Nitrogen (N) fixation by cyanobacterial associates in feather moss carpets is the primary source of N in northern boreal forests; however, there is little know regarding the influence of anthropogenic impacts on this process.  Wet deposition of inorganic N on moss carpets has been found to reduce N fixation and reduce populations of cyanobacteria in the mosses.  Automobile and truck traffic release nitrogen oxides within road corridors and a significant portion of the released N may be deposited in the immediate vicinity or transported offsite.  Plant community studies conducted near highways may be directly influenced by traffic related N deposition, however, there is little data available with which to characterize the potential for such an impact.  We investigated how distance from major highways in this region influenced N deposition rates and how these differences in turn influence N fixation.  Four replicate 200 m transects were established running perpendicular to road segments along four major highways in northern Sweden.  Subplots were established at 0, 5, 10, 50, 100, and 200 m into forest reserves away from the road shoulder.  Similar plots were established on four remote gravel roads which experience little or no monthly traffic. Resin lysimeters were installed at each plot along this transect and left to collect inorganic N over a 12 month period.  Nitrogen fixation rates in feathermoss carpets were measured at three separate points during the 12 month study cycle. 

Results/Conclusions

Nitrate deposition was found to be elevated near major highways roads and decline with distance from the road corridor.  No gradient in N deposition was observed in association with remote roads.  Nitrogen fixation was below detection limit 0 – 10 m away from the highways and appeared to return to natural levels 100 m from the highway.  There was no significant difference in N fixation rates in plots 0 – 200 m away from remote road corridors, demonstrating that this is not a gap phenomenon.  Nitrogen fixation rates are highly likely to be artificially low in carpets that are within 50 m of busy road corridors.  All biological studies conducted on lands in the immediate vicinity of busy roads may be influenced by automobile induced N deposition.