Nitrogen in crop production: An account of global flows

Human activities have roughly doubled the amount of reactive N that enters the element's biospheric cycle. Crop production is by far the single largest cause of this anthropogenic alteration. Inorganic fertilizers now provide 80 Tg N yr −1 (Tg = 10 12 g), managed (symbiotic) biofixation adds about 20 Tg N yr −1 , and between 28 and 36 Tg N yr −1 are recycled in organic wastes. Anthropogenic inputs (including N in seeds and irrigation water) now supply about 85% of 170 (151–186) Tg N reaching the world's cropland every year. About half of this input, 85 Tg N yr −1 , is taken up by harvested crops and their residues. Quantification of N losses from crop fields is beset by major uncertainties. Losses to the atmosphere (denitrification and volatilization) amount to 26–60 Tg N yr −1 , while waters receive (from leaching and erosion) 32–45 Tg N yr −1 . These N losses are the major reason behind the growing concerns about the enrichment of the biosphere with reactive N. The best evidence suggests that in spite of some significant local and regional losses, the world's agricultural land accumulates N. The addition of 3–4 billion people before the year 2050 will require further substantial increases of N input in cropping, but a large share of this demand can come from improved efficiency of N fertilizer use.