Long-term Warming and Litter Additions Affect Moss-associated Nitrogen Fixation in a Subarctic, Moist Heath

Biological nitrogen fixation (N2-fixation) by cyanobacteria is the main source of nitrogen input to nutrient limited ecosystems in the Arctic. Indeed, availability of nitrogen is a main control on other ecosystem key processes such as the cycling of carbon (photosynthesis and mineralization). Hence, N2-fixation can indirectly feed back on the climate through control on the nitrogen limited primary productivity in the Arctic. Some field studies (Zielke et al. 2002, Sorensen et al. 2006) have shown that the rate of N2-fixation is strongly affected by temperature, soil moisture and incoming light, and N2-fixation is therefore expected to be altered by climate change. However, there are only few reports on N2-fixation rates in relation to climate change in the Arctic.

In order to investigate the effects of anticipated global climate change on N2-fixation rates in a subarctic moist heath, a field experiment was carried out in Northern Sweden. Warming was induced by plastic tents, and in order to simulate the effects of future increased tree cover, birch litter was added each fall for 9 years prior to the measurements. We analysed N2-fixation rates on both whole-ecosystem level and specifically on two moss species: Sphagnum warnstorfii and Hylocomium splendens. The whole-ecosystem N2-fixation rates of the warmed plots almost tripled compared to the control plots. However, the in Sphagnum and Hylocomium we observed either no change or occasionally even a decrease in N2-fixation after warming. By both methods litter addition increased N2-fixation rates. The results suggest that warming will lead to a general increased ecosystem nitrogen input, which will increase carbon accumulation in plants, but also that the N2-fixation by cyanobacteria associated with some moss species is likely to decrease. Hence, this study shows that the scale of measurements is crucial when investigating on ecosystem responses to manipulations.

References: Sorensen PL., Jonasson S. and Michelsen A. (2006) Nitrogen fixation, denitrification and ecosystem nitrogen pools in relation to vegetation development in the Subarctic. Arctic, Antarctic and Alpine Research, 38: 263-272 Zielke M., Ekker AS., Olsen RA., Spjelkavik S. and Solheim B. (2002) The influence of abiotic factors on biological nitrogen fixation in different types of vegetation in the high arctic, Svalbard. Arctic, Antarctic and Alpine Research, 34: 293-299