The polyacrylates used to absorb urine are not particularly biodegradable, but they are widely used as a soil amendment, because the water they absorb remains available to plants.
Unfortunately, I think most of the diaper formulations contain sodium, but unless salt is a major problem, it will probably be OK in the soil.
Sodium polyacrylate in action, in diapers it's mixed with paper fluff to distribute moisture better.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.