Ammonia and ammonium fluxes between photolithotrophs and the environment in relation to the global nitrogen cycle

Abstract

summary Ammonium (used here to signify NH4+ plus NH3) is the immediate inorganic precursor of organic nitrogen in photolithotrophs. In marine habitats ammonium is also the major exogenous nitrogen source, and the same is probably true of terrestrial habitats. In addition to this major role of ammonium as exogenous nitrogen source, it is also quantitatively very important as an endogenous nitrogen source with nitrate or N2 as exogenous nitrogen source, and as a recycled nitrogen compound in photorespiration and in phenylpropanoid synthesis. The shoots of terrestrial plants have higher NH3 compensation partial pressures than most natural soils, and especially higher than that of the sea. However, long‐distance transport of gaseous NH3 (e.g. from continents to oceans) is a negligible component of the ‘natural’ global nitrogen cycle. Current concepts of evolution of the atmosphere and biosphere do not involve high partial pressure of NH3 in the atmosphere: any ammonium produced in inorganic or biological processes is removed from the atmosphere by rain‐out, or, over larger time‐scales, by the action of ultraviolet radiation (especially before the ozone screen came into being) and of hydroxyl radicals (especially after oxygenation of the atmosphere). In addition to posing problems for the origin of life and arguing for an early rather than a late evolution of diazotrophy, a low NH3 partial pressure renders implausible arguments that phenylpropanoid synthesis was restricted early in evolution by the effect of high ammonia in reversing phenylalanine ammonia‐lyase.

Impact of gaseous nitrogen deposition on plant functioning

“…The role of GS " is not so clear. It is located in the vascular bundle of root and leaves, where it might play a role in the synthesis of glutamine for transport (Schjøerring et al, 1993) and in recycling of the NH $ released in production of the monomers for lignin biosynthesis (Raven, Wollenweber & Handley, 1992). Expression of GS " was not found in leaf mesophyll cells of Nicotiana tabacum (Carvalho et al, 1992) and Oryza sativa (Kamachi et al, 1992).…”

The influence of atmospheric NH3 on the apoplastic pH of green leaves: a non‐invasive approach with pH‐sensitive microelectrodes

“…NH $ mol −" air ; Farquhar et al, 1980 ;Raven et al, 1992 ;Schjoerring, 1998), NH $ uptake through the leaf stomata constitutes a significant additional nitrogen source for plants (Lockyer & Whitehead, 1986 ;Wollenweber & Raven, 1993). On the other hand, NH $ uptake may also cause negative effects for plants, such as higher susceptibility towards stress conditions including frost, drought and herbivory (reviewed by Fangmeier et al, 1994).…”

Uptake and utilization of atmospheric ammonia in three native Poaceae species: leaf conductances, composition of apoplastic solution and interactions with root nitrogen supply

“…The NH $ compensation point is determined by temperature, NH % + concentration and the pH of the apoplastic solution (Farquhar et al, 1980 ;. Furthermore, the compensation point varies with N supply and may be influenced by the rate of photorespiration (Schjoerring, 1991 ;Raven et al, 1992).…”