Retention of Nitrogen by a Nitrogen‐Loaded Scotch Pine Forest

Abstract

AbstractThe 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 NH4NO3 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 15N 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.