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.

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The issue of carbon (C) neutrality and the environmental advantages and variations in soil organic C (SOC) stocks under biofuel crops need to be addressed thoroughly and objectively. Thus, the aim of this study was to compare the impact of annual biofuel crops (no-till maize, Zea mays L.; sorghum, Sorghum bicolor L.) and perennial lignocellulosic grasses (switch grass, Panicum virgatum L.; miscanthus, Miscanthus  giganteus; and prairie mix) on soil properties and SOC stock in central Ohio. Two years of perennial energy crops improved soil properties in terms of lower soil bulk density, higher porosity, improved water-stable aggregates (WSA), higher mean weight diameter, pH and electrical conductivity compared with those under maize and sorghum. The WSA in the 0-10 and 10-20 cm soil layers were higher under miscanthus (94.7% and 91.8%, respectively) and switch grass (92.7% and 89.4%) than under maize (89.9% and 86.1%) and sorghum (85.1% and 85.4%). Macroaggregates (>0.25 mm diameter) contained higher concentrations of C and nitrogen (N) than microaggregates. Macroaggregates in soil under sorghum and maize contained 17.3% and 14.2% less C and 22.8% and 15.2% less N in 0-10 cm layer, and 29.8% and 24.2% less C and 22% and 7.1% less N in 10-20 cm layer, than macroaggregates under switch grass (15.82 g C kg -1 in 0-10 cm and 14.06 g C kg -1 in 10-20 cm layers), respectively. The SOC stock in the 0-10 cm layer, on an equivalent soil-mass basis, was significantly higher under switch grass (28.5 Mg C ha -1 ) and miscanthus (28 Mg C ha -1 ) than that under sorghum (24.8 Mg C ha -1 ). Thus, only switch grass and miscanthus sequestered C, whereas other species had no or negative effect, with loss of soil C under sorghum in 2 years. There is a need for long-term studies and estimation of SOC stock in deeper layers to establish the SOC balance under biofuel crops.