Research PaperCombined measurements of microscopic leaf wetness and dry-deposited inorganic compounds in a spruce forest
Introduction
Tree leaves, and conifer needles in particular, act as effective filters of aerosol particles (Mayer and Ulrich, 1978), intercepting air pollutants due to their large specific surface areas (Badino et al., 1998). Forests encourage the biological fixation of nitrogen, as well as the dry deposition of gaseous and particulate reactive nitrogen species (e.g., HNO3, NH3, NH4+, and NO3−), which makes our grasp of these processes over the forest important for our overall understanding of the influence of global change on the nitrogen cycle (Sparks et al., 2008).
Hygroscopic particles, which can function as condensation nuclei (Baysens, 1995), may be important in the formation of microscopic leaf wetness (Burkhardt and Hunsche, 2013). The degradation of wax caused by particle deliquescence has been identified as a link between particulate air pollution and drought symptoms in plants (Burkhardt and Pariyar, 2013). Deliquescence also enhances trace gas deposition of easily soluble compounds, such as NH3 and SO2 (Van Hove et al., 1989; Burkhardt and Eiden, 1994; Wichink et al., 2008). Leaf wetness is also known to affect dry deposition and non-stomatal uptake of trace gases such as ozone and peroxyacetyle nitrate (e.g., Zhang et al., 2002; Sun et al., 2016a and b; Clifton et al., 2020).
Currently, leaf wetness is detected by directly monitoring changes in conductance using electrical sensors that must be attached directly to the leaf surface (Burkhardt and Gerchau, 1994; Burkhardt and Eiden, 1994; Burkhardt et al., 2009; Sun et al., 2016b). These sensors respond to changes in the electrical conductance of the mesophyll, the cuticle, and any wetness within the leaf boundary layer. The measured signal usually reflects the wetness level of the leaf surface, but it may also be affected by stomatal aperture, environmental humidity, and the ion concentration in the surface moisture (Burkhardt et al., 1999, 2009). The formation of leaf wetness in response to increasing relative humidity (RH) is the result of water-solid interaction processes (Mauer and Taylor, 2010), and absorption and deliquescence of hygroscopic materials could be the dominant processes in the field (Burkhardt and Hunsche, 2013). Deliquescence is the dissolution of a salt particle in the water vapor that it extracts from the surrounding air, which occurs when the RH of the surrounding air equals or exceeds the deliquescence relative humidity (DRH). Different from the DRH, the efflorescence relative humidity (ERH) indicates the relative humidity at which a solute droplet re-crystallizes. This leads to a hysteresis effect of water uptake and evaporation under increasing or decreasing RH conditions.
On a leaf-level scale, leaf washing (or foliar extraction/rinse) techniques have been widely used to monitor dry deposition in forestry and environmental studies (e.g., Davidson and Wu, 1990; Watanabe et al., 2008; Hara et al., 2014). This relies on experimental extraction methods that were developed specifically to remove only material that was surface-deposited (Lindberg and Lovett, 1985). A longer washing time is generally more effective than a shorter one (Steubing, 1982), so longer washing may be recommended for extracting biomass-incorporated elements (Wyttenbach et al., 1985). For example, a 3-min wash has been suggested to remove dry deposition, while still minimizing interference from leaching, in a study with spruce (Shanley, 1989). To our knowledge, however, there is still no standardized washing procedure even after the review of Oliva and Raitio (2003). In order to target inorganic compounds deposited on the needle surfaces mainly due to dry deposition, a relatively short washing time of 10 s has been applied in previous studies of pine needles (e.g., Wyttenbach et al., 1985). A combination of electrical conductance sensors and such leaf washing techniques has proved effective for investigating the interaction of microscopic leaf wetness and dry-deposited particles (Burkhardt and Eiden, 1994). Although there are many studies using electrical sensors for microscopic leaf wetness in the forest environment (e.g., Burkhardt and Eiden, 1994; Eiden et al., 1994; Klemm et al., 2002; Altimir et al., 2006), combined measurements of leaf wetness and washing are rarely made in the field. Clearly, more investigations focusing on the interactions between leaf wetness and dry deposition are highly relevant.
As a case study, combined measurements of (microscopic) leaf wetness and dry-deposited inorganic compounds with leaf washing techniques were carried out in a coniferous forest in Germany. By comparing the results, we investigated the dominant ion species of dry-deposited compounds in the formation of microscopic wetness on needle leaves. The leaf-level dry deposition rates of oxidized and reduced nitrogen species were also preliminarily estimated from leaf washing measurements.
Section snippets
Study site
Measurements were collected in a Norway spruce canopy at the “Waldstein” site, located in the “Fichtelgebirge” mountains in southeastern Germany (50.142127 N 11.86696 E, 776 m asl.) from July 9–25, 2014, during the Fichtelgebirge–Biogenic Emissions and Aerosol Chemistry (F-BEACh 2014) field campaign. The mean height of the canopy is 20 m. Half-hourly measurements of precipitation, wind speed, air temperature, and relative humidity at heights of 12 and 21 m were used for analysis. Detailed site
Micrometeorology and leaf wetness
Temporal changes in micrometeorological variables and electrical conductance (leaf wetness) are shown in Fig. 1. The average wind speed above the canopy at 32 m above the ground was 2.3 m s−1 and the average air temperature within the canopy at 12 m was 16.9 °C during the measurement period (Fig. 1a and b). The prevailing wind direction varied with time (Fig. 1c). High intensity rain events were observed on July 14 and 22 (Fig. 1d), while light rain (or, more likely, fog and dew) were
Evidence for the presence of dry-deposited salts
As mentioned in section 1, combined measurements using both leaf wetness measurements and leaf washing techniques are rarely applied to forest environments. To our knowledge, Burkhardt (1995) is the only past study using a combination of these techniques to outline the role of deposited hygroscopic particles on the leaf surface in terms of forest decline symptoms. The study revealed that hygroscopic salts on the leaf surface will absorb water vapor mostly from leaf transpiration. Since its
Conclusions
In order to investigate the interaction of microscopic leaf wetness and dry-deposited inorganic compounds, combined measurements of leaf wetness and washing experiments were carried out at a coniferous forest in Germany. Presence of hygroscopic salts on needles, such as ammonium nitrate, was hypothesized from an increase in wetness levels of needle branches exposed to the ambient air, in response to an increase in relative humidity. Leaf washing experiments confirmed the presence of inorganic
Credit author statement
Genki Katata: Conceptualization, Methodology, Data collection, Draft preparation and revision. Andreas Held: Conceptualization, Methodology, Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We express our gratitude to Dr. Mirai Watanabe of the National Institute for Environmental Studies (NIES) and Dr. Masaaki Chiwa of the Kyushu University in Japan, and Dr. Jürgen Burkhardt of the University of Bonn and Dr. Otto Klemm of the University of Münster in Germany for providing useful information and discussion. Technical editing was partially supported by Ms. Chikako Okabe of Ibaraki University in Japan. This work was supported by a Postdoctoral Fellowship for Research Abroad, Leading
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