Effects of nitrogen (N) deposition on N cycling in forest ecosystems have been well studied, often through experimental manipulations. However, an incomplete understanding of how ambient decreases in N deposition are affecting ecosystem N processes remains, and is further complicated by the start of soil recovery from decreased acidic deposition. To address these issues, monitoring data for 1999-2015 were evaluated in North and South Buck Creek watersheds in the western Adirondack region of NY. North Buck is forested with a mixture of hardwoods and conifers, and has organic-rich soil with a 21-cm thick forest floor, whereas South Buck has mostly hardwood vegetation and an 8-cm forest floor. Concentrations of nitrate (NO3-) in 64 streams in the vicinity of these watersheds in 2004 were also compared to values in 2014, and the relationship between soil buffering and stream NO3- was investigated with data from over 400 headwater streams throughout the Adirondack region.
Results/Conclusions
South Buck N export in stream water was 2-to-3 times higher than for North Buck but no trends were observed in either watershed despite a 45% decrease in N deposition. A significant increase in tree N storage from 2005-2015 was observed in South Buck, but not in North Buck. In North Buck, 72% of atmospherically deposited N in the period 2000-2015 could be accounted for by accumulation in the upper soil profile, but South Buck soils lost N in the upper profile over the period. The South Buck response was tied to a loss of forest floor organic carbon, most likely resulting from increased decomposition as these soils adjusted to lower levels of acidic deposition. In contrast, decomposition increases in North Buck may have been constrained by naturally high levels of organic acidity. Regional stream sampling results were consistent with the Buck Creek results, showing no significant overall difference in NO3- concentrations between 2004 and 2014, and a significant negative correlation between NO3- concentrations and watershed buffering capacity. These results indicate that Adirondack streams in hardwood-dominated watersheds with low-carbon soils may continue to demonstrate acidification in the coming decades, despite reduced atmospheric N deposition.