Satoru Hobara

Nitrogen Fixation in Surface Soils and Vegetation in an Arctic Tundra Watershed: A Key Source of Atmospheric Nitrogen

Abstract Atmospheric nitrogen (N) fixation is a key N input to arctic ecosystems, but relatively few estimates of annual N-fixation rates are available. We measured N-fixation of plant-soil cores by the acetylene reduction technique at different topographic positions in an upland tundra watershed, Imnavait Creek, through two growing seasons in order to evaluate spatial and temporal variation in N-fixation. We also examined the effects of light and temperature on N-fixation to estimate annual N-fixation rates of surface soil in this watershed using field meteorological data. Surface soil at Imnavait Creek had significant acetylene reduction potential throughout the watershed (generally 6 to 10 μmol C2H4 m−2 h−1), indicating that N-fixing organisms were present everywhere. Although acetylene reduction potential was roughly constant through the growing season, moisture, temperature and light intensity strongly affected the measured acetylene reduction rates in laboratory incubations. In addition, the relatively few samples that included the lichen, Peltigera apthosa, had significantly greater acetylene reduction potential, although the overall influence of Peltigera on N-fixation in this watershed seems to be small. The N input via N-fixation at Imnavait Creek was estimated at 80 to 131 mg N m−2 yr−1, indicating that N-fixation contributed 85 to 90% of total watershed N inputs.

Nitrogen and phosphorus enrichment and balance in forests colonized by cormorants: Implications of the influence of soil adsorption

Although much concern has been directed at nitrogen (N) cycling in terrestrial ecosystems with bird colonies, little has been clarified on the processes of phosphorus (P) cycling itself, and few comparisons between P and N cycling in bird colonies have been made. On the Isaki Headland and Chikubu Island, which are located on or near the shore of Lake Biwa, Central Japan, a dramatic increase in the population of cormorants has occurred since the 1980s. There has been a concomitant increase in the transport of nutrients from the lake to the waterside ecosystems. We compared the pools and dynamics of N and P in the cormorant-colony forests in order to clarify the effects of differences in soil N and P dynamics on the N–P balance of these colony forests. The total N concentration in the forest floor at excrement-influenced sites was not significantly different from that at sites without such influence, in spite of the heavy load of cormorant-derived N. In contrast to N, forest floor P concentration at the sites with excrement influence was significantly higher compared to sites without such influence, resulting in the lower forest floor N/P ratio at the excrement-influenced sites even after colony abandonment. The site pattern of total N and P concentrations and N/P ratio for mineral soil was similar to that for the forest floor. It seems that the leaky character for N and the accumulative character for P are due to the high mobility of nitrate in soils and the tight absorption of inorganic P to clay minerals, respectively. The site pattern of N/P ratios observed for Chamaecyparis obtusa Sieb. et Zucc. leaves is consistent with that for the forest floor and/or mineral soil, suggesting that the soil geochemical property was reflected in the foliar N/P ratio. The chemistry of throughfall and soil solution was also changed due to deposition of cormorant excrement, and the changes continued for a few years after abandonment of the colony. The quantitative analyses for N and P suggested that the major part of N and P transported by cormorants was not retained in plant matter and the surface soil beneath the colony but instead leached into deeper soil layers. The influence of cormorant excrement on nutrient balance of the whole colony ecosystem is also discussed.

