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| Titel |
N-15 tracing helps explaining N leaching losses from contrasting forest ecosystems |
| VerfasserIn |
J. Staelens, T. Rütting, D. Huygens, C. Müller, K. Verheyen, P. Boeckx |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023391
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| Zusammenfassung |
| Despite chronically enhanced nitrogen (N) deposition to forest ecosystems in Europe and
NE America, considerable N retention by forests has been observed, reducing N
leaching losses. Organic and mineral soil layers typically immobilize more N than
the aboveground biomass, but it is unclear which factors determine N retention
in forest ecoystems. However, this knowledge is crucial to assess the impact of
changing anthropogenic N emissions on future N cycling and N loss of forests.
For coniferous and deciduous forest stands at comparable sites, it is known that
both N deposition onto the forest floor as well as N loss by leaching below the
rooting zone are significantly higher in coniferous stands. In addition, the N loss in
coniferous stands is often more enhanced than can be explained by the higher N input
only. This suggests lower N retention by coniferous stands, and may be related to
differences in litter and soil characteristics, microbial activity, and N uptake by plant
roots.
To test this hypothesis, we studied the effect of forest type on N retention using 15N
tracing techniques: a field tracer experiment and a combination of in situ isotope pool dilution
and a tracing model. The N dynamics were examined for two adjacent forest stands
(pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.)) on a well-drained
sandy soil and with a similar stand history, located in a region with high N deposition
(Belgium). Input-output N budgets were established by quantifying atmospheric deposition
and leaching below the rooting zone, and confirmed the above finding of higher N deposition
and disproportionately higher N loss for the pine stand compared to the oak stand. First, the
fate of inorganic N within the ecosystems was studied by spraying three pulses of dissolved
15N, either as ammonium or as nitrate, onto the forest floor in 12 plots of 25 m2. The
organic and mineral soil layers, tree roots, soil water percolate, ferns, and tree foliage
were sampled and analyzed for total N and 15N four times in the year after 15N
application. Here we present results of the 15N recovery four months after the first
application, and compare the recovery between the two forest stands and the two N
treatments.
Second, gross N transformation rates in the undisturbed mineral forest soils were
determined via 15N pool dilution and advanced trace modelling. Using five spatial replicates
per stand, three 15N treatments were applied in the field to ‘virtual’ soil cores (0-10 cm) that
were disturbed only at sampling. Each treatment solution contained ammonium, nitrate, and
nitrite, with one of the N forms labelled with 15N at 99% at. excess. Intact soil cores were
sampled at six time intervals over a 12-day period, and analyzed for N and 15N content in
different mineral and organic pools. The parameters of different simultaneously occurring
process rates were optimized using a Markov Chain Monte Carlo algorithm. In
both stands, heterotrophic nitrification of the organic soil pool was more important
than autotrophic nitrification of ammonium. Significantly different process rates
between the two forest stands were found for mineralization, heterotrophic and
autotrophic nitrification, and ammonium and nitrate immobilization. Gross mineralization
and ammonium immobilization rates were higher in the oak soil than in the pine
soil. Gross nitrate production, in contrast, was faster in the pine soil, while nitrate
immobilization was slower. These different soil nitrate dynamics likely contribute to
the observed higher nitrate leaching loss in the pine than oak stand. In addition to
the faster nitrate immobilization in the oak soil, our results strongly suggested the
occurrence of a second N-conserving process under oak, i.e. dissimilatory nitrate
reduction to ammonium (DNRA). This is unexpected for a temperate forest soil under
enhanced N deposition, as this process has mainly been described for unpolluted soils. |
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