NITROGEN-FIXATION - ANTHROPOGENIC ENHANCEMENT-ENVIRONMENTAL RESPONSE

In the absence of human activities, biotic fixation is the primary sou rce of reactive N, providing about 90-130 Tg N yr(-1) (Tg = 10(12) g) on the continents. Human activities have resulted in the fixation of a n additional approximate to 140 Tg N yr(-1) by energy production (appr oximate to 20 Tg N yr(-1)), fertilizer production (approximate to 80 T g N y(-1)), and cultivation of crops (e.g., legumes, rice) (approximat e to 40 Tg N yr(-1)). We can only account for part of this anthropogen ic 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 whi ch is buried or denitrified. Open oceans receive 18 Tg N yr(-1) by atm ospheric deposition, which is incorporated into oceanic n pools (e.g., NO3, N-2). The remaining 80 Tg N yr(-1) are either retained on contin ents in groundwater, soils, or vegetation or denitrified to N-2. Field studies and calculations indicate that uncertainties about the size o f each sink can account for the remaining anthropogenic n. Thus althou gh anthropogenic n is clearly accumulating on continents, we do not kn ow rates of individual processes. We predict the anthropogenic N-fixat ion rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil-fuel combustion. Aobut two-thirds of the increase will occur in Asia, which by 2020 will account for ov er half of the global anthropogenic N fixation.