Responses of a nitrogen-saturated forest to a sharp decrease in nitrogen input

• The reversibility of induced N saturation was investigated in a 46- yr-old pine (Pinus sylvestris L.) forest in northern Sweden Ammonium nitrate has been applied annually since 1971 to plots (30 by 30 m) at average dosages of 36 (N1), 72 (N2), and 108 (N3) kg N ha(-1) yr(-1): with or without P and K addition (background N deposition is <4 kg ha(-1) yr(-1)). In 1990, after two decades of treatment, the largest N application (N3) was suspended, while N1 and N2 still received ammonium nitrate applications. Seven gears after the last application in N3, the N availability measured as N concentration in plants spine roots and needles and in leaves of the grass Deschampsia flexuosa (L.) Trin.] and activity of the enzyme nitrate reductase in leaves of D. flexuosa, and N-15 uptake by excised pine roots, was at the same levels as in N1, although more than twice the amount of N has been applied in total to N3. The arginine concentrations in pine needles, concentrations of exchangeable mineral N in the organic layer and the uppermost 20 cm of the mineral soil were at the same levels as in the control plots. Thus, an experimentally induced N excess was, according to these measurements, to a high degree reversed 7 yr after the last N application. However, the composition of the understory vegetation still differed markedly from the untreated control 8 yr after the last N3 application.

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