Fertilizer Nitrogen in Global Crop Production

The most accurate way to calculate the nitrogen taken up by the global crop harvest is to multiply the output of every major crop by the nitrogen contents of harvested parts (grains, stalks, leaves, roots, fruits). Global crop harvests of the mid-1990s amounted to about 2.65 Gt of dry matter per year, and they contained 50 Mt N (appendixes O and P). Cereals accounted for 60% of total dry crop 

Quantification of nitrogen incorporated by crop residues is not as straightforward, as no country keeps statistics on their production. My calculations resulted in the most likely total of 3.75 Gt of dry residual phytomass. Cereal straws, stalks, and leaves accounted for % of all residual phytomass, and sugarcane tops and leaves were the second largest contributor. As already explained in chapter 2, substantial inter- and intraspecific variations in nitrogen content result in a rather broad range of the nutrient incorporated annually in crop residues.  

Planted phytomass (seeds and roots) and irrigation water are two minor sources of nitrogen, the first one returning annually only about 2 Mt N, the second one contributing about twice as much. Nitrates (mainly from oxidation of NO released by combustion of fossil fuels) and volatilization of NHj (from animal wastes, soils, and plant tops) are the two largest sources of fixed atmospheric nitrogen, whose total annual deposition on the world s agricultural land is about 20 (18-22) Mt N.  

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