Nitrogen fixation rates and cyanobacteria cover in a subarctic biological soil crust are enhanced by warming, wetting and light

biocrust, N-fixation, metagenomes, warming, microbAbstract textIntroduction. Areas of high latitude with volcanic andosol and winter snow cover commonly have a 3-5 mm thick biological soil crust (biocrust) characterized by the leafy liverwort Anthelia juratzkana. This biocrust can be long term or transient, especially as snow cover decreases. Although Antheliais the primary producer, the biocrust is a complex microbial community with substantial biomass and biological activities associated with bacteria and fungi. Available nitrogen is thought to bethe limiting nutrient.Methods. Acetylene reduction assays (ARA) were used as a proxy in assessing biocrust responses to temperature, moisture and light. Soil carbon and nitrogen isotopes were measured and shotgun metagenomic DNA and RNA libraries were sequenced. Decomposition rates were assessed using the tea bags index. Respiration was measured with a LI-6400 system.Results. ARA measurements yielded estimates of nitrogen fixation of 1-4 kg ha-1 yr-1. Nitrogen fixation increases with temperature up to 25 oC, with light up to ~150 moles photons m-2 s-1 as well as with moisture. In situ summer daylight is 300-3000 moles photons m-2 s-1, moisture is from 4% to saturation, and temperatures show strong diurnal cycles from 5 oC to 30 oC on sunny days. Warming effects on decomposition varied with depth and type of tea. Respiration measurements gave an estimate of ~100 kg C ha-1yr-1.There is a highly negative isotope discrimination of 15N vs. 14N in the biocrust, possibly due to fractionation between multiple compartments, from bacteria to fungi and liverworts. The microbial community is diverse, with small differences between locations and habitats. Ascomycete fungi are abundant in the surface layer, particularly the orders Heliotales and Chaetothyriales.Conclusions. Integration of in situ measurements, meteorological information and data from laboratory experiments allows us to estimate nitrogen fixation from year to year, and what effects predicted climate changes are likely to have.