1 In a 4-year fertilization experiment we studied the effects of increased
nitrogen (N) or phosphorus (P) supply on productivity and potential decay i
n the acrotelm of Sphagnum-dominated Swedish peatlands at northern (low atm
ospheric N input: low-N site) and southern (high atmospheric N input: high-
N site) sites.
2 During the experimental period, there was a severe summer drought at the
high-N site which led to strong limitations on Sphagnum growth, especially
in the fertilized treatments. There were no significant effects of the nutr
ient treatments on cumulative length growth and productivity at either site
. There were, however, significant effects of site and nutrient treatment o
n nutritional variables of the Sphagnum species.
3 Nitrogen concentration in living capitula was lower, but P concentration
was higher at the low-N site than at the high-N site. However, at both site
s N and P concentration in the living capitula showed only minor responses
to the nutrient additions.
4 Litter chemistry variables differed significantly among sites and treatme
nts. Sphagnum litter had lower N concentration, higher P concentration, a h
igher C : N ratio, a lower C : P ratio and a lower N : P ratio at the low-N
site. Litter chemistry was significantly affected by the nutrient treatmen
ts only at the high-N site, where litter P concentrations increased signifi
cantly in the P-fertilized treatment, whereas C : P and N : P ratios showed
a significant reduction.
5 Although potential rates of decay of Sphagnum litter were higher at the h
igh-N site than at the low-N site, they were not significantly affected by
nutrient additions. Potential decay rates did, however, show significant re
lations with all the litter chemistry variables we had determined.
6 An increase in nitrogen input to Sphagnum-dominated bogs appears to lead
to higher N concentrations in litter and, as a result of dilution, to lower
P concentrations. This change in litter chemistry leads to a higher potent
ial decay rate of the litter and this may seriously affect the carbon balan
ce in the acrotelm of these systems.
7 Our results also show that carbon balance characteristics of Sphagnum bog
s can be strongly affected by unpredictable extreme climatic conditions. Th
is suggests that the occurrence and consequences of these events need more
attention when long-term responses of ecosystems to global change are studi