| Nitrogen (N) fertilization, both intended and unintended, interacts with carbon cycling in
terrestrial ecosystems, because the major processes of carbon (C) turnover depend on
enzymes and thus on N availability. Comparisons between annual carbon dioxide flux (CO2)
budgets and wet N deposition in forests showed a very strong linear increase of CO2
sequestration with increased N deposition. After considering total rather than only wet N
deposition the ratios between increased carbon uptake and atmospheric N input were closer to
C/N that can be found in wood. This suggested that the observed ecosystems responses to
enhanced N inputs were mainly driven by plant responses. Finally, looking at changes in soil
organic matter changes indicated even lower sensitivities of carbon sequestration to N
addition. The objective of this study is to describe the mechanisms of the responses and the
fate of the N in the ecosystem based on results from intensively investigated forest
sites.
Within the European NitroEuope-IP project the annual fluxes and pool sizes of C and N
were estimated in four so-called forest supersites, including temperate coniferous forests in
Southern Germany (Höglwald) and in the Netherlands (Speulderbos), one temperate beech
forest close to Sorø on Zealand in Denmark and a boreal pine forest (Hyytiälä,
Southern Finland). Due to differences in vegetation, bedrock and climate history,
soils differed in acidity, organic matter content and biological activity; the levels
of atmospheric N deposition varied from very low (Hyytiälä) to high (the other
sites).
Comparisons of N and C budgets of plants and soils confirmed a simple and
stoichiometric effect dCuptake/dNdep = constant and in the order of magnitude of (C/N)wood
for plants but not for soils and thus not for the forest ecosystems as a whole. Differences in
soil processes as indicated by the differing C/N of SOM, differing amounts of N stored in the
soil and considerable differences in N leaching rates even at comparable N deposition
levels, showed clearly that the diversity of soils play a large role in the N use for C
sequestration and thus for the beneficial effects of additional N loads on climate change
mitigation effects in forests. An important conclusion of the study for intended forest
fertilization is to consider N leaching to the ground water, which might even enhance
the greenhouse effect through increased N2O emissions from streams, estuaries
and coasts rather than mitigating it via increased CO2 sequestration at the forest
site.
Acknowledgements
This work has been funded by the European Commission via the NitroEurope and
CarboEurope integrated projects. |