It's not any one conquest, though the Muslim Caliphate did damage because it had a politcy of cultural destruction and Mongols because they were big on destroying resistance and actual... stuff. Then you have the Christians, who stated killing philosophers around 300AD or so, burning books and generall being a nuissence.

Overall that region has been fought over and screwed over since around 200 AD at best. Each invasion takes a bit away and more is taken away the harder it is to recover. I though it was salinification, but either, way it's a process I believe the Mesopotamians were aware of. Spreading ash and ground bone onto land to increase pH is as old as the hills. I imagine you pick those tricks up when people keep deliberately salting you cropland.