The people of ancient egypt and nubia were completely isolated from others because of the surrounding desert. is it true or false

That statement is true
A lot of countries prefer not to invaded ancient Egypt and Nubia because of their harsh Topography. They did not want to lose so much resources to invade those countries and obtaining so little in return ( you cannot grow your crops and wheat in the desert)

Indeed it is true - if you look at any map, you will see that Egypt had a natural wall in the form of the desert through which no army wanted to pass due to the sheer issues of logistics it would take if one would decide to go through. Such a journey would massively undermine their desire to take Egypt. 

Clean air provides nitrogen