Environ. Sci. Technol. 1996, 30, 246-251
Concentration and Distribution of Metal Elements at the Surface of Balsam Fir Foliage by Micro-Proton Induced X-ray Emission ZHI-QING LIN* AND PETER H. SCHUEPP Department of Natural Resource Sciences, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue, Quebec, H9X 3V9 Canada
Metal contaminations at the surface of balsam fir (Abies balsamea (L.) Mill.) needles collected at two locations in southern Quebec were determined by micro-PIXE (micro-proton induced X-ray emission) with scanning depth of 3-4 or 45-50 µm and scanning spacing from 15 to 250 µm. At adaxial surfaces, concentrations of Al, Si, Ca, Ti, Fe, and Zn were significantly (P < 0.05) higher at the near-urban site than at the mountainous location. Accumulation of Mg, Al, Si, K, Mn, Fe, and Zn was significantly (P < 0.01) higher at surfaces of unwashed needles as compared to chloroform-washed needles, which shows significant metal accumulation in/on epicuticular wax layers. Metal deposits, except Mn, in epicuticular wax layers were significantly affected by washing with acidic solution. Distributions of Mg, K, Mn, and Zn deposits at the surface were lower around central regions than near foliar edges, while Si was higher around the central area. Scanning to the 45-50 µm depth showed significant accumulation of Ni on either side of the foliar center groove.
Introduction Heavy metals have been suggested to play a role in the forest decline in North America (1) and knowledge of pathways of heavy metals deposited from the atmosphere to forests is essential in estimating biogeochemical cycling rates (2, 3). Concern over the biological significance of metal deposition to forests stems from the potential longterm accumulation of trace metals in forests and their known biotoxicity (4). Accumulation and distribution of atmospheric metal elements deposited on forest surfaces are primarily dependent on the metal concentrations in air (5), the deposition pathways (6), and the surface structures (7, 8). The surface structures of foliage vary under different growing conditions (9) and the degradation of foliar surfaces is enhanced by air pollutants (10). Gu ¨ nther et al. (11) reported that plant surfaces covered with epicuticular wax * Corresponding author e-mail address:
[email protected].
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were less contaminated by deposited particles than those without wax. While large surface deposits (>1 µm in size) are easily washed off from foliage by precipitation, aerosols or submicron particles might be adsorbed and embedded in degraded epicuticular wax layers or in epistomatal chambers (12, 13). Substantial amounts of metal elements have been found on the surface of Norway spruce (Picea abies) needles by chloroform extraction after washing the needles in water (5). Overall, elemental composition of surface deposits on Norway spruce needles has been intensively investigated by electron probe microanalysis (EDX) (14), X-ray photoelectron spectroscopy (XPS) (13), and extraction of epicuticular wax layers from needles in organic solutions (15). By directly scanning needle surfaces, Grill and Golob (14) reported significant differences in concentrations of Na, Mg, Al, Si, P, S, K, Ca, Mn, and Fe at needle surfaces from environments with different levels of air pollution. In the semiquantitative XPS analysis of Simmleit et al. (13), the permanently adsorbed particles on spruce needle surfaces were found to consist mainly of organic materials with Si, Al, and Pb accounting for 3%, 1%, and 0.01% (on a mass basis), respectively, and Fe and Mn only present in trace amounts. Acid fog deposition has been observed to affect metal input from the atmosphere to subalpine forests in southern Quebec (6, 16). Balsam fir (Abies balsamea) has been found with greater fog deposition on the upper than the lower surface, but with least efficiency in fog collection compared with red spruce (Picea rubens) and Norway spruce (17). Considering that such disparity is mainly due to differences in the surface structure and needle orientation, the concentration and spatial distribution of metals deposited on balsam fir needles must be further explored when pathways of metal deposits are to be considered in such forests co-dominated by balsam fir and red spruce (18). The PIXE (proton induced X-ray emission) technique with microscanning facility is ideally suited for the study of spatial distribution of metal pollutants on forest surfaces (19, 20). Micro-PIXE has been used in determining the element concentrations at different depths below apple skin (21) and the elemental distribution within roots by line-scanning along the diameter of the cross section (22). By scanning needle surfaces with micro-PIXE, our study determined metal concentrations in and on epicuticular wax layers and characterized their relationship with surface microstructure, especially the distribution of stomata. It also evaluated the effects of simulated acid rain on accumulation and distribution of metal deposits in/on epicuticular wax layers at the surface of balsam fir needles.
Materials and Methods Site Description and Sampling. Roundtop Mountain is located near Sutton, 95 km SE of Montreal with peak elevation of 970 m, and Morgan Arboretum is a near-urban low elevation (40 m) site on the Montreal island. Roundtop Mountain is affected by acidic fog deposition, and the percentage of time for immersion of forests in acidic fog along the mountain slope increases as a function of elevation (23). The reported 1985-1991 mean pH in fog at the 845-m elevation of Roundtop Mountain was 3.9 (24), while the Morgan Arboretum site had an annual pH of 4.3-
0013-936X/96/0930-0246$12.00/0
1995 American Chemical Society
TABLE 1
TABLE 2
Meteorological Observations during the 30 Days Preceding Sampling Date (October 15-16, 1993) at Roundtop Mountain and Morgan Arboretum Locations
Leaching of 54Mn and 65Zn from Internal Foliar Tissues of 1-Year Old Balsam Fir Shoots Washed in Chloroform Solution for 15 sa
Roundtop Mountain total rainfall (mm) 127 rain events (>1 mm) Sep 24, 25, 27-29; Oct 2-5, 7-9, 11, 12 wind direction SW -1 wind speed (m s ) 4.6 ( 1.3
54Mn
65Zn
Morgan Arboretum 58 Sep 16, 24, 27, 28; Oct 2 SW 3.2 ( 1.2
FIGURE 1. Scanning electron micrographs of adaxial and abaxial surfaces of balsam fir needles. Ad, adaxial surface; Ab, abaxial surface; S, stomata.
4.4 in precipitation (25). These two sampling locations have been previously described with detailed characterization of soil and tree species by Lin et al. (18). Median age of sampled mature trees was approximately 100 yr at the Roundtop site and about 30 yr at the Morgan Arboretum, with median tree heights of 7-8 and 10-11 m, respectively, at the two sites. Sampling was carried out on October 15-16, 1993, at the 840-m elevation of the mountain site and at the Morgan Arboretum. Shoots were collected from branches near the middle part of the canopy, about 5 m above ground, on the side of prevailing wind direction (SW) and stored in plastic bottles. The meteorological observations for the 30 days preceding the sampling date are summarized in Table 1, showing more precipitation at the Roundtop Mountain location than at the Morgan Arboretum location. Balsam fir needles are generally 300-350 µm thick, 1.41.6 mm wide, and 18-22 mm long. Stomata are mainly distributed on the abaxial surface, with six to eight rows on either side of the mid-rib (Figure 1). The thickness of the epicuticular layer is about 3-4 µm (26). Metal concentrations at the foliar surfaces were determined in samples from
activity in unwashed shoots (Bq) activity in washing solution (Bq) leaching ratio (%) a
4144 0.1
