Retention of nitrogen by a nitrogen-loaded Scotch pine forest

The rapacity of forests to store added N and how this excess N might intera ct 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 thi s study. Leaching of inorganic N was measured beneath lysimeters with and w ithout living tree roots inside them, and the fate of N-15 injected into th e mot layer was traced by sampling understory vegetation, soil to 20-cm dep th, and leachates, Leaching of inorganic N increased with increasing N dose and was dominated by NO3- on plots to which N had been added In the suspen ded N3 treatment, leaching of N was lower than in the N2 treatment. The pre sence of tree roots greatly reduced leaching of N, The occurrence of labele d NO3- under lysimeters to which labeled NH4+ had been added showed that ni trification was important in treatment N2, but not in N1, suggesting that a dditions of >30 kg N ha(-1) yr(-1) are needed to induce larger net nitrific ation at this site. Our data showed (i) that trees were an important sink f or N, (ii) that additions of P and K had a marginal positive effect on N re tention, and (iii) that leaching from previously N-loaded forest rapidly de creased when the N load was removed.