Nitrogen Losses in Modern Farming

Long-term experiments with winter wheat on Broadbalk field at Rothamsted (fig. 9.3) show fairly steady recovery shares for more than a century of cropping, between 

Aerial view of Broadbalk field at Rothamsted (Hertfordshire), the site of the world s longest continuous agronomic experiments. Courtesy of Rothamsted Experimental Station, Harpen- 

Partitioning of nitrogen fertilizer applied to a rice field. 

We may thus conclude with a high degree of confidence that half of all the nitrogen added annually to the world s croplands is lost from the world s agroecosystems, most of it before it can get incorporated in the harvested biomass. Apportioning this loss among the handful of major causes cannot be done accurately, but even an imprecise attempt is useful as it reveals at least the relative magnitudes of the principal flows, and hence the potential for particular environmental impacts. 

The most likely total denitrification flux—the aggregate of N2, N2O, and NO— would then be around 20 Mt N/year, but uncertainties surrounding these emission estimates mean that the actual rate may be as low as 13-15 and as high as about 30 Mt N/year. Obviously, much more N2 is eventually returned from agroecosystems to the atmosphere by denitrification of nitrogen compounds that were removed from fields by leaching and soil erosion and carried away in harvested feed and food crops. 

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