Predicting the effects of atmospheric nitrogen deposition in conifer stands: evidence from the NITREX ecosystem-scale experiments

Emmett B.A. 1, Boxman D. 2, Bredemeier M. 3, Gundersen P. 4, Kjønaas O.J. 5, Moldan F. 6, Schleppi P. 7, Tietema A. 8, Wright R.F. 9

1 Inst. Terr. Ecol., Bangor Res. Unit, Bangor LL57 2UP, Gwynedd, Wales
2 Univ. Nijmegen, Dept. Ecol., NL-6500 GL Nijmegen, Netherlands
3 Univ. Gottingen, Forest Ecosyst. Res. Ctr., D-37077 Göttingen, Germany
4 Danish Forest and Landscape Res. Inst., DK-2970 Hørsholm, Denmark
5 Agr. Univ. Norway, Norwegian Forest Res. Inst., N-1432 Ås, Norway
6 Swedish Environm. Res. Inst., S-40285 Gothenburg, Sweden
7 Swiss Fed. Inst. Forest Snow and Landscape Res., Dept. Forest Ecol., CH-8903 Birmensdorf, Switzerland
8 Univ. Amsterdam, Landscape and Environm. Change Res. Grp., NL-1018 VZ Amsterdam, Netherlands
9 Norwegian Inst. Water Res., N-0411 Oslo, Norway

Ecosystems 1 (1998): 352-360

DOI: 10.1007/s100219900029


The NITREX project, which encompasses seven ecosystem-scale experiments in coniferous forests at the plot or catchment level in northwestern Europe, investigates the effect of atmospheric nitrogen (N) deposition in coniferous forests. The common factor in all of the experiments is the experimentally controlled change in N input over a period of 4-5 years. Results indicate that the status and dynamics of the forest floor are key components in determining the response of forests to altered N inputs. An empirical relationship between the carbon-nitrogen (C/N) ratio of the forest floor and retention of incoming N provides a simply measured tool through which the likely timing and consequences of changes in atmospheric N deposition for fresh waters may be predicted. In the terrestrial ecosystem, a 50% increase in tree growth is observed following the experimental reduction of N and sulfur inputs in a highly N-saturated site, illustrating the damaging effects of acidifying pollutants to tree health in some locations. Few biotic responses to the experimental treatments were observed in other NITREX sites, but the rapid response of water quality to changes in N deposition, and the link to acidification in sensitive areas, highlight the need for N-emission controls, irrespective of the long-term effects on tree health. The observed changes in ecosystem function in response to the experimental treatments have been considered within the framework of the current critical-load approach and thus contribute to the formulation of environmental policy.

Keywords: NITREX, conifer, forest, nitrogen deposition, nitrate leaching, carbon/nitrogen ratio, critical-loads