The quantitative role of ammonia/ammonium transport and metabolism by plants in the global nitrogen cycle

Estimate of global yearly N assimilation by photolithotrophs are 417 Tmol N in the oceans and 167 Tmol on land and in freshwater, of which diazotrophy contributes 1 (sea) and 10 (land plus freshwater) Tmol N. More than half of the combined N assimilated (416 and 157 Tmol N year^?1 in the sea and on land plus freshwater, respectively) is due to reduced N, i. e. organic N and, mainly, NH₃/NH⁺₄. Assimilation of reduced N amounts to up to 334 Tmol N year^?1 in the oceans and at least 79 Tmol N year^?1 in freshwater and on land. Reassimilation of NH₃/NH⁺₄ within the plant which is related to photorespiration is at least as great as primary NH₃/NH⁺₄ assimilation in the sea, and 8 times greater on land. The less frequently considered reassimilation of NH₃/NH⁺₄ that is related to phenyl-propanoid (mainly lignin) synthesis in land plants is similar (111 Tmol N) to the primary assimilation of NH₃/NH⁺₄ on land each year. Shoots of terrestrial plants have higher NH₃ compensation partial pressures than most natural soils, and especially than have ocean-surface biota. However, gaseous transfer of NH₃/NH⁺₄ from land to the oceans is a negligible component of the global N cycle. Consideration of area-based N assimilation rates, diffusion distances and diffusion coefficients can rationalise why steady-state NH₃/NH⁺₄ concentrations in the sea are lower than in the soil solution. The possibility that photolithotrophs can catalyse the oxidation of NH₃/NH⁺₄, or organic N at the same redox level, to N₂, N₂O, NO, –NO₂, NO^?, ₂, NO₂ or NO₄⁺, is critically assessed. The tentative conclusions are that such oxidation probably occurs, but is not a major component of the global conversion of reduced N to N₂ and more oxidized N species. More work is needed, especially to determine if NO generated from reduced N (conversion of arginine to citrulline plus NO) has a regulatory role in plants analogous to that established in metazoa. Relative to NO₃^? (or N₂) as N sources, growth using NH₃/NH⁺₄ as N source has a number of potential advantages in terms of cost of other resources. Mechanistically predicted economies for NH⁺₄ as N source are: (1) lower cost of photons used and, in transpiring plants, (2) less water lost per unit C assimilated, and (3) lower costs of catalytic Fe, Mn and Mo (unit C assimilated)^?1 s^?1, as well as (4) a higher maximum growth rate. The lower photon costs are frequently borne out by experimentation and the predicted higher maximum growth rates sometimes occur, while the predicted lower water costs are invariably contradicted. Few data are available for the cost of Fe, Mn or Mo as a function N source.