Soil nitrogen transformations in beech and maple stands along a nitrogen deposition gradient

A gradient of atmospheric nitrogen deposition exists across the northeaster n United States due to the concentration of urban and industrial sources of nitrogen oxides in the southern and western parts of the region. We examin ed possible effects of N deposition on N cycling in forests by measuring po tential net mineralization and nitrification of soils under single-species plots of sugar maple and American beech along this gradient. The total atmo spheric deposition of nitrogen was estimated to range from 11.1 kg N.ha(-1) .yr(-1) at a site in southern New York to 4.2 kg N.ha(-1).yr(-1) at a site in western Maine. Although potential net mineralization and nitrification r ates were extremely variable, highly significant positive correlations were observed between N deposition and mineralization and nitrification rates i n organic horizons in maple plots, but not in beech plots. The correlation between deposition and N cycling variables was weaker in mineral horizons t han in organic horizons for maple plots, and no significant correlations be tween these variables were found for beech mineral horizons. Many beech soi ls showed no net nitrification even under the higher deposition conditions. Percentage nitrogen (%N) of the organic horizons increased with increasing deposition in sugar maple, but not in beech plots. In organic horizons of both species, mineralization and nitrification increased with increasing %N , although the slopes of the increases were steeper for maple than for beec h. Nitrogen deposition, mean annual temperature, and mean annual precipitat ion were intercorrelated across the sites of this study, but the data indic ate that the observed patterns of N mineralization in maple plots resulted from the N deposition gradient rather than the climate gradient. These resu lts suggest that the two species respond differently to N accumulation from atmospheric deposition. Species differences in the responses of forests to N deposition should be considered in both the prediction of forest respons e and the management of forest composition which could affect that response .