Nitrous Oxide and Other Nitrogen Trace Gases from Intensively Managed Landscapes: A Global Perspective

Concentrations of N20 in the global atmosphere have been rising over the last 20 yr at about 0.8 parts per billion volumetric (ppbv) or 0.25% yr1 to the 310 ppbv present in today's atmosphere (Fig. 6-1; Elkins and Rossen, 1989). Ice core data that indicate concentrations of around 280 ppbv in the preindustrial atmosphere (e.g., Pearman et al., 1986) suggest that this rise reflects a long-term trend (Fig. 6-2; IPCC, 1990). In fact, this 0.8 ppbv increase represents a 3.5 Tg addition of N20-N to the global atmosphere each year, and if one assumes that the present photodissociation rate of N20 in the stratosphere (11 Tg yr I N) is typical of preindustrial steady-state loading rates, then the preindustrial loading rate of N20-N was 11 Tg yr 1-suggesting that we are today adding another 40% (3.5 Tg N) of N20 to the atmosphere each year than was the case 100 yr ago (Cicerone, 1987; Robertson et al., 1989). This loading rate is significant primarily for two reasons. First, N20 is one of the major greenhouse gases, accounting for 6 to 8% of the present greenhouse forcing rate ascribed to anthropically derived gases (C02 , CFC's, CH4 , and N20; Hansen et al., 1990; IPCC, 1990). On a molar basis N20 is about 250 times more potent than C02 as an absorber of infrared radiation-in part due to its molecular structure but largely due to the fact that it absorbs in a portion of the infrared transmission window that in our present atmosphere is relatively clean (Duxbury et al., 1993). Second, N20 is the major natural regulator of stratospheric 0 3, which effectively controls the earth's ultraviolet-B (UV8 ) radiation balance. Through a series of reactions elucidated over the last 20 yr, the oxidation of N20 to NO via reaction with photolytically produced atomic oxygen 0( 1 D) in the upper stratosphere ( > 25 km) produces NO that in turn reacts with 0 3 to form