Impact of traffic pollution on the nitrogen metabolism and zinc content of roadside vegetation
The impact of traffic pollution on the nitrogen metabolism and zinc content of roadside vegetation was investigated by measuring a range of physiological responses and surface Zn in trees. Dry deposition in the form of NOx ranged from 14.8 to 40.7 Kg N ha yr"1 within the woodland. The amount of deposition was spatially related to the distance from the motorway that runs adjacent to the woodland. Maximum NOx deposition was found next to the motorway in the rural downwind environment At mis site trees exhibited significantly higher growth rates as assessed by yearly shoot extension as well as exhibiting more positive foliar 815N signatures compared to trees at a control site 600m away. These two results are taken as an indication that atmospheric NOx produced by motorway traffic is contributing to the nutrition and growth of roadside trees. In vivo activity of the N assimilatory enzyme, NR was assayed in the foliage of S. nigra, C. monogyna, B. pendula and Quercus spp. growing at the woodland. Elevated NR activity close to the road was only demonstrated for one species, S. nigra suggesting the assimilation of traffic-derived NOx following foliar uptake. High NR activities in this species may mean mat leaves act as NOx sinks, effectively 'mopping up' NOx from the atmosphere. In general, proximity to the traffic appeared to have few effects of the motorway on tissue metabolite pools. Total N and tissue NO3" were not affected by proximity to the motorway. Similarly, no significant differences were found for total P concentrations. It is suggested that at this lowland deciduous woodland, soil nutrients are non-limiting and thus allow growth to occur in response to inputs from traffic-derived N. It is proposed that this increase in biomass has the effect of diluting elements or metabolites such as total N and NO3" so that they remain at a constant level. It is recommended that at similar sites, rather than using metabolites as biomarkers, foliar 8,5N signatures in combination with simple growth measurements are used as to demonstrate traffic effects. Whereas results indicated that tissue Zn was unaffected by proximity to the road, surface Zn was found to accumulate during dry weather, only to be washed off during rainfall events. Amounts of surface Zn were examined on a 5 tree species growing in central London, namely S. nigra, C. monogyna, F. excelsior, A. hippocastanum and P.acerifolia. Surface Zn was highest on the leaves of C. monogyna which contained 13.8 mg Zn m"2 on its leaves. Scanning electron microscopy was able to show that species differences may be related to epicuticular wax characteristics. Results are discussed in relation to the potential use of trees for ameliorating atmospheric NOx and particulate pollution levels.