Long-term stabilization of N-15-labeled experimental NH4+ deposition in a temperate forest under high N deposition

High nitrogen (N) deposition levels, currently present in many industrial and agricultural regions of the world, can strongly affect the functioning of forest ecosystems. In a pine forest with strong N leaching, located in the Netherlands, we studied the long-term fate of a year-long NH4+ deposition cohort labeled with 15N. A high ambient and a low N deposition treatment had been established at the site by means of a roof and sprinklers. Resampling the N pools 19 years after labeling and 11 years after the last sampling, we found similar 15N deltas in needles, twigs and the LF1 organic soil layer of each treatment, indicating intensive N cycling among these pools. In the last 11 years, label recovery decreased in these labile pools, while recovery remained constant in wood and increased in bark. Together these aboveground vegetation pools retained less than 3% of the labeled N. In the organic layers, label recovery after 19 years decreased to 23% in both treatments, while in the mineral soil it increased from 4% to 13% (high N) and from 3% to 29% (low N treatment). Within the mineral soil of the high N treatment the labeled N was mainly found in fine roots, while in the low N treatment most N was incorporated in the two soil density fractions, shifting to the high density fraction with depth. This suggests a low capacity of the mineral soil at high N deposition to incorporate N. After the labeled N had been lost substantially in previous years, especially in the first, its presence remained constant in the last 11 years at 38% (high N) and 54% (low N treatment). Apparently, even in this strongly N leaching ecosystem, N once incorporated, was retained well and did not affect the input-output fluxes of the system.