Masaki Uchida

Contribution of soil moisture to seasonal and annual variations of soil CO2 efflux in a humid cool-temperate oak-birch forest in central Japan

To quantify the contribution of soil moisture to seasonal and annual variations in soil CO2 efflux in a cool-humid deciduous broadleaf forest, we measured soil CO2 efflux during the snow-free seasons of 2005–2008 using an automated chamber technique. This worked much better than manual chambers employing the samexa0steady-state through-flow method. Soil CO2 efflux (gxa0Cxa0m−2xa0period−1) during the snow-free season ranged from 979.8xa0±xa049.0 in 2005 to 1131.2xa0±xa056.6 in 2008 with a coefficient variation of 6.4xa0% among the 4xa0years. We established two-parameter (soil temperature and moisture) empirical models, finding that while soil temperature and moisture explained 69–86xa0% and 10–13xa0% of the temporal variability, respectively. Soil moisture had the effect of modifying the temporal variability of soil CO2 efflux, particularly during summer and early fall after episodic rainfall events; greater soil moisture enhanced soil CO2 efflux in the surface soil layers. High soil moisture conditions did not suppress soil CO2 efflux, leading to a positive correlation between normalized soil CO2 efflux (ratio of the measured to predicted efflux using a temperature-dependent Q10 function) and soil moisture. Therefore, enhanced daily soil CO2 efflux following heavy rainfall events could significantly reduce net ecosystem exchange (i.e. daily net ecosystem production) by 32xa0% on some days. Our results highlight the importance of precisely estimating the response of soil CO2 efflux to changes in soil moisture following rainfall events when modeling seasonal carbon dynamics in response to climate change, even in humid monsoon regions.

Characteristics of ammonia oxidation potentials and ammonia oxidizers in mineral soil under Salix polaris–moss vegetation in Ny-Ålesund, Svalbard

AbstractnAlthough nitrification is a unique and important process in the nitrogen cycle with respect to ammonium consumption and nitrate production, limited information on this process is available for high-Arctic soils. We elucidated the ammonia oxidation potentials (AOPs) and characteristics of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in mineral soils under climax vegetation, i.e., Salix polaris (polar willow)–moss vegetation, on a coastal hill in Ny-Ålesund, Svalbard. AOPs at 10xa0°C were determined by incubation with sufficient substrate (2xa0mM ammonium). The ammonia monooxygenase subunit A (amoA) genes of AOB and AOA were analyzed by using quantitative polymerase chain reaction and pyrosequencing. AOPs ranged from 1.1 to 14.1xa0ngxa0Nxa0g−1xa0dry soilxa0h−1—relatively low but of a similar order to the gross nitrification rates reported in another Svalbard study. AOP was positively correlated with thickness of the moss layer (Pxa0<xa00.01), soil water content, and ammonium nitrogen content (Pxa0<xa00.05). The population sizes of both AOB and AOA were not significantly related to AOP or edaphic factors. For AOB-amoA, six major operational taxonomic units (OTUs) were identified, all of which were classified into the Nitrosospira Mount Everest cluster. For AOA-amoA, six major OTUs were also identified, five of which were grouped with sequences from cold environments within clade A of the Nitrososphaera cluster, i.e., species known to have low, or no, AOP. It is, therefore, possible that the AOPs measured at the study site were driven mainly by psychrotolerant AOB.n

Rhytisma polaris: morphological and molecular characterization of a new species from Spitsbergen Island, Norway

Rhytisma polaris, which causes tar spot disease on Salix polaris on Spitsbergen Island, is described. The most characteristic morphological feature of this new species are ascospores distinctly broader than those of other Rhytisma species. rDNA ITS and LSU sequence analysis also indicated that R. polaris is sufficiently distinct from other Rhytisma species to justify the new species status.

Abundant deposits of nutrients inside lakebeds of Antarctic oligotrophic lakes

Most freshwater lakes in continental Antarctica are in a paradoxical situation as they are in nutrient-poor conditions despite luxuriant vegetation growth covering the entire lakebed. Although the phytobenthos possibly take up nutrients from inside lakebeds, the amount of nutrients and their utilization by these phytobenthos are unclear. Sediment cores were collected from 17 freshwater lakes in East Antarctica, then dissolved inorganic nitrogen (DIN) and phosphate of the lake waters, and the vertical profiles of the interstitial water in the sediment cores were analyzed. Here we revealed that there are abundant nutrients inside lakebeds surface with 3–220 times the amount of DIN and 2–102 times concentration of phosphate than those in lake water, and the nutrient profile inside the sediment suggested that the phytobenthos can utilize the much nutrients from lakebeds. We also show that nitrogen stable isotope ratios of shallower phytobenthos lying on the small amount of nutrients in a lake are similar to that of terrestrial cyanobacteria possessing N2 fixation ability.

