Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short-term (o15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long-term (30 years) experiment with annual N loading on an unpolluted boreal forest. Ammonium nitrate was added to replicated (N 5 3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha À1 yr À1 , respectively. A third treatment, N3, 108 kg N ha À1 yr À1 , was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m 3 ha À1 stemwood in N2 and a gain of 100 m 3 ha À1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca 2 1 , Mg 2 1 , K 1 ) in the mineral soil, along with decreases in pH and increases in exchangeable Al 3 1 . In contrast, the organic mor-layer (forest floor) in the N-treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N-treated plots, providing evidence of up-lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N-treated plots, but was corrected at an early stage. Extractable NH 4 1 and NO 3 À were high in mor and mineral soils of on-going N treatments, while NH 4 1 was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N-load than predicted. Our long-term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long-term effects of N deposition at lower rates.

An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ15N, of N during the transport of N from the soil through the fungus into the plant. Non‐ mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi, Paxillus involutus, Suillus bovinus and S. variegatus. Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH4+ or NO3−. Fractionation against 15N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH4+ was the source. At high ratios of Nuptake/Nsupplied there was no (NO3−), or little (NH4+), fractionation. There seemed to be no difference in fractionation between ECM and non‐mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in 15N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus) in δ15N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively high 15N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows that the δ15N of plant N is a good approximation of δ15N of the available N source(s), provided that N is limiting growth.

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 years after the last application in N3, the N availability measured as N concentration in plants [pine 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 15N 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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