Synthetic fertilizers provide about 40 percent of all the nitrogen taken up by [cultivated plants]. Because they furnish—directly as plants and indirectly as animal foods—about 75 percent of all nitrogen in consumed proteins (the rest comes from fish and from meat and dairy foodstuffs produced by grazing), about one third of the protein in humanity’s diet depends on synthetic nitrogen fertilizer.
While the article includes references to four books/articles as "further reading," this is an interesting and uncited assertion. I assume "depends on" to mean that the nitrogen in a third of the protein in humanity's diet is fixated through synthetic, industrial processes. In other words, "derived from" synthetic processes.
At first glance, 33 percent is such a large number! How can humanity be so dependent on an "artificial" chemical processes? Then with more thinking, the number is not obviously wrong. And then with even more thinking, it's a small number! Only 33 percent? I always thought synthetic fertilizers were more significant to mankind. They are everywhere and reporting is consistent on its negative effects (environmental damage from runoff, etc.).
A separate journal article states:
Now, over 80% of the nitrogen in the average human body originates from the Haber–Bosch process (Howarth 2008).
As the Haber-Bosch process is used to (synthetically) produce ammonia for plant fertilizers, I would assume that the above statement is equivalent to
80% of the nitrogen in humanity's diet is derived from synthetic nitrogen fertilizer. (my own inference)
And, according to the Royal Society of Chemistry's General Briefing Sheet (emphasis added):
Nitrogen is an important part of your DNA, which defines what you are like in many ways. We cannot survive without nitrogen in our diet – we get [nitrogen] in the form of protein.
So, I thus refined my earlier statement
80% of the protein in humanity's diet is derived from synthetic nitrogen fertilizer. (my own inference)
This statement does not agree with Smil's statement, yet it seems to reflect cited science.
Do "one third (33%) of the protein in humanity’s diet depend on synthetic nitrogen fertilizer?" 80%? Some other number?
Of course, this number may have changed between July 1997 (Smil article) and December 2008 (Howarth article). I am interested in Smil's statement.
Note (based on comments):
@sumelic correctly points out:
Although the quotation uses the word "depends", it might be easier to avoid opinion if you used a different phrasing such as "Is the nitrogen in a third/80% of the proteins we consume derived from synthetic fertilizer?"
Hence, the question is more simply stated as "Is 33% of the protein in humanity's diet derived from synthetic nitrogen fertilizer?"
If the nitrogen is fixated through rhizobia, it is "natural."
If the nitrogen is fixated through the Haber-Bosch process, it is "synthetic."
If some corn is grown using synthetic fertilizer, a cow eats the corn, and a human eats the beef, the nitrogen in the beef proteins is "synthetic." (credit @Jan)
@Jan also notes: "Currently vegetal sources of protein dominate protein supply globally (57%), with meat (18%), dairy (10%), fish and shellfish (6%) and other animal products (9%) making up the remainder." (source)