~0.2 Tg N/yr
||Galloway et al., Nitrogen Cycles: Past, Present, and Future,
September 2004, Volume 70, Issue 2, pp 153-226 p.156 top paragraph
||Smil V. 2000. Feeding the World: A Challenge for the Twenty-First Century. MIT Press, Cambridge, MA.
||"The only sources for ‘new’ N
at that time ([the beginning of the 20th century,] in addition to cultivation) were guano deposits on arid islands and evaporite nitrate deposits in South America (e.g., Chile), which supplied about
0.2 Tg N/yr (primary source)."