Abstract
Stomatal conductance, one of the major plant physiological controls within NH3 biosphere-atmosphere exchange models, is commonly estimated from semi-empirical multiplicative schemes or simple light- and temperature-response functions. However, due to their inherent parameterization on meteorological proxy variables, instead of a direct measure of stomatal opening, they are unfit for the use in climate change scenarios and of limited value for interpreting field-scale measurements. Alternatives based on H2 O flux measurements suffer from uncertainties in the partitioning of evapotranspiration at humid sites, as well as a potential decoupling of transpiration from stomatal opening in the presence of hygroscopic particles on leaf surfaces. We argue that these problems may be avoided by directly deriving stomatal conductance from CO2 fluxes instead. We reanalysed a data set of NH3 flux measurements based on CO2 -derived stomatal conductance, confirming the hypothesis that the increasing relevance of stomatal exchange with the onset of vegetation activity caused a rapid decrease of observed NH3 deposition velocities. Finally, we argue that developing more mechanistic representations of NH3 biosphere-atmosphere exchange can be of great benefit in many applications. These range from model-based flux partitioning, over deposition monitoring using low-cost samplers and inferential modelling, to a direct response of NH3 exchange to climate change.
Highlights
Excessive dry deposition of reactive nitrogen has long been recognized as a major threat to both the environment and human health alike
Averages of stomatal conductance of NH3 derived from CO2 flux measurements were relatively constant until the last week of March 2014, after which they exhibited a strong and seemingly linear increase up to more than twice their initial magnitude, whereas a selection of empirical models (Emberson et al, 2000; Wesely, 1989) show an initial increase at the beginning of the measurement campaign and remain on a relatively low level below 0.1 cm/s with an only slightly positive trend (Figure 2c)
CO2-derived stomatal conductance does not appear to be correlated with the strong decrease in observed deposition velocity (Figure 2c), while empirically modelled stomatal conductance seems to be anti-correlated to a certain degree
Summary
Excessive dry deposition of reactive nitrogen has long been recognized as a major threat to both the environment and human health alike. Ammonia (NH3) is considered to be one of the most important constituents of total reactive nitrogen, with global emission estimates ranging from 46 to 85 Tg N/year, likely more than half of which originate from agricultural production (Sutton et al, 2013). The accurate representation of NH3 biosphere–atmosphere exchange within models is of great importance to build an adequate set of tools necessary to assess the whole lifecycle and impacts of reactive nitrogen compounds—from emission sources, over transport and chemical reactions in air, to their deposition. There have been considerable efforts to improve the parameterization of the bidirectional NH3 exchange estimates in recent years (Massad, Nemitz, & Sutton, 2010; Personne et al, 2009; Wichink Kruit et al, 2010; Zhang, Wright, & Asman, 2010, and others); major uncertainties still prevail
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