Nitrogen turnover in the leaf litter and fine roots of sugar maple

In order to better understand the nitrogen (N) cycle, a pulse of 15 NO 3 − was applied in 1998 to a sugar maple ( Acer saccharum ) dominated northern hardwood forest receiving long‐term (1994–2008) simulated atmospheric N deposition. Sugar maple leaf litter and live fine‐root 15 N were quantified for four years prior to labeling and for 11 subsequent years. Continuous sampling of 15 N following addition of the tracer enabled calculation of leaf litter and fine‐root N pool turnover utilizing an exponential decay function. Fine‐root 15 N recovery peaked at 3.7% ± 1.7% the year the tracer was applied, while leaf litter 15 N recovery peaked in the two years following tracer application at ∼ 8%. These results suggest shoots are primarily constructed from N taken up in previous years, while fine roots are constructed from new N. The residence time of N was 6.5 years in leaf litter and 3.1 years in fine roots. The longer residence time and higher recovery rate are evidence that leaves were a stronger sink for labeled N than fine roots, but the relatively short residence time of tracer N in both pools suggests that there is not tight intra‐ecosystem cycling of N in this mature forest.