Nitrous Oxide: Emission from Soils During Nitrification of Fertilizer Nitrogen

Tropospheric sources of NOx: lightning and biology

Laboratory experiments to quantify the global production of NOx (NO + NO2) in the troposphere due to atmospheric lightning and biogenic activity in soil are presented. These laboratory experiments, as well as other studies, suggest that the global production of NOx by lightning probably ranges between 2 and 20 MT(N)y-1 of NO and is strongly dependent on the total energy deposited by lightning, a quantity not well-known. In our laboratory experiments, nitrifying micro-organisms is soil were found to be a significant source of both NO and nitrous oxide (N2O). The measured production ratio of NO to N2O averaged 2-3 for oxygen partial pressures of 0.5-10%. Extrapolating these laboratory measurements to the global scale, which is somewhat risky, suggests that nitrifying micro-organisms in soil may account for as much as 10 MT(N) y-1 of NO. Additional experiments with denitrifying micro-organisms gave an NO to N2O production ratio ranging from 2 to 4 for an oxygen partial pressure of 0.5% and a ratio of less than unity for oxygen partial pressures ranging from 1 to 20%. The production of NO and N2O, normalized with respect to micro-organism number indicates that the production of both NO and N2O by denitrifying micro-organisms is at least an order of magnitude less than production by nitrifying micro-organisms for the micro-organisms studied.

Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

We used a combination of 15 N tracer methods and a C 2 H 2 blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm −3 , increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm −3 · day −1 ) was similar to the average denitrification rate (44 ng of N · cm −3 · day −1 ). In situ N 2 O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm −3 · day −1 . Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.

Denitrification Associated with Periphyton Communities

Scrapings of decomposing Cladophora sp. mats (periphyton) covering stream bed rocks produced N 2 O when incubated under N 2 plus 15% C 2 H 2 . Denitrification (N 2 O formation) was enhanced by NO 3 − and was inhibited by autoclaving, Hg 2+ , and O 2 . No N 2 O was formed in the absence of C 2 H 2 (air or N 2 atmosphere). Chloramphenicol did not block N 2 O formation, indicating that the enzymes were constitutive. In field experiments, incubation of periphyton scrapings in the light inhibited denitrification because of algal photosynthetic O 2 production. The diurnal periphyton-associated denitrification rate was estimated to be 45.8 μmol of N 2 O·m −2 ·day −1 , as determined by averaging light, aerobic plus dark, and anaerobic rates over a 24-h period.

Nitrous Oxide Emissions from an Irrigated Cornfield

During the 1978 growing season, nitrous oxide (N(2)O) emissions from a typical well-managed northern Colorado field of corn (Zea mays L.) totaled approximately 2.6 kilograms of nitrogen per hectare, or about 1.3 percent of the applied fertilizer nitrogen. Thirty percent of the loss occurred during the 3 weeks following fertilization while ammonia was being rapidly nitrified, and 59 percent was emitted during the week after the field's first irrigation, when restricted oxygen diffusion favored denitrification. Considering the large spatial and temporal variability of N(2)O emissions from soil, micrometeorological estimates of vertical N(2)O flux density compared favorably with estimates based on a simple soil cover method.