Abstract
Industrially produced N-fertilizer is essential to the production of cereals that supports current and projected human populations. We constructed a top-down global N budget for maize, rice, and wheat for a 50-year period (1961 to 2010). Cereals harvested a total of 1551 Tg of N, of which 48% was supplied through fertilizer-N and 4% came from net soil depletion. An estimated 48% (737 Tg) of crop N, equal to 29, 38, and 25 kg ha−1 yr−1 for maize, rice, and wheat, respectively, is contributed by sources other than fertilizer- or soil-N. Non-symbiotic N2 fixation appears to be the major source of this N, which is 370 Tg or 24% of total N in the crop, corresponding to 13, 22, and 13 kg ha−1 yr−1 for maize, rice, and wheat, respectively. Manure (217 Tg or 14%) and atmospheric deposition (96 Tg or 6%) are the other sources of N. Crop residues and seed contribute marginally. Our scaling-down approach to estimate the contribution of non-symbiotic N2 fixation is robust because it focuses on global quantities of N in sources and sinks that are easier to estimate, in contrast to estimating N losses per se, because losses are highly soil-, climate-, and crop-specific.
Highlights
A powerful approach for assessing the fate of N is to construct an N budget at a global scale[13,14,15]
A two-phase trend in N fertilizer rates was used with a faster rate from X to Y until 1980s followed by a slower rate thereafter, which is consistent with actual trends in the USA and Europe[19]
Of the total 1551 Tg of crop N harvested by the three cereals, the amount of N derived from applied fertilizer-N (NdF) was 746 Tg, with confidence limit (CL) of 717 to 775 Tg (48% of total crop N harvest), and this represented 47% of the 1594 Tg of applied fertilizer-N (Table 1)
Summary
A powerful approach for assessing the fate of N is to construct an N budget at a global scale[13,14,15]. We construct global cumulative N budgets for 1961 to 2010 for the production of maize, rice, and wheat from data on crop production area, yield, and N content of harvested grain and straw, fertilizer-N input, and changes in soil-N reserves (Table S1). Changes in N reserves in soil, mostly in the soil organic nitrogen (SON), are difficult to estimate because of the lack of a global database We base these latter estimates on directly measured changes in soil-N in 114 long-term field experiments that are representative of the world’s major cereal production systems. These long-term experiments are located in 21 countries that account for 67% of the global production of the three cereals[18]
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