Break in human nitrogen input in 1990s
First, don't take that graph too literally, it is just a cartoon version and isn't quantitatively correct.  

The USSR heavily subsidized fertilizer until 1988.  When they ended the subsidy, fertilizer prices increased to market prices and use of fertilizer decreased.  Fertilizer use decreased further after the USSR fell a couple years later. 

Fertilizer use in France, Germany, UK and Japan peaked in the 1980s and contributed to the decline.