Ecosystem nitrogen fixation throughout the snow-free period in subarctic tundra: effects of willow and birch litter addition and warming.

Nitrogen (N) fixation in moss-associated cyanobacteria is one of the main sources of available N for N-limited ecosystems such as subarctic tundra. Yet, N 2 fixation in mosses is strongly influenced by soil moisture and temperature. Thus, temporal scaling up of low-frequency in situ measurements to several weeks, months or even the entire growing season without taking into account changes in abiotic conditions cannot capture the variation in moss-associated N 2 fixation. We therefore aimed to estimate moss-associated N 2 fixation throughout the snow-free period in subarctic tundra in field experiments simulating climate change: willow (Salix myrsinifolia) and birch (Betula pubescens spp. tortuosa) litter addition, and warming. To achieve this, we established relationships between measured in situ N 2 fixation rates and soil moisture and soil temperature and used high-resolution measurements of soil moisture and soil temperature (hourly from May to October) to model N 2 fixation. The modelled N 2 fixation rates were highest in the warmed (2.8 ± 0.3 kg N ha -1 ) and birch litter addition plots (2.8 ± 0.2 kg N ha -1 ), and lowest in the plots receiving willow litter (1.6 ± 0.2 kg N ha -1 ). The control plots had intermediate rates (2.2 ± 0.2 kg N ha -1 ). Further, N 2 fixation was highest during the summer in the warmed plots, but was lowest in the litter addition plots during the same period. The temperature and moisture dependence of N 2 fixation was different between the climate change treatments, indicating a shift in the N 2 fixer community. Our findings, using a combined empirical and modelling approach, suggest that a longer snow-free period and increased temperatures in a future climate will likely lead to higher N 2 fixation rates in mosses. Yet, the consequences of increased litter fall on moss-associated N 2 fixation due to shrub expansion in the Arctic will depend on the shrub species' litter traits.

(© 2016 John Wiley & Sons Ltd.)