PARENT SESSION
Contributed Oral Session 153: Forest Soils and Nutrient Cycling
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 514 A, Level 5, Palais des congrès de Montréal

Increased export of dissolved organic nitrogen from forest watersheds across a gradient of chronic atmospheric loading.

Brookshire, E.N.J.^*,1, Valett, H.M.¹, Webster, J.R.¹, Thomas, S.A.², ¹ Department of Biology, Blacksburg, VA, USA² Department of Ecology and Evolutionary Biology, Ithaca, NY, USA

ABSTRACT- Available nitrogen (N) is often scarce in terrestrial and aquatic ecosystems and thus limits biotic activity. High demand by microbe-plant systems and incorporation into slow-turnover pools can regulate losses of bioavailable N from watershed ecosystems. Humans have greatly increased inputs of available N to many temperate forest ecosystems via atmospheric deposition, resulting in non-linear shifts in terrestrial N cycling and elevated dissolved inorganic N (DIN) in stream water. In contrast, dissolved organic N (DON) often dominates N losses from unpolluted watersheds, but far less is known about its production, cycling, and retention in response to chronic N loading. 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^-1yr^-1) in the Appalachian Mts., USA. Stream DON and DIN concentrations increased significantly and non-linearly with higher deposition, shifting abruptly in absolute and relative abundance at a threshold deposition of 10 kg N ha^-1yr^-1. Soil C: N ratios also showed steep declines with N deposition (from 30 to 17). Consequently, DON and DIN concentrations were negatively associated with soil C: N ratios. Dissolved organic matter (DOM) C: N ratios also declined with increased deposition, suggesting a shift in DOM quality. This pattern was not attributable to a change in DOC but rather in DIN loading. Elevated N loading appeared to result in a steep increase in the slope of the DON vs. DOC curve above that expected based on strict stoichiometric patterns observed across a global range of soil and stream DOM. While mechanisms of increased DON fluxes are unclear, our results point to atmospheric pollution and associated shifts in soil pools and processes as key drivers of DON quantity and possibly quality. Increases in DIN and DON export with N loading may have large implications for elemental cycling in terrestrial and aquatic ecosystems.

Key words: don, watershed, stream, soil