Jürgen Burkhardt

Vegetation atmosphere exchange of ammonia: Canopy cycling and the impacts of elevated nitrogen inputs

SummaryMicrometeorological measurements of atmospheric ammonia (NH3) exchange with semi-natural and agricultural plant communities were made using sensitive new instrumentation capable of determining NH3 fluxes at <0.1 μg m−3. The results are used to test hypotheses concerning the canopy cycling of reduced nitrogen (NHx) and the existence of potential feedbacks between total N inputs (from agricultural sources or atmospheric deposition) and the net NH3 flux. The measurements over cropland, together with a model calculating the ‘canopy compensation point’ for NH3 indicate the importance of stomatal NH3 emission and recapture of NH3 by plant cuticles and water-layers. In contrast, measurements at an extremely clean upland moorland suggest that cuticular desorption of NH3 is also possible at low concentrations. Interpretation of dew measurements suggests that leaf uptake of NH44+ may occur as a result of pH gradients between the leaf surface and apoplast. The combined conceptual model of NHx exchange provides a useful basis for developing quantitative resistance models to predict NH3 fluxes.

The ion concentration of dew condensed on Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) needles.

SummaryDew droplets collected with pipettes from coniferous needles were analysed for their ionic composition. Almost all samples of dew taken from Scots pine trees (Pinus sylvestris) showed significantly higher ion concentrations than those taken from Norway spruce trees (Picea abies). This can be explained by the micromorphology of the needle surface. The higher microscale roughness of the wax layer of a pine needle causes a more efficient flux of atmospheric aerosol particles compared to the spruce needle surface. Dew on coniferous needles is shown to be capable of maintaining pH values below 3 for several hours.

Deliquescence of Deposited Atmospheric Particles on Leaf Surfaces

A large fraction of deposited aerosol particles on leafsurfaces represents hygroscopic material in a high humidity environment, likely to become deliquescent within the water vapour transpired by the leaf. Microscopic observations on leaf surfaces of beech, kohlrabi and elder leaves grown in a particle-free environment and/or treated with defined particle load are presented. Spreading of deliquescent particles, formation of salt crusts, and encoating of and by waxes was observed. Deliquescence of NaNO3 particles (deliquescence point 74% RH at 25 °C) was observed on and near the stomata at 35% relative humidity (RH) of the surrounding air, illustrating the influence of the boundary layer. Aerosols are difficult to identify on leaf surfaces, as similar patterns may be produced by deliquescent particles and processes like ‘salt creep’. These problems are especially important on leaves from the natural environment if only scanning electron microscopy (SEM) is used. These patterns could appear similar to the fused waxes described in forest decline research. Strong gradients are formed between deliquescent particles and the apoplastic solution within the leaf, promoting cuticular and stomatal uptake.

