Plant—atmosphere exchange of ammonia

Abstract

The results of recent controlled environment and micrometeorological measurements of NH$_{3}$ fluxes are presented to highlight the processes controlling NH$_{3}$ plant-atmosphere exchange. The presence of NH$^{_{4}^{+}}$ in leaf tissues results in the existence of an NH$_{3}$ `compensation point' concentration for substomatal tissues ($\chi _{\text{s}}$), so that both emission and deposition are possible from stomata. In addition, NH$_{3}$ may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emission, so that a `canopy compensation point' ($\chi _{\text{c}}$) may be defined that is smaller than $\chi _{\text{s}}$. Ammonia is generally deposited to nitrogen limited ecosystems, indicating a small $\chi _{\text{s}}$ and small leaf cuticle resistance (R$_{\text{w}}$). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger $\chi _{\text{s}}$ as a consequence of greater N supply. The paper discusses the processes defining R$_{\text{w}}$ (humidity, acidic pollutants) and $\chi _{\text{s}}$ (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates $\chi _{\text{s}}$ and R$_{\text{w}}$ into one model. Estimating long term bidirectional NH$_{3}$ fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH$_{3}$ fluxes over diurnal time scales.