Extensive Microbial Processing of Atmospheric Nitrate Inputs Along a Nitrogen Deposition Gradient.

Chronic elevated atmospheric nitrate deposition to forests can alter the terrestrial nitrogen cycle and lead to ecosystem nitrogen saturation.  From August 2012 through July 2013, we measured concentrations and nitrate stable isotopic compositions (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) of precipitation and stream waters collected monthly from hardwood-dominated catchments at Coweeta, Fernow, and Hubbard Brook Experimental Forests.  Based on long-term (35 years) average atmospheric nitrate deposition rates, these study sites form a nitrogen deposition gradient, ranging from 10.8 kg ha^-1 yr^-1 to 17.3 kg ha^-1 yr^-1 of nitrate.  During the study period, temporal trends in precipitation nitrate concentrations among sites were variable.  Temporal trends in Δ¹⁷O of precipitation nitrate were similar among the sites, with the highest values occurring in the colder months and lower values during warmer months.  Mean stream nitrate concentrations among the sites were significantly different over the study period (α=0.05; Fernow=1.45 mgL^-1; Hubbard Brook=0.21 mgL^-1; Coweeta=0.04 mgL^-1), and displayed different seasonal trends.  Mean Δ¹⁷O of nitrate values were significantly different (α=0.05; Fernow=-0.36‰; Hubbard Brook=+0.39‰; Coweeta=+3.95‰), but little seasonal variability in stream water Δ¹⁷O values was evident at any of the sites.  Comparisons of mean precipitation and stream water Δ¹⁷O of nitrate values at each site indicate that most of the nitrate (between 84 and 100%) in streams draining these catchments is derived from microbial nitrification rather than direct inputs of unprocessed atmospheric nitrate.  Mean percentages of unprocessed atmospheric nitrate in streams were inversely related to mean stream water nitrate concentrations and long-term average rates of atmospheric nitrate deposition, suggesting greater microbial processing of atmospheric nitrate inputs at sites with higher atmospheric deposition rates.  We explore potential explanations for the observed trends in nitrate concentrations and isotopic compositions, and discuss the utility of Δ¹⁷O in assessing catchment-scale nitrogen biogeochemical cycling.