Reductions in Atmospheric Nitrogen Deposition Linked to Increased Stream Silica on Five Appalachian Forest Basins

David R. DeWalle
School of Forest Resources,
Penn State University,
Univ. Park, PA USA

Reductions in atmospheric nitrogen (N) deposition appear to be linked to increases in stream silica (Si) on five unglaciated Appalachian forest basins in Pennsylvania. Possible causes for this linkage being investigated are: (1) reduced N deposition is causing reduced productivity and thus reducing assimilation of Si by diatoms and terrestrial vegetation and/or (2) reduced N deposition is leading to increased respiration by soil heterotrophs, which is increasing silicate weathering. NADP and CASTNet data for the region clearly show downward trends in N deposition over the past two decades. Concomitantly, stream nitrate levels have been declining and Si levels have been increasing on the five basins being studied as part of the EPA Long-Term Monitoring program. Molar Si/N ratios computed for the five streams are >>1 indicating that N is limiting to diatom production (critical ratio 1), thus reduced N deposition could have caused reduced assimilation of Si by diatoms. Declining dissolved organic carbon found in these streams may also be a manifestation of declining ecosystem productivity caused by reduced N deposition. However, trends in stream Si/N are positive, indicating that reduced diatom productivity and assimilation, which would release Si in direct proportion to N declines, probably was not the sole cause of increasing Si. Furthermore, increasing trends in stream dissolved inorganic carbon and sodium also indicate that increased weathering of sodium-aluminosilicate minerals has taken place. Long-term stream monitoring on these five basins suggests that a shift in ecosystem Si cycling, and perhaps C cycling, has occurred due to reduced N deposition. Continued monitoring of atmospheric deposition and stream chemistry along with critical experimentation are needed to fully understand these previously unreported impacts of reduced N deposition.