The possible role of aerosols on stomatal conductivity for water vapour

Summary Stomatal conductance, which controls both the exchange of water vapour and CO 2 , is thought to be strictly determined by stomatal aperture. However, water transport processes might be modified by deposition of hygroscopic salts on the leaf surface. For this reason, simultaneous measurements of gas exchange and stomatal aperture were performed on Sambucus nigra leaves before and after treatment with sub-micrometer NaNO 3 aerosol. Aerosol treatment of the leaves led to higher transpiration, which was particularly pronounced at small apertures, leading to an enhancement of minimum leaf conductance between 45 and 90%. CO 2 -uptake of Sambucus leaves was not enhanced by aerosol treatment, and consequently water use efficiency decreased at small stomatal apertures. Artificial leaves consisting of small water reservoirs tapped with filter membranes also showed an increase in evaporation after aerosol treatment, which indicates a mainly physical explanation for the findings with Sambucus leaves. Absorption and desorption of water by salt deposits due to changing thickness of the laminar sublayer and entering turbulent elements -‘bursts’- is suggested to be a possible explanation of the results. The cyclic elementary process, condensation and evaporation of water on the leaf surface, was observed microscopically under changing boundary layer conductance at 50% relative humidity (RH). Hygroscopic particles might therefore act as intermediate short-term storage or as primary evaporation sites after liquid water transport along films of salt solution through stomatal pores. A dynamic model describes the effects of cycles of short-term storage of salts. Possible consequences for plant water relations are discussed. Wasserdampf- und CO 2 -Austausch von Blattern werden durch die stomatare Leitfahigkeit reguliert, welche bislang als eine Funktion alleine der Spaltoffnungsweite gesehen wurde. Hygroskopische Salze auf der Blattoberflache konnten diese eindeutige Beziehung aufgrund ihrer Wechselwirkung mit Wasser(dampf) beeinflussen. Aus diesem Grund wurden gleichzeitig Gasaustausch und Spaltoffnungsweiten an Blattern von Holunder ( Sambucus nigra ) gemessen. Die Ergebnisse dieser Messungen vor und nach Bespruhung der Blatter mit NaNO 3 -Aerosol wurden verglichen. Nach der Aerosolbehandlung war die Transpiration signifikant erhoht. Die relative Zunahme war bei kleinen Offnungsweiten besonders stark und die mimimale Leitfahigkeit erhohte sich um 4590%. Dagegen stieg die CO 2 -Aufnahme der Sambucus -Blatter durch die Aerosolbehandlung nicht an, wodurch sich bei kleinen Spaltoffnungsweiten eine geringere Wassernutzungseffizienz ergab. Auch fur Modellblatter (kleine mit Membranfiltern verschlossene Wassergefase) wurde nach Aerosolbehandlung ein Verdunstungsanstieg gemessen, was auf eine uberwiegend physikalische Ursache der Ergebnisse an Sambucus hindeutet. Die Ursache konnte in der wiederholten Ab- und Desorption von Wasser an den Salzablagerungen liegen und auf Veranderungen der Grenzschichtdicke und das Eindringen von Turbulenzelementen (“bursts”) in die Grenzschicht zuruckgehen. Der elementare Zyklus, Kondensation und Evaporation von Wasser auf der Blattoberflache bei sich andernder Grenzschichtleitfahigkeit, wurde bei 50% relativer Feuchte mikroskopisch beobachtet. Analog konnten hygroskopische Partikel als kurzzeitige Zwischenspeicher fungieren oder, nach dem Transport flussigen Wassers entlang von Filmen aus Salzlosung durch die Spaltoffnungen, als primare Verdunstungsorte wirken. Ein dynamisches Modell beschreibt die Wirkung wiederholter kurzzeitiger Speicherung durch das Salz. Die mogliche Bedeutung der Ergebnisse fur den pflanzlichen Wasserhaushalt wird diskutiert.

Atmospheric aerosol particles and their role in the formation of dew on the surface of plant leaves

Abstract Measurements of the electrical conductivity along the surface of leaves or conifer needles show that the wetting of such surfaces coated with wax already starts below a relative humidity of 100%. To explain the observations, the deposition of water soluble atmospheric aerosol particles on these surfaces is taken into account. It is shown that the capillary space between particle and settling surface and, as an additional indispensable condition, the water solubility of at least some constituents of the atmospheric aerosol particle (i.e. a combined capillary-solution effect), permits the formation of condensate or dew water significantly below a r.h. of 100%.

The synergism between SO2 oxidation and manganese leaching on spruce needles — A chamber experiment

Four year old spruce (Picea abies (L.) Karst.) seedlings were planted in sand pots and supplied with nutrient solution. Three groups were formed, differing only in manganese nutrition (0.5 ppm, 2.5 ppm, 12.5 ppm, respectively). After three months, five individuals of each group were transferred to a dew chamber. For the next seven weeks the trees were sprayed in the evenings, the relative humidity overnight was kept high and the droplets were collected directly from the needles in the mornings. The trees were sprayed with HNO3 (pH 3.4) during the first three weeks to reduce the natural buffering capacity of the needles. After this time, the trees were sprayed with KCl (1 mM) solution, and NaHSO3 was added to the chamber resulting in SO2 concentrations usually between 50 and 150 microg m(-3). Needles and water samples were analysed. Foliar Ca seemed to be only a short-time buffer even under optimal Ca supply. A highly significant influence of managanese supply on manganese in needles and droplets was observed, as well as on sulphate, H+ and calcium concentrations in the droplets. The SO2 flux to trees treated with 12.5 ppm Mn was about twice as high as to trees treated with 0.5 ppm Mn. The conclusion is that this is due to a synergism between manganese leaching and catalysis of the SO2 oxidation by the leached Mn2+ ions. The results suggest a positive feedback between (moderate) acidification of soils and SO2 and NH3 inputs to terrestrial ecosystems.

