Retention of Nitrogen by a Nitrogen‐Loaded Scotch Pine Forest

The capacity of forests to store added N and how this excess N might interact with other nutrients is not well understood. We studied N retention in a Scotch pine ( Pinus sylvestris L.) forest that had received two decades of annual additions of N as NH 4 NO 3 at four rates (N0‐N3, equivalent to 0, 36, 72 and 108 kg N ha ‐1 yr ‐1 ), with and without supplementary additions of P and K. The highest N treatment (N3) was suspended three years before this study. Leaching of inorganic N was measured beneath lysimeters with and without living tree roots inside them, and the fate of 15 N injected into the mor layer was traced by sampling understory vegetation, soil to 20‐cm depth, and leachates. Leaching of inorganic N increased with increasing N dose and was dominated by NO ‐ 3 on plots to which N had been added. In the suspended N3 treatment, leaching of N was lower than in the N2 treatment. The presence of tree roots greatly reduced leaching of N. The occurrence of labeled NO ‐ 3 under lysimeters to which labeled NH + 4 had been added showed that nitrification was important in treatment N2, but not in N1, suggesting that additions of >30 kg N ha ‐1 yr ‐1 are needed to induce larger net nitrification at this site. Our data showed (i) that trees were an important sink for N, (ii) that additions of P and K had a marginal positive effect on N retention, and (iii) that leaching from previously N‐loaded forest rapidly decreased when the N load was removed.