ATMOSPHERIC DEPOSITION, ORGANIC MATTER STOICHIOMETRY, AND LONG-TERM NUTRIENT LOSSES FROM FOREST WATERSHED ECOSYSTEMS

The flow of water links complex interactions and feedbacks among the atmosphere, soils, nutrients, and biota and in turn shapes forest ecosystem biogeochemical cycling over long time scales. Here we present results from a survey of soils and streams in forested watersheds spanning a wide atmospheric N deposition gradient (5 to 45 kg N ha^-1 yr^-1) in the Appalachian Mts., USA and analysis of long-term (30-yr) hydrochemical data from reference watersheds in the Coweeta Hydrologic Laboratory to understand long-term regulation of watershed N losses. Across the loading gradient, stream DON and DIN increased significantly and non-linearly with higher deposition, shifting abruptly in quantity and relative proportion at a threshold deposition of ~ 10 kg N ha^-1 yr^-1. Dissolved organic matter (DOM) C: N ratios also declined with increased deposition. This pattern was not attributable to a change in DOC but rather in DIN loading and accumulation of soil N. Stream water C: N drops substantially below the 1:1 line relating bulk soils and stream solution as soil C: N declines, suggestive of a proportionally larger N enrichment in dissolved compared to solid organic pools over the course of ecosystem N saturation. At the low end of the N gradient, further exploration of long-term hydrochemical data from Coweeta revealed decadal shifts in precipitation and stream chemistry. Over the last 30 years, precipitation Cl- inputs have declined in concentration and flux. Because 90% of Cl- inputs here derive from marine aerosols, this pattern suggests either a shift in dominant weather patterns or possible dilution of marine sources. In contrast, analysis of cation: Si ratios as an index of weathering losses and hydrologic flowpaths revealed a pronounced hyperbolic pattern of cation losses relative to rock-derived silica over the three decades of study. These patterns are similar to those reported for long-term hydrologic records and suggest decadal-scale shifts in dominant watershed flowpaths associated with droughts. Unlike cations, patterns of N were characterized by steep increases in NO3-N inputs but not losses even after accounting for losses in organic forms, reflecting the highly retentive nature of these forests. We explore implications of climate change and shifts in atmospheric chemistry for long-term N cycling in forest watersheds.