When I go to Europe, the drinks on the flight there are sweetened with HFCS. On the way back, they are sweetened with cane sugar. "Corn in everything" is mostly and American thing. (And we're so healthy because of it!)
But the OP said that the amount of naturally occurring Nitrogen is enough to feed 10bn people if we move towards a more vegetarian diet. That is, no need for artificial nitrogen.
Which brought me to my question that Corn is the #1 direct consumer of nitrogen. So unless livestock is consuming an awful lot of Corn, reducing the livestock headcount isn't going to reduce the amount of Corn per se.
That's probably true, as the corn industry, at least in the US, will find another consumer for its quantities (since, if Pollan's research is to be believed, the corn industry and the government essentially created and supported the market that exists now).
If you are growing vegetables though, you can easily just rotate your crops year by year between crops that fix nitrogen into the soil (like peas, beans, clover, grass) and other crops that take nitrogen (like corn, berries, etc).
"Legumes are an example of a nitrogen fixing plant. These should be rotated with your vegetable crop in order to keep a proper amount of nitrogen in your soil. Crops most often paired by farmers are rice and cotton, and soybeans and maize. These crop pairings are essential for the continuing health of the soil."