Forest canopy uptake of atmospheric nitrogen deposition at eastern US conifer sites: Carbon storage implications?

Dry deposition determinations, along with wet deposition and throughfall (T F) measurements, at a spruce fir forest in central Maine were used to estim ate the effect of atmospherically deposited nitrogen (N) uptake on forest c arbon storage. Using nitric acid and particulate N as well as TF ammonium a nd nitrate data, the growing season (May-October) net canopy uptake of atmo spheric, predominantly anthropogenic, N deposition was found to be 1-5 kg N ha(-1). The ratio of growing season net canopy N uptake to that of recycle d root N uptake (10-30 kg N ha(-1) during the growing season) suggests a su bstantial modification of the N cycle at this Maine spruce fir forest over the past decade. The growing season 1-5 kg ha(-1) canopy N uptake may induc e an enhanced annual carbon (C) storage of 250-1350 kg C ha(-1) yr(-1). Thi s magnitude of N-stimulated C storage may be compared with measured annual C sequestration of similar to 2000 kg C ha(-1) for each of the years 1996-1 998 at this Maine site. Consideration of four other eastern U.S. forest sit es for which net canopy N uptake data are available suggests that from 285 to 2950 kg C ha(-1), annual C sequestration may be occurring at these conif er sites, consistent with the Maine site results.