CENPERM seminar – University of Copenhagen

CENPERM seminar

Nutrient limitation of moss-associated nitrogen fixation in the Subarctic

Kathrin Rousk, Postdoc, Dept of Biology,UCPH/CENPERM

Nitrogen (N2) fixation performed by moss-associated cyanobacteria is one of the main sources of new N input in pristine, high latitude ecosystems. Yet, the nutrients that limit N2 fixation remain elusive. Here, we tested if N2 fixation in mosses is limited by the availability of molybdenum (Mo), phosphorus (P) or both. We measured N2 fixation in dominant mosses at different time intervals following Mo and P additions, in both laboratory microcosms with mosses from a boreal spruce forest, and in field plots in subarctic tundra. We further used a 15N2 tracer technique to assess the acetylene reduction to N2 fixation conversion ratios at our subarctic site. Nitrogen fixation was up to 4-fold higher shortly after the addition of Mo, in both the laboratory and field experiments. A similar positive response to Mo was found in moss colonizing cyanobacterial biomass. As the growing season progressed, N2 fixation activity became progressively more P-limited. The ARA:15N2 ratios increased with increasing Mo additions. Our findings show that N2 fixation activity and cyanobacterial biomass in dominant feather mosses is limited by Mo availability.

Besides being limited by the availability of nutrients, N2 fixation in mosses is also strongly influenced by soil moisture and temperature. Previous attempts to temporally scaling up from low frequency in situ measurements to several 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 therefore aimed to estimate 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. 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 (hourly from May – October) to model N2 fixation. 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.

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