The hypothesis was tested that O₃-induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy-level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O₃ regime, as prevailing at the forest site (control), or under an experimental twice-ambient O₃ regime (elevated O₃), as released through a free-air canopy O₃ fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O₃. As this outcome only partly accounts for the decline in stem growth, O₃-induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth.

Pre-requisite for reliable O₃ risk assessment for plants is determination of stomatal O₃ uptake. One unaddressed uncertainty in this context relates to transpiration-induced molecular collisions impeding stomatal O₃ influx. This study quantifies, through physical modelling, the error made when estimating stomatal O₃ flux without accounting for molecular collisions arising from transpiratory mass flow of gas out of the leaf. The analysis demonstrates that the error increases with increasing leaf-to-air water vapour mole fraction difference (Δw), being zero in water vapour saturated air and 4.2% overestimation at Δw of 0.05. Overestimation is approximately twice as large in empirical studies quantifying stomatal O₃ flux from measured leaf or canopy water flux, if neglecting both water vapour-dry air collisions (causing overestimation of leaf conductance) and collisions involving O₃. Correction for transpiration-induced molecular collisions is thus relevant for both empirical research and for large-scale modelling of stomatal O₃ flux across strong spatial Δw gradients.

Orange trees are widely cultivated in regions with high concentrations of tropospheric ozone. Citrus absorb ozone through their stomata and emit volatile organic compounds (VOC), which, together with soil emissions of NO, contribute to non-stomatal ozone removal. In a Valencia orange orchard in Exeter, California, we used fast sensors and eddy covariance to characterize water and ozone fluxes. We also measured meteorological parameters necessary to model other important sinks of ozone deposition. We present changes in magnitude of these ozone deposition sinks over the year in response to environmental parameters. Within the plant canopy, the orchard constitutes a sink for ozone, with non-stomatal ozone deposition larger than stomatal uptake. In particular, soil deposition and reactions between ozone, VOC and NO represented the major sinks of ozone. This research aims to help the development of metrics for ozone-risk assessment and advance our understanding of citrus in biosphere-atmosphere exchange.

The interaction of atmospheric sulphur (S) was investigated within the canopies of two boreal forests in Québec, Canada. The net canopy exchange approach, i.e. the difference between S–SO₄ in throughfall and precipitation, suggests high proportion of dry deposition in winter (up to 53%) as compared to summer (1–9%). However, a 3.5‰ decrease in δ¹⁸O–SO₄ throughfall in summer compared to incident precipitation points towards a much larger proportion of dry deposition during the warm season. We suggest that a significant fraction of dry deposition (about 1.2 kg ha⁻¹ yr⁻¹, representing 30–40% of annual wet S deposition) which contributed to the decreased δ¹⁸O–SO₄ in throughfall was taken up by the canopy. Overall, these results showed that, contrary to what is commonly considered, S interchanges in the canopy could be important in boreal forests with low absolute atmospheric S depositions.

The O₃ uptake in 17 adult trees of six urban species was evaluated by the sap flow-based approach under free atmospheric conditions. The results showed very large species differences in ground area scaled whole-tree ozone uptake ( [Formula: see text] ), with estimates ranging from 0.61 ± 0.07 nmol m⁻² s⁻¹ in Robinia pseudoacacia to 4.80 ± 1.04 nmol m⁻² s⁻¹ in Magnolia liliiflora. However, average [Formula: see text] by deciduous foliages was not significantly higher than that by evergreen ones (3.13 vs 2.21 nmol m⁻² s⁻¹, p = 0.160). Species of high canopy conductance for O₃ ( [Formula: see text] ) took up more O₃ than those of low [Formula: see text] , but that their sensitivity to vapour pressure deficit (D) were also higher, and their [Formula: see text] decreased faster with increasing D, regardless of species. The responses of [Formula: see text] to D and total radiation led to the relative high flux of O₃ uptake, indicating high ozone risk for urban tree species.

An in-depth knowledge of solutes advection and turbulent diffusion is crucial to estimate dispersion area and retention time (tR) of pollutants within seagrass habitats. However, there is little knowledge on the influence of seagrass habitat fragmentation on such mechanisms. A set of dye tracer experiments and acoustic Doppler velocimeter measurements (ADV) were conducted. Solute transport conditions were compared in between fragmented (FM) vs homogeneous (HM) intertidal meadows, and in vertical gradients (canopy vs overlaying flow). Results showed the highest horizontal diffusion coefficient (Ky, c.a. 10⁻³m²s⁻¹) on FM and at the canopy-water column interface, whereas tR (2.6–5.6min) was not affected by fragmentation. It suggests that (1) FM are more vulnerable to pollution events in terms of dispersion area and (2) at low tide, advection rather than turbulent diffusion determines tR. Furthermore, Taylor’s theorem is revealed as a powerful tool to analyze vertical gradients on Ky within seagrass canopies.

Tree canopies are believed to act as a sink of atmospheric ammonia (NH₃). However, few studies have compared the uptake efficiency of different tree species. This study assessed the uptake of ¹⁵N-labelled NH₃ at 5, 20, 50 and 100 ppbᵥ by leaves and twigs of potted silver birch, European beech, pedunculate oak and Scots pine saplings in June, August and September 2008. Additionally, foliar uptake of ¹³C-labelled carbon dioxide (¹³CO₂) and leaf stomatal characteristics were determined per species and treatment date and the relation with ¹⁵NH₃ uptake and estimated stomatal ¹⁵NH₃ uptake were assessed. Both ¹⁵NH₃ and ¹³CO₂ uptake were affected by tree species and treatment date, but only ¹⁵NH₃ uptake was influenced by the applied NH₃ concentration. Depending on the treatment date, ¹⁵NH₃ uptake by leaves and twigs was highest at 5 (September), 20 (June) or 50 (August) ppbᵥ. Birch, beech and oak leaves showed the highest uptake in August, while for pine needles this was in June and, except at 5 ppbᵥ in June, the ¹⁵NH₃ uptake was always higher for the deciduous species than for pine. For all species except beech ¹³CO₂ uptake was highest in August and on every treatment date the ¹³CO₂ uptake by leaves of deciduous species was significantly higher than by pine needles. Leaf characteristics and ¹³CO₂ uptake did not provide a strong explanation for the observed differences in ¹⁵NH₃ uptake. This study shows that on the short-term a high interspecific variability exists in NH₃ uptake, which depends on the time in the growing season.

Over the past few years the number of biogas slurries, which are generally used as nitrogen fertilisers, have seen a steady increase in Germany. A mechanistic ammonia volatilisation model was developed to predict the ammonia losses of these slurries when applied to bare soil, maize, wheat and rye grass canopies. Data for model development were collected from several field measurements carried out at two locations in Northern Germany between the years of 2007 and 2008. Additionally, the behaviour of the slurries on and in the soil was investigated through the use of infiltration pot experiments. The model includes three main compartments: slurry, atmosphere and soil. The soil compartment model is relatively simple, as the slurry infiltration, nitrification and ploughing dislocation into the soil determined in the experiments showed quantitatively no significant differences between the tested slurries (mono-fermented, co-fermented and pig slurry) and soils (sand soil and loamy sand). Hence, instead of a complex soil model, stable reduction factors, as derived from the experiments, were implemented in the model. Simulated ammonia emissions were statistically compared (root mean square error (RMSE), modelling efficiency (ME), linear regression) to the observed emissions. All evaluations showed an acceptable model performance (RMSE = 1.80 kg N ha−1), although there were a few number of anomalies which could not be modelled in an adequate way. A model sensitivity analysis showed that temperature and slurry pH value are the main drivers of NH3 volatilization in the model. Following a change of +1°C or of +0.1 pH unit ammonia volatilization will increase by about 1% and 1.6% of the applied total ammoniacal nitrogen, respectively. We were able to show that a simple model approach could explain most factors of ammonia volatilization in biogas crop rotations.