I'm having a little bit of difficulty picturing your setup. It sounds like your raised beds should be excellent--compost, manure, and topsoil. If the flat area doesn't drain well, you can till in a number of ammendments to loosen it -- much like the raised beds you made before.

I wouldn't till in uncomposted oak leaves, since they are 'browns' and would suck the nitrogen out of the soil as they decompose.

The most important thing with iris, I think, is let the rhizome see the sun (no mulch), keep the bases cleaned up of dead leaves, etc. and avoid heavy nitrogen fertilizers that encourage rot.

Good luck!

(You can hit the iris forum for better advice)

High Carbon materials do not "suck" Nitrogen from your soil but if that high Carbon material is worked into the soil the soil bacteria will get busy digesting it and they will utilize all the available Nitrogen to do that and concentrate on digesting that material and not on feeding your plants. But once they die off that Nitrogen they used will be put back into your soil for future use.
Since my sandy soil, in the higher areas, drains so well I use shredded oak leaves as a mulch on the beds the Iris are planted in, but not in the lower areas where the high water table has soil moisture nearer the surface, until in mid July (usually) when the water table drops way below the roots zone.

Mulch or unfinished compost might promote rot & borers in iris.

I'd post the same question on the Iris Forum;
there are a lot of very knowledgeable & very helpful iris growers over there.

Thank you all very much for your input.
Kay

I don't grow Irises, but I do use oak leaves for mulch on all of the plants that I do grow. Including bulbs like daffs and tulips...

But do think mulches and bulb type plants may have rot issues depending upon the climate that you live in...

I probably would not till the leaves in, but rather use htem for mulch...mainly because that is what I do with everything else.

I second sylvia's suggestion to check the iris forum.

kimmsr is right, it is bacteria that do the actual sucking. ;) And it's also true that when the leaves have left, and the bacterial population recedes (dies), their little bacteria bodies will return that nitrogen back to the soil. That's my small understanding of the Soil Food Web.

It takes a while for this digestion, and if you're going to plant iris now, that nitrogen would be unavailable to your iris.

I do grow some iris, so my other advice is pretty sound.

Yes, you do need to know about your soil. Mulches on clay soils can, if that clay soil is not well amended with organic matter and that clay soil does not drain well, can hold too much moisture in the soil and can cause rot. But if you have a well drained soil, whether it is clay or sand, mulches will not cause rot, whether the mulch is shredded Oak leaves or other tree leaves.