Galloway, J. N., W. H. Schlesinger, H. Levy II, A. Michaels, and J. L. Schnoor, 1995: Nitrogen fixation: anthropogenic enhancement-environmental response. Global Biogeochemical Cycles, 9(2), 235-252.

Abstract: In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90-130 Tg N yr^-1 (Tg = 10¹²g) on the continents. Human activities have resulted in the fixation of an additional ~140 Tg N yr^-1 by energy production (~20 Tg N yr^-1), fertilizer production (~80 Tg N yr^-1), and cultivation of crops (e.g., legumes, rice) (~40 Tg N yr^-1). We can only account for part of this anthropogenic N. N₂O is accumulating in the atmosphere at a rate of 3 Tg N yr^-1. Coastal oceans receive another 41 Tg N yr^-1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr^-1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO₃, N₂). The remaining 80 Tg N yr^-1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N₂. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N-fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil-fuel combustion. About two-thirds of the increase will occur in Asia, which by 2020 will account for over half of the global anthropogenic N fixation.