Effects Of Atmospheric Deposition On Sulfur And Nitrogen Content Of Four Urban Tree Species

Materials and Methods

One-yr.-old dormant seedlings of black locust (Robinia pseudoacacia), black cherry (Prunus serotina), Siberian elm (Ulmus pumila) and yellowpoplar (Liriodendron tulipifera) were planted in 15-cm plastic containers in a potting media containing bog soil and sand (2:1), and placed in a preconditioned environmental chamber (25/20 C day/night thermoperiod; 50 ± 10% relative humidity; 16 hr photoperiod at 250 Em^-2S^-1 photosynthetically active radiation, PAR). During this pretreatment period, all seedlings were watered thoroughly as required and fertilized biweekly with 100 ml of commercial fertilizer solution containing nitrogen, phosphoric acid and soluble potash (20-20-20). When 60 seedlings of each species had broken dormancy, 15 plants were randomly assigned to one of the following fumigation treatments: 1) 0.1 ppm SO₂; 2) 0.1 ppm NO₂; 3) 0.1 ppm SO₂ + 0.1 ppm NO₂; 4) control. Fumigations of black cherry, Siberian elm and yellow-poplar were conducted in a single pass chamber system similar to that described by Heck,et al. (9). Fumigation of black locust seedlings was carried out in chambers similar to those described by Wood et al. (20).

Environmental conditions during fumigation were the same as those previously mentioned. Seedlings of black locust were fumigated 7 days each week. Black cherry, Siberian elm and yellowpoplar seedlings were fumigated only 5 days per week. All plants were fumigated for 7 hr every day over a period of 8 weeks. During fumigation, SO₂and/or NO₂ were added to a charcoal-filtered air stream and were sampled automatically in each chamber at 20 minute intervals. Sulfur dioxide was sampled with a Beckman model 90 A SO₂analyzer, and NO₂ with a Monitor Labs model 8840 Nitrogen Oxides analyzer. Experiments with black locust and black cherry seedlings were conducted once; those involving Siberian elm and yellow-poplar seedlings were repeated a second time.

Following the 8 week fumigation period, the leaves, stems, and roots of each seedling were oven-dried at 100 C for 48 hr and ground together in a Wiley mill to pass a 40-mesh screen. From this dried plant tissue, individual 0.1 g samples were taken for sulfur and nitrogen analyses. Each sample was analyzed for sulfur content using a modification of the iodometric sulfur determination described by Bremanis et al. (3), and for total nitrogen content using an automated Keldahl nitrogen technique (1).

Results and Discussion

The sulfur and nitrogen content of seedlings fumigated with SO₂, NO₂, and a combination of both pollutants is found in Tables 1 and 2, respectively. Since the primary way in which vegetation removes gases from the atmosphere is by stomatai absorption, it was expected that the sulfur and nitrogen content in plant tissue exposed to SO₂ or NO₂ would be greater than comparable tissue grown in the absence of these pollutants. Although this relationship was true for black locust and yellow-poplar seedlings fumigated with 0.1 ppm SO₂ (Table 1), it was not true for any of the species fumigated with 0.1 ppm NO₂ (table 2). The nitrogen data were unexpected, but may be explained by the different rates at which NO₂ is metabolized. Tingey (18) has reported NO₂ uptake in various herbaceous plants can change from a linear to a nonlinear relationship as the length of exposure to the pollutant increases. The concentration of NO₂ during fumigation may also influence the amount of uptake (13).

The quantity of elemental sulfur in black locust and yellow-poplar seedlings fumigated with SO₂ alone was always greater than in unfumigated (control) plants of the same species (Table 1). For seedlings fumigated with a combination of SO₂ + NO₂, the sulfur content was either the same (Siberian elm and yellow-poplar) or significantly less (black locust and black cherry) than for comparable plants exposed to SO₂ alone (Table 1). These results were unexpected and suggest the possibility of an antagonistic or inhibitory effect on sulfur absorption by certain species when contaminants other than SO₂ are present. Elkiey and Ormrod (6) reported similar findings for petunia plants exposed to combinations of SO₂ and ozone. As expected, there was no effect of NO₂ fumigation on the sulfur content of any of the species used in this study, i.e. no significant difference in sulfur content was found among plants fumigated with NO₂ only and among those receiving no fumigation (Table 1).

For the species used in this study, yellowpoplar was found to have the highest endogenous level of sulfur, followed by black locust, elm and black cherry (Table 1). In general, these results agree with published values for these species (16). Comparing the sulfur content of SO₂ fumigated and unfumigated seedlings (Table 1), black locust appears to be the most effective sink plant among the species tested in these experiments. Sulfur contents more than doubled in black locust seedlings over the 8 week fumigation period, while it increased only 14 to 31 % for the remaining three species. However, yeliow-popiar, elm, and cherry were fumigated only 5 days per week while black locust was fumigated 7 days each week.

As stated previously, the total nitrogen content of seedlings fumigated with NO₂ was no greater than similar seedlings without NO₂ (controls). In this respect, the species used in this study were less effective as nitrogen sinks in atmospheres containing NO₂ than they were as sulfur sinks in atmospheres containing SO₂. When a combination of both pollutants was used, the nitrogen content of Siberian elm and black cherry seedlings was actually higher than it was for similar seedlings fumigated with NO₂ only (table 2). These results are unlike those observed for sulfur uptake, where the data suggested a possible competitive effect between the two pollutants. For nitrogen uptake, the data indicate the possibility of an additive effect when plants are fumigated with both pollutants together. One possible explanation for these results may be the influence of SO₂ on stomatai opening. Stomatai conductance has been shown to increase for certain woody plants in the presence of SO₂ (2), and this increase in stomatai opening may be sufficient to allow for the absorption of additional NO₂.

The data from this study present additional evidence to suggest that trees have considerable potential to filter gaseous pollutants from the atmosphere. There is a need for additional research to identify the species which can function effectively in the capture and recycling of atmospheric contaminants. As yet we lack the necessary quantitative data to demonstrate that trees are capable of significantly reducing air pollution levels below the threshold values which harm urban vegetation.