with PWF plywood.
PWF plywood is considerably more heavily treated than the standard pressure treated lumber of plywood available for above ground use.
If you cut the plywood, be advised that the exposed wood is NOT treated and will rot. So, plan your work so that the factory treated edge goes into the ground. The exposed cut edge can be treated with an end cut preservative.
You should also be aware that various copper naphthenate wood preservatives and end cut preservatives will have varying levels of the active ingredient copper naphthenate in them. The highest I've ever found is 8.0 percent copper naphthenate in Copper II Green from Recochem, but if you Google "8% Copper Naphthenate" you'll find a lot of wood preservatives with that level of copper naphthenate in them.
This PDF file compares the effectiveness of various wood preservatives in prevention Southern Yellow Pine sapwood from rotting when buried in the ground: