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Much controversy exists about whether or not NH(+)(4) is translocated in the xylem from roots to shoots. In this paper it is shown that such translocation can indeed take place, but that interference from other metabolites such as amino acids and amines may give rise to large uncertainties about the magnitude of xylem NH(+)(4) concentrations. Elimination of interference requires sample stabilization by, for instance, formic acid or methanol. Subsequent quantification of NH(+)(4) should be done by the OPA-fluorometric method at neutral pH with 2-mercaptoethanol as the reducing agent since this method is sensitive and reliable. Colorimetric methods based on the Berthelot reaction should never be used, as they are prone to give erroneous results. Significant concentrations of NH(+)(4), exceeding 1 mM, were measured in both xylem sap and leaf apoplastic solution of oilseed rape and tomato plants growing with NO(-)(3) as the sole N source. When NO(-)(3) was replaced by NH(+)(4), xylem sap NH(+)(4) concentrations increased with increasing external concentrations and with time of exposure to NH(+)(4). Up to 11% of the translocated N was constituted by NH(+)(4). Glutamine synthetase (GS) incorporates NH(+)(4) into glutamine, but root GS activity and expression were repressed when high levels of NH(+)(4) were supplied. Ammonium concentrations measured in xylem sap sampled just above the stem base were highly correlated with NH(+)(4) concentrations in apoplastic solution from the leaves. Young leaves tended to have higher apoplastic NH(+)(4) concentrations than older non-senescing leaves. The flux of NH(+)(4) (concentration multiplied by transpirational water flow) increased with temperature despite a decline in xylem NH(+)(4) concentration. Retrieval of leaf apoplastic NH(+)(4) involves both high and low affinity transporters in the plasma membrane of mesophyll cells. Current knowledge about these transporters and their regulation is discussed.

Abstract.A new biophysical model SURFATM-NH3, simulating the ammonia (NH 3 ) exchange between terrestrial ecosystems and the atmosphere is presented. SURFATM-NH3 consists of two coupled models: (i) an energy budget model and (ii) a pollutant exchange model, which distinguish the soil and plant exchange processes. The model describes the exchanges in terms of adsorption to leaf cuticles and bi-directional transport through leaf stomata and soil. The results of the model are compared with the flux measurements over grassland during the GRAMINAE Integrated Experiment at Braunschweig, Germany. The dataset of GRAMINAE allows the model to be tested in various meteorological and agronomic conditions: prior to cutting, after cutting and then after the application of mineral fertilizer. The whole comparison shows close agreement between model and measurements for energy budget and ammonia fluxes. The major controls on the ground and plant emission potential are the physicochemical parameters for liquid-gas exchanges which are integrated in the compensation points for live leaves, litter and the soil surface. Modelled fluxes are highly sensitive to soil and plant surface temperatures, highlighting the importance of accurate estimates of these terms. The model suggests that the net flux depends not only on the foliar (stomatal) compensation point but also that of leaf litter. SURFATM-NH3 represents a comprehensive approach to studying pollutant exchanges and its link with plant and soil functioning. It also provides a simplified generalised approach (SVAT model) applicable for atmospheric transport models.Correspondence to: E. Personne

Ammonium (NH4+) is a central intermediate in the N metabolism of plants, but the quantitative importance of NH4+ in transporting N from root to shoot and the capability of plants to store NH4+ in leaves are still matters of substantial controversy. This paper shows that some of these controversies have to be related to the use of inadequate analytical procedures used for extraction and quantification of NH4+ in plants. The most frequently used methods for determination of NH4+, viz. colorimetric methods based on the classical Berthelot reaction, suffered severely from interference caused by amino acids, amines, amides and proteins. For some of these metabolites the interference was positive, while for others it was negative, making correction impossible. Consequently, colorimetric analysis is inapplicable for determination of NH4+ in plants. Results obtained by ion chromatography may overestimate the NH4+ concentration due to co‐elution of NH4+ with amines like methylamine, ethylamine, ethanolamine and the non‐protein amino acid Γ‐aminobutyric acid. Derivatization of NH4+ with o‐phthalaldehyde at alkaline pH and subsequent quantification of NH4+ by fluorescence spectroscopy was also associated with interference. However, when pH was lowered to 6.8 during derivatization and 2‐mercaptoethanol was used as reductant, NH4+ could be determined with a high selectivity and sensitivity down to a detection limit of 3.3 μM in a 10‐μl sample volume. Derivatization was performed on‐line using a column‐less HPLC system, enabling rapid quantification of NH4+ in a few minutes. Flow injection analysis with on‐line gas dialysis was, likewise, free from interference, except when applied on highly senescent plant material containing volatile amines. Labile N metabolites in leaf tissue extract, xylem sap and apoplastic fluid were degraded to NH4+ during extraction and subsequent instrumental analysis if the samples were not stabilised. A simple and efficient stabilisation could be obtained by addition of 10 mM ice‐cold HCOOH to the plant extraction medium or to the samples of apoplastic fluid or xylem sap. We conclude that significant concentrations of NH4+, exceeding 1 mM, may occur in xylem sap, leaf apoplastic fluid and leaf tissue water of nitrate‐grown tomato and oilseed rape plants. The measured NH4+ concentrations were not a result of excessive N supplies, as even plants grown under mildly N‐deficient conditions contained NH4+.