A new device for the study of water vapour condensation and gaseous deposition to plant surfaces and particle samples

Abstract This note describes the construction and application possibilities of newly developed surface wetness sensors. At the present state of sensor development the absolute values are not reproducible. However, even the relative conductance measurements allow the detection of non-visible water films on samples in the laboratory as well as in the open field. Applications to spruce needles and to aerosol samples deposited on artificial collecting surfaces are presented. Using special experimental conditions, the sensors are also suited for the determination of gas deposition to surfaces.

Hygroscopic salts on the leaf surface as a possible cause of forest decline symptoms

Forest decline has been observed for about 20 years, especially in Central Europe and eastern North America. Whereas the direct effects of acid rain, acid fog and several trace gases on aerial parts of trees have been investigated, deposited atmospheric particles on leaf surfaces have been regarded as inert so far. By conductance measurements on the surface of coniferous needles an invisible wetness was observed over extended times. Washing experiments revealed this to be due to hygroscopic salts on the leaf surface, which will absorb water vapour mostly from leaf transpiration. The possible influence of salts on the microclimatic conditions on the leaf surface is outlined and it is concluded that they will act similar to a wick or a drying agent. This could form a direct link between air pollution and those forest decline symptoms which are connected with the water status of the trees.

Plants and Atmospheric Aerosols

Atmospheric aerosols are liquid, solid, or mixed suspensions of heterogeneous chemical composition, ranging from a few nanometers to almost 100 μm in diameter. Plants are sources and sinks of these diverse aerosols. Vegetation is influenced by aerosols through the water cycle, radiation balance, and nutrient transport, on global and regional scales, but direct interactions of aerosols with plant ecophysiology have not been considered in sufficient detail. Plant surface characteristics and aerodynamic factors control deposition. These factors may be manipulated in efforts to mitigate aerosol concentrations using urban vegetation as efficient aerosol collectors. Hygroscopic aerosols deposited on leaves generate concentrated solutions that reduce surface tension and generate thin liquid films. These films are shown to enter the stomatal pores, facilitating foliar nutrient uptake and enhancing liquid water loss that is poorly controlled by stomata. Aerosol pollution can reduce plant drought tolerance and alter nutrient balance. Anthropogenic aerosols now exceed natural aerosols, particularly in urban areas. The effects of these aerosols on plants require a focused research effort.

Hydraulic resistances in seedlings of Coffea arabica accessions under contrasting shade regimes in southwestern Ethiopia

The study was carried out to determine the variations among different Coffea arabica germplasm lines in hydraulic resistances under controlled nursery settings at the Jimma Agricultural Research Center in southwest Ethiopia. The experimental treatments included contrasting shade conditions (moderate shading v. full sunlight) and seedlings of 12 arabica coffee accessions of varying geographical areas in Ethiopia. Root hydraulic conductance and hydraulic resistances in the whole-shoot and different shoot parts were measured using a high-pressure flow meter. The results depicted significantly lower hydraulic resistances in the whole-shoot and in various shoot segments from the full sunlight exposed seedlings. The contribution of root and shoot resistances varied significantly in response to shade treatments. Likewise, seedlings of coffee accessions exhibited significant variation in the resistance contribution of the main stem-cut to whole-shoot resistances. The maximum hydraulic resistances in main stem-cut were noted in the order of Bonga>Berhane-Kontir>Yayu>Harenna coffee populations, suggesting a direct relationship between growth and hydraulic characteristics. The resistance contributions declined across seedling growth parts: roots>leaf>whole-shoot>lateral branch>petiole, which is consistent with hydraulic gradients and thus sensitivity to drought stress. Moreover, the findings indicate the possibility of predicating the latter stage performances of coffee genotypes at specific field locations. In support of the hypothesis, the effects of both environmental and genetic factors need to be considered in fully understanding drought tolerance strategies in coffee genotypes. In view of the continuous multifaceted threats on the untapped coffee genetic resources, due mainly to, among others, anthropogenic activities coupled with climate change, there is an urgent need for global collaborative actions for future development of the coffee sector in Ethiopia and worldwide.