Watershed-scale changes in terrestrial nitrogen cycling during a period of decreased atmospheric nitrate and sulfur deposition
During a period of decreased atmospheric N deposition, we assessed changes in forest N cycling by evaluating trends in tree-ring d15N values (between 1980 and 2010; n ¼ 20 trees per watershed), stream nitrate yields (between 2000 and 2011), and retention of atmospherically-deposited N (between 2000 and 2011) in the North and South Tributaries (North and South, respectively) of Buck Creek in the Adirondack Mountains, USA. We hypothesized that tree-ring d15N values would decline following decreases in atmospheric N deposition (after approximately 1995), and that trends in stream nitrate export and retention of atmospherically deposited N would mirror changes in tree-ring d15N values. Three of the six sampled tree species and the majority of individual trees showed declining linear trends in d15N for the period 1980e2010; only two individual trees showed increasing trends in d15N values. From 1980 to 2010, trees in the watersheds of both tributaries displayed long-term declines in tree-ring d15N values at the watershed scale (R ¼ 0.35 and p ¼ 0.001 in the North and R ¼ 0.37 and p <0.001 in the South). The decreasing d15N trend in the North was associated with declining stream nitrate concentrations (0.009 mg N L1 yr1 , p ¼ 0.02), but no change in the retention of atmospherically deposited N was observed. In contrast, nitrate yields in the South did not exhibit a trend, and the watershed became less retentive of atmospherically deposited N (7.3% yr1 , p < 0.001). Our d15N results indicate a change in terrestrial N availability in both watersheds prior to decreases in atmospheric N deposition, suggesting that decreased atmospheric N deposition was not the sole driver of tree-ring d15N values at these sites. Other factors, such as decreased sulfur deposition, disturbance, long-term successional trends, and/or increasing atmospheric CO2 concentrations, may also influence trends in tree-ring d15N values. Furthermore, declines in terrestrial N availability inferred from tree-ring d15N values do not always correspond with decreased stream nitrate export or increased retention of atmospherically deposited N.