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.