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0.20
Field 0
Field 0N
0.15
0.10
0.05
0.00
0.20
Field 4N
Field 4
0.15
0.10
0.05
0.00
0 N4 N
0 N4 N
Treatment during short-term laboratory measurements
Figure 6.8 Microbial respiration rates associated with decomposing leaf litter in
response to experimental warming and nitrogen enrichment. Standard litter samples
were collected from control (0), experimentally heated (4), nitrogen-enriched (0N), and
heated and enriched (4N) enclosures, and respiration of all these samples was measured
in the laboratory at ambient lake temperature (0), at elevated temperature (4) with
nitrogen added, and at elevated temperature with nitrogen added (4N). Data shown in
each of the four panels refer to conditions in the field, while treatments indicated on the
x -axis refer to conditions during measurements in the laboratory.
attributed to the inhibition of lignin-degrading enzymes. An alternative hypothesis
is that increased N supply stimulates processes other than litter decomposition
(e.g. algal growth), thereby exacerbating deprivation of litter decomposers of
potentially limiting nutrients such as phosphorus. Irrespective of the underlying
mechanism, the observed inhibition of heterotrophic processes by NO 3 - enrichment
suggests that increased nitrogen availability, resulting from continuing high rates
of atmospheric N deposition, might buffer against the acceleration of litter
decomposition caused by warming in the future.
Understanding changes in the regulation of decomposition rates under warmer
and nutrient-richer conditions may be more intricate than suggested by our results on
leaf decomposition. Neither warming nor nitrate enrichment affected the
decomposition of stem litter of P. australis . This may be due to the intrinsic recalcitrance
of stem litter and/or because environmental conditions other than temperature and
nitrate availability were overwhelming in determining stem decomposition rate.
One of these factors could be oxygen, since litter placed in unfenced control plots in
the reed stand, which freely exchanged water with the open lake, clearly decomposed
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