Ammonia was necessary for the origin of life. However, NH3 would not have been a significant component of the neutral or mildly reducing (N-2, CO2, H2O) atmosphere which characterized the Hadean earth(> 3.8 Gyr ago), especially in view of the u.v. lability of NH3, the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH3 source. Lightning, or bolides (meteorites and comets), generated NOx., and Fe2+ in the sea could have reduced NO2- resulting from the NOx. to NH4+; this disequilibrium NH4+ level could have supported the production of the nitrogenous organic building blocks of life. NOx. and NHy thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH4+, and biological N-2 fixation could have evolved in its present Fe-demanding, O-2-sensitive forms in the Archaean O-2-free, Fe2+-rich environment. Organic matter could have driven biological denitrification reactions based on NO2- and NO(3)(-)generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NOx. and producing N2O. Any atmospheric NH3 leaking from oceanic biology would have been subject to u.v. breakdown and rain-out. Although O-2-evolving photosynthesis probably began in the Archean some 3.5 Gyr ago, any O-2 accumulation was local until 2.0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe2+ and S2-. However, localized O-2 accumulation before 2.0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O-2-consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NOx. production. Oxygen accumulation globally from 2.0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification-like aerobic production of NO. by nitric oxide synthetase, while lightning and bolides could now generate NO. from N-2 and O-2. Evidence for terrestrial ecosystems is found from 1.0 Gyr onwards; NOx. and NH3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO2 levels fell and O-2 levels rose, NH3 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity and N cycling on land, with greater quantitative possibilities for NOx. and NH3 escape to the atmosphere. The evolution of lignin land related phenylpropanoids) at least 400 Myr ago, with associated NH3 recycling in vascular land plant, does not seem ton present evidence) to increase NH3 loss to the atmosphere significantly. Biomass burning occurred at least 350 Myr ago with lightning as the likely ignition source; such burning yielded NO., NH3, N2O and N-2 from organic N. As in earlier times, the existence of terrestrial embryophyte vegetation has been punctuated by major bolide impacts, with the Cretaceous-Tertiary boundary impact generating perhaps 10(4) as much NO. as a year's thunderstorms do today, although the toxicity of NOx. per se to vegetation might not have been the major effect of the NOx. on biota. Terrestrial plants suffered fewer extinctions at this time than did many other major taxa. Despite such very significant generators of atmospheric combined N as major bolide impacts, the mean levels of NHy, NOx. and N2O land O-3) in the atmosphere during the 450 Myr existence of embryophyte vegetation was lower than the current, globally averaged, anthropogenically influenced values. Current globally averaged levels of atmospheric combined N are thus not without precedent in the history of life, globally for NOx., and locally for NHy, so vegetation has had evolutionary experience of high atmospheric combined N. However, this should not make us complacent about the impact of current wide-spread and continuing anthropogenic inputs of combined N, in view of the rate and extent of the inputs, and their combination with other aspects of local anthropogenic influence (e.g. SO, from burning of high-S coal), stratospheric O-3 depletion, and global environmental change (increasing CO2, temperature and sea-level).
