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| Titel |
Ammonia sources and sinks in an intensively managed grassland canopy |
| VerfasserIn |
M. David, B. Loubet, P. Cellier, M. Mattsson, J. K. Schjoerring, E. Nemitz, R. Roche, M. Riedo, M. A. Sutton |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 6, no. 9 ; Nr. 6, no. 9 (2009-09-23), S.1903-1915 |
| Datensatznummer |
250003993
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| Publikation (Nr.) |
 copernicus.org/bg-6-1903-2009.pdf |
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| Zusammenfassung |
Grasslands represent canopies with a complex structure where sources and
sinks of ammonia (NH3) may coexist at the plant level. Moreover,
management practices such as mowing, hay production and grazing may change
the composition of the sward and hence the source-sink relationship at the
canopy level as well as the interaction with the atmosphere. There is
therefore a need to understand the exchange of ammonia between grasslands
and the atmosphere better, especially regarding the location and magnitude
of sources and sinks.
Fluxes of atmospheric NH3 within a grassland canopy were assessed in
the field and under controlled conditions using a dynamic chamber technique
(cuvette). These cuvette measurements were combined with extraction
techniques to estimate the ammonium (NH4+) concentration and the
pH of a given part of the plant or soil, leading to an estimated ammonia
compensation point (Cp). The combination of the cuvette and the
extraction techniques was used to identify the potential sources and sinks
of NH3 within the different compartments of the grassland: the soil,
the litter or senescent "litter leaves", and the functioning "green
leaves". A set of six field experiments and six laboratory experiments were
performed in which the different compartments were either added or removed
from the cuvettes.
The results show that the cuvette measurements agree with the extraction
technique in ranking the strength of compartment sources. It suggests that
in the studied grassland the green leaves were mostly a sink for NH3 with a
compensation point around 0.1–0.4 μg m−3 and an
NH3 flux of 6 to 7 ng m−2 s−1. Cutting of the grass did
not increase the NH3 fluxes of the green leaves. The litter was found
to be the largest source of NH3 in the canopy, with a Cp of up to
1000 μg m−3 NH3 and an NH3 flux up to
90 ng m−2 s−1. The litter was found to be a much smaller
NH3 source when dried (Cp=160 μg m−3 and
FNH3=35 ng m−2 s−1 NH3). Moreover emissions from
the litter were found to vary with the relative humidity of the air. The
soil was a strong source of NH3 in the period immediately after cutting
(Cp=320 μg m−3 and
FNH3=60 ng m−2 s−1), which was nevertheless always
smaller than the litter source. The soil NH3 emissions lasted, however,
for less than one day, and were not observed with sieved soil. They could
not be solely explained by xylem sap flow extruding NH4+. These
results indicate that future research on grassland-ammonia relationships
should focus on the post-mowing period and the role of litter in interaction
with meteorological conditions. |
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