The Most Productive Agroecosystems

Erench, Dutch, and English winter wheats, whose yields average close to 8 t/ha, receive close to 200 kg N/ha from synthetic fertihzers, and 60-80 kg N/ha from recycled crop residues, biofixation and atmospheric deposition. Fertilizer nitrogen is thus the source of 70-75% of the total supply. With corn monocropping there is only a small input of nitrogen from free-living diazotrophs. Atmospheric deposition adds much less than 10 kg N/ha in the driest parts of the Corn Belt, and where no manures are applied, crop residues are the only substantial nonfertilizer input of 

Maximum population densities supportable by vegetarian diets would also be quite high in North America and Western Europe, but these highly carnivorous societies now use a great deal of fertilizer nitrogen in order to support that particular dietary habit. Naturally, this reality must be taken into account when trying to estimate the extent of the world s existential dependence on Haber-Bosch ammonia synthesis, the principal goal of the next chapter. 

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