Journal of Conference Abstracts

Nitrogen - Too Much of a Good Thing?
A Synthesis of the NITREX Project

B. A. Emmett¹ (b.emmett@ite.ac.uk), A. W. Boxman² (driesb@sci.kun.nl),
M. Bredemeier³ (mbredem@gwdg.de, P. Gundersen⁴ (pgu@fsl.dk),
O. J. Kjønaas⁵ (janne.kjonaas@nisk.nlh.no), F. Moldan⁶ (filip.moldan@ivl.s),
P. Schleppi⁷ (patrick.schleppi@ch.ethz.wsl), A. Tietema⁸ (atietema@fgb.frw.uva.nl) &
R. F. Wright⁹ (richard.wright@niva.no)

¹ Inst.Terrestrial Ecology, UWB, Bangor, Gwynedd LL57 2UP, U.K.

² University of Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands.

³ Georg-August Universität, Habichtsweg 55, D37075 Göttingen, Germany.

⁴ Danish Forest and Landscape Research Inst., DK-2970 Hoersholm, Denmark.

⁵ NISK, Høgskoleveien 12, N-1432 Ås, Norway.

⁶ Swedish Environmental Research Inst., Box 47086, S-47086 Göteborg, Sweden.

⁷ WSL, FNP, Zücherstr. 111, CH-8903, Birmensdorf, Switzerland.

⁸ University of Amsterdam, NL-1018 VZ Amsterdam, The Netherlands.

⁹ NIVA, Box 173 Kjelsås, N-0411 Oslo, Norway.

Recent results from ecosystem-scale manipulation experiments in European coniferous stands within the EC-funded NITREX (Nitrogen Saturation Experiments) project has enabled the fate and impact of pollutant nitrogen in these systems to be more clearly understood. The common factor amongst all the experiments has been a simulation of either increased or decreased nitrogen deposition to mature forest stands over a period 4-5 years. Ambient loadings to the stands were between 13 and 59 kg N ha^-1 yr^-1 with treatments representing a change in deposition between -95 % and + 300 %. Attention was paid to both the terrestrial and freshwater components of the ecosystem as both a decline in tree health and eutrophication and acidification of drainage waters in response to enhanced N deposition had been proposed. A summary of the major conclusions from NITREX (NITREX 1995, & In Press) are outlined below and will be presented at BIOGEOMON with a comparative look at results from similar studies.

Within the NITREX project, retention of applied N was observed to vary between 0 and 95 % depending on the initial nitrogen status of the site and the form of N applied. Stands could be separated into three categories: (1) low N status; both ammonium and nitrate were retained with high efficiency with little available for leaching, (2) intermediate N status; a decline in retention efficiency of nitrate was observed due to saturation of the N requirements and preferential utilisation of ammonium by both the soil microbial population and vegetation, and (3) high N status sites; limited/no nitrate retention and reduced retention of ammonium due to the onset of nitrification. The N status of the site could be described by several or a combination of individual components of the ecosystem, e.g. N concentration of the forest floor, foliage or leaf litter and N flux in drainage water. Whilst all were indicators of the N status, changes in the efficiency of N retention by the forest floor with increased N loading and decreasing C/N ratio was identified as a key factor determining N retention in these mature stands. In contrast, vegetation and the mineral soil were relatively unresponsive pools to large changes in N deposition loading with similar retention efficiencies of incoming-N in all sites, irrespective of treatments.

The continuing supply of deposited N was observed to be essential to the loss of N to drainage water in high N status sites. All studies simulating a decrease in N deposition by ca. 95% reported a rapid reduction in drainage water N content. This indicated that a tight N cycle was re-established between internally produced N and vegetation despite continuing high rates of N mineralisation at pre-treatment rates for several years. Similarly, there were delays of 4­5 years before significant shifts in tree nutritional status or tree growth were observed, with sensitivity of the stand dependent on both the tree species involved and the form of N deposited. For example, weekly N applications at 75 N ha^-1 yr^-1 in the form of a neutral nitrate salt for 6 years to a N-saturated, but undamaged, spruce stand in Wales was found to have no effect on spruce growth or health. At other sites in which ambient or applied N was dominated by ammonium, significant effects on trees above and/or below ground were observed if input levels were changed. Thus, high ammonium deposition appeared to have greater negative influence on the functioning of coniferous trees possibly due to the detrimental effect of high concentrations of ammonium on uptake of base cations in the soil.

In combination with regional studies, the results and derived empirical relationships from the NITREX project enable predictions to be made as to the likely consequence of changes in N loading to European coniferous stands and have been used to test the current critical load values for N in coniferous stands and to develop a new process-based catchment nitrate leaching model.

References

NITREX Special Issue, For. Ecol. Manage. 71 (1995) & In Press.