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, NO₃⁻ concentrations were also compared between 2004–2005 and 2014–2015, and a link between soil calcium and stream NO₃⁻ was investigated using data from 387 Adirondack streams that were sampled in either 2003–2005 or 2010–2011. No trends in N export or NO₃⁻ 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 NO₃⁻ concentrations in Buck watersheds bracketed regional results and were consistent with the regional streams that showed no overall change in NO₃⁻ concentrations between 2004 and 2014. A negative correlation observed between NO₃⁻ concentration and watershed buffering capacity expressed as the ratio of Ca²⁺ to SO₄²⁻ also suggested that stream NO₃⁻ concentrations were elevated where soil Ca depletion had occurred.