Carbon Cost of Applying Nitrogen Fertilizer

When the addition of nitrogen (N) fertilizer leads to increased crop biomass, it also augments carbon (C) inputs to the soil and, hence, often increases soil organic matter. Consequently, the efficient use of fertilizer N to increase crop production has also been found valuable for sequestering atmospheric carbon in soil. William H. Schlesinger, however, in his Policy Forum "Carbon sequestration in soils" (Science's Compass, 25 June 1999, p. 2095) analyzes results from a 20-year experiment in Kentucky on conventional-till and no-till corn (1) and concludes that "the full carbon cost of N fertilizer…would effectively negate any net carbon sink as a result of the application of the fertilizer." These costs include the CO2-C emitted during fertilizer manufacture, storage, transport, and application. The three carbon cost factors (moles of CO2-C emitted per mole of N applied) documented by Schlesinger are 0.375 (stoichiometry of Haber-Bosch reaction), 0.58 (carbon cost of fertilizer manufacture) (2), and 1.436 (carbon cost of fertilizer manufacture, storage, transport, and application) (3, 4).