Foliar sulfur and nitrogen along an 800-km pollution gradient

Emissions of sulfur (S) and nitrogen (N) oxides in the midwestern and northeastern United States result in pronounced regional gradients of acidic deposition. The objective of this study was to determine the extent to which atmospheric deposition alters the uptake and cycling of S and N in five analogous northern hardwood forests located along one of the most pronounced regional gradients of SO42−-S and NO3−-N deposition in the United States. We tested the hypothesis that acidic deposition would alter foliar S and N ratios and nutrient cycling in aboveground litter fall. Sulfate in both wet deposition and throughfall increased by a factor of two across the 800-km deposition gradient. The July concentration of S in sugar maple (Acersaccharum Marsh.) leaves increased from about 1600 μg•g−1 at the northern research sites to 1800–1900 μg•g−1 at the southern sites. Differences in leaf litter S concentration were even more pronounced (872–1356 μg•g−1), and a clear geographic trend was always apparent in litter S concentration. The 3-year average S content of leaf litter was 63% greater at the southern end of the pollution gradient. Nitrate and total N deposition were also significantly greater at the southern end of the gradient. The concentration of N in both summer foliage and leaf litter was not correlated with N deposition, but the content of N in leaf litter was significantly correlated with N deposition. The molar ratios of S:N in mid-July foliage and leaf litter increased as atmospheric deposition of SO42−-S increased. Ratios of S:N were always much greater in leaf litter than in mid-July foliage. The molar ratios of S:N retranslocated from the canopies of these northern hardwood forests were less than those in mid-July foliage or litter fall and showed no geographic trend related to deposition, suggesting that S and N are retranslocated in a relatively fixed proportion. Significant correlations between SO42−-S deposition and foliar S concentration, S cycling, and the molar ratio of S:N in foliage suggest that sulfate deposition has altered the uptake and cycling of S in northern hardwood forests of the Great Lakes region.