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

Positive correlation between wood δ15N and stream nitrate concentrations in two temperate deciduous forests

“…These results indicate that a variety of nitrate concentration records may be used to compare against future records of wood δ 15 N. Similar to our results, a recent study found that wood δ 15 N records from a single species, tulip poplar (Liriodendron tulipifera), could explain inter-annual variation of stream nitrate concentrations at a catchment in Fernow Experimental Forest [Burnham et al 2016]. Other observational studies have suggested similar relationships through time [McLauchlan et al 2007, Sabo et al 2016a. In combination, our results and previous studies suggest that wood δ 15 N models can be used to fill in temporal gaps in, and potentially extend, water quality records back in time.…”

Recovery of Soils From Acidic Deposition May Exacerbate Nitrogen Export From Forested Watersheds

“…Therefore, watershed export of dissolved inorganic N (DIN) was determined by calculating a monthly mean concentration for NO 3 − and NH 4 + from the two to three water samples collected in each month, which was then multiplied by the total daily flow to compute watershed dissolved inorganic N export as kilograms of nitrogen per hectare per day. This method of calculating watershed export values gave similar results to those presented in Sabo et al (). Estimates of monthly mean DON concentrations were used in the same manner to determine watershed DON export.…”

“…The Adirondack region has experienced pronounced changes in both terrestrial and aquatic ecosystems from some of the highest levels of S and N deposition in the United States (Driscoll et al, ; Lawrence et al, ) and has also begun to show indications of recovery in soils (Lawrence et al, ) and streams (Lawrence, Burns, & Riva‐Murray, ) from declining S deposition. In a previous study, export of N (largely in the form of NO 3 − ) from these same two watersheds unexpectedly did not exhibit trends from 2000 to 2011 ( P > 0.10) despite a marked decrease in wet N deposition (Sabo et al, ). However, the cause of the continued watershed export of N was unclear, as was the relationship of this response to other forested watersheds that have been impacted by S and N deposition.…”

“…Increased decomposition rates from lower acidic deposition would be expected to increase nutrient availability, and the high watershed N export suggested that N availability exceeded demand over the study period. Sabo et al () observed declining d 15 N values that suggested lower N availability before decreases in N deposition occurred in these watersheds (before 1995). The cause of this relationship is unclear but could perhaps be tied to decreases in S deposition that improved tree growth but had not yet enhanced forest floor mineralization.…”

Continental scale variability of foliar nitrogen and carbon isotopes in Populus balsamifera and their relationships with climate

“…Of particular interest to understanding plant-environment relationships are the isotopic composition of nitrogen and carbon in leaves. Foliar δ 15 N, which serves as an indicator of terrestrial N cycling 5 , 6 , varies systematically between species 7 – 9 , mycorrhizal associations 10 , 11 and along gradients in climate 12 , and, when measured over time, can be used to infer ecosystem response to disturbance and climate change 13 – 16 . Local to regional-scale anthropogenic impacts on N cycling can also be important influences on foliar δ 15 N 17 – 20 .…”