The interactive effects of temperature and moisture on nitrogen fixation in two temperate-arctic mosses

Nitrogen (N) fixation in moss-cyanobacteria associations is one of the main sources of 'new' N in pristine ecosystems like subarctic and arctic tundra. This fundamental ecosystem process is driven by temperature as well as by moisture. Yet, the effects of temperature and moisture stress on N-2 fixation in mosses under controlled conditions have rarely been investigated separately, rendering the interactive effects of the two climatic factors on N-2 fixation unknown. Here, we tested the interactive effects of temperature and moisture on N-2 fixation in the two most dominant moss species in a temperate heath, subarctic tundra and arctic tundra: Pleurozium schreberi and Tomentypnum nitens. Mosses with different moisture levels (25, 50, 100%) were kept at different temperatures (10, 20, 30 A degrees C) and N-2 fixation was measured at different times after exposure to these conditions. T. nitens had the highest nitrogenase activity and this increased with moisture content, while effects were moderate for P. schreberi. Nitrogenase activity increased with temperature in all mosses, and the temperature optimum (T-opt) was between 20 A degrees C and 30 A degrees C for all mosses. Quick acclimatization towards higher temperatures occurred. Our results suggest that the contemporary and not the historical climate govern the response of moss-associated N-2 fixation to changes in the abiotic environment. Thus, climate change will have substantial impacts on N-2 fixation in dominant mosses in temperate, subarctic and arctic habitats.