What controls moss-associated nitrogen fixation in the Subarctic?

Background/Question/Methods

Nitrogen (N₂) fixation performed by moss-associated cyanobacteria is one of the main sources of new N in pristine ecosystems like subarctic tundra. However, N₂ fixation in mosses is strongly influenced by abiotic conditions as moisture, temperature and nutrient availability. Previous attempts to upscale N₂ fixation in mosses from low frequency measurements to weeks, months or even the entire growing season without taking into account changes in abiotic conditions could not capture the variation in moss-associated N₂ fixation. We modelled moss-associated N₂ 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. The litter additions were sought to simulate shrub expansion into the Arctic. We established relationships between measured in situ N₂ fixation rates and soil moisture and soil temperature and used high-resolution measurements of soil moisture and soil temperature (May – October) to model N₂ fixation throughout the growing season. In a second set of experiments, we tested if moss-associated N₂ fixation is limited by molybdenum (Mo) or phosphorus (P) availability. For this, N₂ fixation in mosses was measured at different time intervals following Mo and P additions in laboratory and field experiments.

Results/Conclusions

The modelled N₂ 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). Our findings suggest that a longer snow-free period and increased temperatures in a future climate will likely lead to higher N₂ fixation rates in mosses. Yet, the consequences of increased litter fall on moss-associated N₂ fixation due to shrub expansion in the Arctic will depend on the shrub species’ litter traits. In the nutrient (Mo, P) addition experiments, moss-associated N₂ fixation was up to 4-fold higher shortly after the addition of Mo, in both the laboratory and field experiments. As the growing season progressed, N₂ fixation became progressively more P-limited. Our results support previous findings showing that N₂ fixation is strongly influenced by nutrient availability, in particular Mo, thereby extending the geographical distribution of Mo-limitation of N₂ fixation to the boreal biome.