Abstract. The exchange of ammonia between crop canopies and the atmosphere depends on a range of plant parameters and climatic conditions. However, little is known about effects of management factors. We have here investigated the stomatal ammonia compensation point in response to cutting and fertilization of a grass sward dominated by Lolium perenne. Tall grass had a very low NH 3 compensation point (around 1 nmol mol −1 ), reflecting the fact that leaf nitrogen (N) concentration was very low. During re-growth after cutting, leaf tissue concentrations of NO concentration of the newly emerging leaves to increase dramatically. The NH 3 compensation point peaked between 15 and 25 nmol mol −1 the day after the fertiliser was applied and thereafter decreased over the following 10 days until reaching the same level as before fertilisation. Ammonium concentrations in leaf apoplast, bulk tissue and litter were positively correlated (P=0.001) throughout the experimental period. Bulk tissue NH

Abstract. Improved data on biosphere-atmosphere exchange are fundamental to understanding the production and fate of ammonia (NH 3 ) in the atmosphere. The GRAMINAE Integrated Experiment combined novel measurement and modelling approaches to provide the most comprehensive analysis of the interactions to date.Major intercomparisons of micrometeorological parameters and NH 3 flux measurements using the aerodynamic gradient method and relaxed eddy accumulation (REA) were conducted. These showed close agreement, though the REA systems proved insufficiently precise to investigate vertical flux Correspondence to: M. A. Sutton (ms@ceh.ac.uk) divergence. Grassland management had a large effect on fluxes: emissions increased after grass cutting (−50 to 700 ng m −2 s −1 NH 3 ) and after N-fertilization (0 to 3800 ng m −2 s −1 ) compared with before the cut (−60 to 40 ng m −2 s −1 ).

Abstract. Ammonia exchange fluxes between grassland and the atmosphere were modelled on the basis of stomatal compensation points and leaf surface chemistry, and compared with measured fluxes during the GRAMINAE intensive measurement campaign in spring 2000 near Braunschweig, Germany. Leaf wetness and dew chemistry in grassland were measured together with ammonia fluxes and apoplastic NH + 4 and H + concentration, and the data were used to apply, validate and further develop an existing model of leaf surface chemistry and ammonia exchange. Foliar leaf wetness which is known to affect ammonia fluxes may be persistent after the end of rainfall, or sustained by recondensation of water vapour originating from the ground or leaf transpiration, so measured leaf wetness values were included in the model. pH and ammonium concentrations of dew samples collected from grass were compared to modelled values.The measurement period was divided into three phases: a relatively wet phase followed by a dry phase in the first week before the grass was cut, and a second drier week after the cut. While the first two phases were mainly characterised by ammonia deposition and occasional short emission events, regular events of strong ammonia emissions were observed during the post-cut period. A single-layer resistance model including dynamic cuticular and stomatal exchange could describe the fluxes well before the cut, but after the cut the stomatal compensation points needed to numerically match Correspondence to: J. Burkhardt (j.burkhardt@uni-bonn.de) measured fluxes were much higher than the ones measured by bioassays, suggesting another source of ammonia fluxes. Considerably better agreement both in the direction and the size range of fluxes were obtained when a second layer was introduced into the model, to account for the large additional ammonia source inherent in the leaf litter at the bottom of the grass canopy. Therefore, this was found to be a useful extension of the mechanistic dynamic chemistry model by keeping the advantage of requiring relatively little site-specific information.

Abstract. Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH 3 ) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks.Fluxes of atmospheric NH 3 within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH + 4 ) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (C p ). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH 3 within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes.The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartCorrespondence to: B. Loubet (loubet@grignon.inra.fr) ment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH 3 with a compensation point around 0.1-0.4 µg m −3 and an NH 3 flux of 6 to 7 ng m −2 s −1 . Cutting of the grass did not increase the NH 3 fluxes of the green leaves. The litter was found to be the largest source of NH 3 in the canopy, with a C p of up to 1000 µg m −3 NH 3 and an NH 3 flux up to 90 ng m −2 s −1 . The litter was found to be a much smaller NH 3 source when dried (C p =160 µg m −3 and F NH3 =35 ng m −2 s −1 NH 3 ). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH 3 in the period immediately after cutting (C p =320 µg m −3 and F NH3 =60 ng m −2 s −1 ), which was nevertheless always smaller than the litter source. The soil NH 3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH + 4 . These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.