Sara E. Scanga | Geology Reports

USGS2020

Recovery of soils from acidic deposition may exacerbate nitrogen export from forested watersheds

Effects of ambient decreases in N deposition on forest N cycling remain unclear as soils recover from acidic deposition. To investigate, repeated soil sampling data were related to deposition, vegetation, and stream data, for 2000–2015 in North and South Buck Creek watersheds, in the Adirondack region of New York, USA. In 63 other Adirondack streams, NO 3 − concentrations were also compared between 2004–2005 and 2014–2015, and a link between soil calcium and stream NO 3 − was investigated using data from 387 Adirondack streams that were sampled in either 2003–2005 or 2010–2011. No trends in N export or NO 3 − concentrations were observed in either Buck watershed despite a 45% decrease in N deposition, although South Buck N export was 2 to 3 times higher than in North Buck, where 48% of deposited N was accounted for by accumulation in the upper soil. In marked contrast, the upper profile in South Buck showed a net loss of N. Increased decomposition appeared likely in South Buck as those soils are adjusted to lower levels of acidifying S deposition, whereas decomposition increases in North Buck were likely suppressed by high levels of natural organic acidity. Stream NO 3 − concentrations in Buck watersheds bracketed regional results and were consistent with the regional streams that showed no overall change in NO 3 − concentrations between 2004 and 2014. A negative correlation observed between NO 3 − concentration and watershed buffering capacity expressed as the ratio of Ca 2+ to SO 4 2− also suggested that stream NO 3 − concentrations were elevated where soil Ca depletion had occurred.

JGR: Biogeosciences↗

USGS2016

Watershed-scale changes in terrestrial nitrogen cycling during a period of decreased atmospheric nitrate and sulfur deposition

Recent reports suggest that decreases in atmospheric nitrogen (N) deposition throughout Europe and North America may have resulted in declining nitrate export in surface waters in recent decades, yet it is unknown if and how terrestrial N cycling was affected. During a period of decreased atmospheric N deposition, we assessed changes in forest N cycling by evaluating trends in tree-ring δ 15 N values (between 1980 and 2010; n = 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 δ 15 N 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 δ 15 N values. Three of the six sampled tree species and the majority of individual trees showed declining linear trends in δ 15 N for the period 1980–2010; only two individual trees showed increasing trends in δ 15 N values. From 1980 to 2010, trees in the watersheds of both tributaries displayed long-term declines in tree-ring δ 15 N values at the watershed scale (R = −0.35 and p = 0.001 in the North and R = −0.37 and p <0.001 in the South). The decreasing δ 15 N trend in the North was associated with declining stream nitrate concentrations (−0.009 mg N L −1 yr −1 , p = 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% yr −1 , p < 0.001). Our δ 15 N 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 δ 15 N values at these sites. Other factors, such as decreased sulfur deposition, disturbance, long-term successional trends, and/or increasing atmospheric CO 2 concentrations, may also influence trends in tree-ring δ 15 N values. Furthermore, declines in terrestrial N availability inferred from tree-ring δ 15 N values do not always correspond with decreased stream nitrate export or increased retention of atmospherically deposited N.

Atmospheric Environment↗