Sensitivity analysis of ecosystem CO2 exchange to climate change in High Arctic tundra using an ecological process-based model

Arctic terrestrial ecosystems are extremely vulnerable to climate change. A major concern is how the carbon balance of these ecosystems will respond to climate change. In this study, we constructed a simple ecological process-based model to assess how the carbon balance will be altered by ongoing climate change in High Arctic tundra ecosystems using in situ observations of carbon cycle processes. In particular, we simulated stand-level photosynthesis, root respiration, heterotrophic respiration, and hence net ecosystem production (NEP) of a plant community dominated by vascular plants and mosses. Analyses were carried out for current and future temperature and precipitation conditions. Our results showed that the tundra ecosystem was a CO2 sink (NEP of 2.3–18.9xa0gCxa0m−2 growing season−1) under present temperature conditions. Under rising temperature (2–6xa0°C), carbon gain is significantly reduced, but a few days’ extension of the foliage period caused by their higher temperatures compensated for the negative effect of temperature on NEP. Precipitation is the major environmental factor driving photosynthetic productivity of mosses, but it had a minor influence on community-level NEP. However, NEP decreased by a maximum 15.3 gC m−2xa0growing season−1 under a 30-day prolongation of the moss-growing season, suggesting that growing season extension had a negative effect on ecosystem carbon gain, because of poorer light conditions in autumn. Because the growing season creates a weak CO2 sink at present, lengthening of the snow-free season coupled with rising temperature could seriously affect the future carbon balance of this Arctic tundra ecosystem.

Morphological and molecular characterization of Rhytisma filamentosum sp. nov. from Nagano Prefecture, Japan

We describe a new fungal species, Rhytisma filamentosum, which causes tar spot disease on Salix integra in Nagano Prefecture, Japan. Its most distinctive morphological features are asci and ascospores that are distinctly longer than those of other Rhytisma species on Salix. rDNA ITS and LSU sequence analyses also indicate that this fungus is sufficiently distinct from other Rhytisma species to justify new species status.

Occurrence pattern of the parasitic fungus Rhytisma polare (Ascomycota) on the polar willow ( Salix polaris ) under limited water conditions in a high-Arctic semi-desert

The parasitic fungus Rhytisma polare is a common parasite on leaves of the polar willow (Salix polaris) in the high-Arctic polar semi-desert of Spitsbergen, Norway. Because Rhytisma spp. generally requires saturation with free water to develop ascospores, it is unclear how R. polare has ecologically adapted to the Arctic desert, where such water is very limited. In this study, the response of R. polare to different water conditions on Spitsbergen was investigated during the summer months of June–August in 2012. Field and laboratory experiments demonstrated that free water availability from rainfall or snowmelt is essential to facilitate ascostromal maturation and ascospore dispersal in R. polare. The field experiments also revealed that the dispersal of ascospores produced on fallen leaves did not extend beyond a few meters. These results suggest that the free water requirement combined with the short spore-dispersal distance constrains the local occurrence of R. polare in thexa0Arctic desert to locations where free water from rainfall and snowmelt is present.

The effect of tar spot pathogen on host plant carbon balance and its possible consequences on a tundra ecosystem

In Arctic tundra, plant pathogens have substantial effects on the growth and survival of hosts, and impacts on the carbon balance at the scale of ecological systems. To understand these effects on carbon dynamics across different scales including plant organ, individual, population and ecosystem, we focused on two primary factors: host productivity reduction and carbon consumption by the pathogen. We measured the effect of the pathogen on photosynthetic and respiratory activity in the host. We also measured respiration and the amount of carbon in the pathogen. We constructed a model based on these two factors, and calculated pathogenic effects on the carbon balance at different organismal and ecological scales. We found that carbon was reduced in infected leaves by 118% compared with healthy leaves; the major factor causing this loss was pathogenic carbon consumption. The carbon balance at the population and ecosystem levels decreased by 35% and 20%, respectively, at an infection rate of 30%. This case study provides the first evidence that a host plant can lose more carbon through pathogenic carbon consumption than through a reduction in productivity. Such a pathogenic effect could greatly change ecosystem carbon cycling without decreasing annual productivity.

Factors affecting water availability for high Arctic lichens

Lichen symbiosis is a successful nutritional strategy that drives lichen distributions in high Arctic ecosystems. However, the in situ effects of fungal partners (mycobionts) on water availability for the photosynthetic partners (photobionts) and their necessity for survival remain unclear. We investigated the factors creating differences in water availability in high Arctic lichen assemblages using observational and experimental analyses of substrates and thallus morphology in the high Arctic, near Ny-Ålesund, Svalbard (79°N) during the snow-free season. We used five lichen species found on five substrate types: moss litter, vascular plant litter, mixed litter, biological soil crust (BSC) and gravel. BSC had significantly higher water content than the other substrates; although it had high levels of surface evaporation, BSC took up more ground water. Moreover, the structure of BSC supported greater water retention than the four other substrates in our study, providing the moistest environment for lichens. In fact, 60xa0% of the surface area of the crustose lichen Ochrolechia frigida, which was mainly distributed on BSC, was in contact with the substrate. In contrast, the four fruticose lichens had larger surface areas than Och frig and roughly 90xa0% air exposure. Initial rates of absorption and evaporation increased with greater thallus surface area, suggesting that water availability for photobionts is strongly affected by both morphological characteristics and substrate water properties, both of which depend on mycobionts. We conclude that lichens show preferences for both morphologies and substrates that promote autotrophic nutrition in the water-limited glacier foreland of the high Arctic region.