Nutritional constraints on Sphagnum-growth and potential decay in northern peatlands

1 In a 4-year fertilization experiment we studied the effects of increased nitrogen (N) or phosphorus (P) supply on productivity and potential decay in the acrotelm of Sphagnum-dominated Swedish peatlands at northern (low atmospheric 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 nutrient treatments on cumulative length growth and productivity at either site. There were, however, significant effects of site and nutrient treatment on 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 sites 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 treatments. Sphagnum litter had lower N concentration, higher P concentration, a higher 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 treatments only at the high-N site, where litter P concentrations increased significantly 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 high-N site than at the low-N site, they were not significantly affected by nutrient additions. Potential decay rates did, however, show significant relations 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 potential decay rate of the litter and this may seriously affect the carbon balance in the acrotelm of these systems. 7 Our results also show that carbon balance characteristics of Sphagnum bogs can be strongly affected by unpredictable extreme climatic conditions. This suggests that the occurrence and consequences of these events need more attention when long-term responses of ecosystems to global change are studied.