Abundance, diversity, and species composition of fungal communities in a temperate forest affected by excreta of the Great Cormorant Phalacrocorax carbo

The possible effects of excreta of the Great Cormorant Phalacrocorax carbo on abundance, diversity, and species composition of fungal communities were investigated in a temperate evergreen coniferous forest near Lake Biwa in central Japan. Samples were collected at three study sites that had the same vegetation composition, but which had been influenced by different stages of breeding colony establishment: Site C (control site), Site 2 (colonizing site), and Site 3 (post-colony site). In forest floor samples and in needles and twigs of Chamaecyparis obtusa, total hyphal length was lowest at Site 3, and clamp-bearing hyphal length (biomass of basidiomycetous fungi) was lower at Sites 2 and 3 than at Site C. Dark-pigmented hyphal length was highest at Site 2. Dilution plating of forest floor samples and mineral soil revealed: (i) species richness was higher at Sites 2 and 3 than at Site C, (ii) diversity was higher at Site 3 than at Sites C and 2, and that (iii) species composition differed among the sites. Surface sterilization of needles and twigs of C. obtusa revealed (i) with the exception of species richness in twigs, species richness and diversity were higher at Site 3 than at Sites C and 2, and that (ii) species composition differed markedly among the sites. In twig samples white rot Marasmius-like fungus and Geniculosporium sp. 1 were dominant at Site C and reduced at Sites 2 and 3. A coprophilous species, Sordaria sp. 1, showed a marked increase at Site 2 in needle and twig samples.

Pattern of natural 15N abundance in lakeside forest ecosystem affected by cormorant-derived nitrogen

Waterbirds are one of the most important groups of organisms inhabiting the land–water interface, especially with regard to mediating the transport of materials from the aquatic to the terrestrial environment. The great cormorant (Phalacrocorax carbo) is a colonial piscivorous bird that transports nutrients from fresh water to forest. We measured cormorant-derived nitrogen at two nesting colonies on the Isaki Peninsula and Chikubu Island at Lake Biwa, Japan, and analyzed the long-term effects of cormorant colonization on the forest nitrogen cycle, and the mechanisms of nitrogen retention. Three sites were examined in each colony: a currently occupied area, a previously occupied but now abandoned area, and a control area never colonized by cormorants. High nitrogen stable isotope ratios of cormorant excreta, the forest floor, mineral soil, and living plants showed cormorant-derived nitrogen in both occupied and abandoned areas. The relationship between δ15N and N content showed that the high δ15N of the excreta and N turnover in the soil were important at the occupied sites, whereas high δ15N of litter was important at the abandoned sites. Physiological changes of various organisms are also important for the N decomposition process. In conclusion, cormorant-derived nitrogen remains in the forest ecosystem as a result of two cormorant activities: heavy deposition of excreta and collection of nitrogen-rich nest material. Colony stage (occupied, abandoned, or never inhabited) and historical change of N decomposition process of an area can be identified from the relationship between δ15N and N content.

The roles of microorganisms in litter decomposition and soil formation

Much has been learned about the microbial decomposition of plant litter, but relatively little is known about microbial contributions to litter and soil chemistry. We conducted a 3-year litterbag experiment and measured hydrolyzable amino acids (AA) and amino sugars (AS) to gain insights about microbial contributions to the chemical characteristics of decomposing litter and soil. Microscopic observations of hyphae were used to estimate fungal contributions to litter. The carbon (C)-normalized yields of AA and AS increased during decomposition along with nitrogen (N), indicating a shift in chemical characteristics from C-rich plant-derived biopolymers to N-rich, microbially-derived biochemicals. The contributions of fungal biomass to C and N were minor, but necromass of fungi as melanized and clamp-bearing hyphae increased during litter decomposition. Yields of glucosamine and galactosamine in litter approached those in microorganisms, particularly bacteria, suggesting major contributions of bacterial residues to litter during decomposition. The microbial contributions to decomposing litter were consistent with those observed in organic and mineral soils. Microorganisms play important roles in the organization and stabilization of soil organic matter as well as N immobilization and organic C preservation.

Selective lignin decomposition and nitrogen mineralization in forest litter colonized by Clitocybe sp.

Abstract Fruiting bodies of Clitocybe sp. were encountered in association with partly decomposed litter materials that were bleached due to colonization by mycelia of this fungus. We clarified the impact of this fungus on the chemical and fungal properties of litter materials and quantitatively evaluated the potential ability of this fungus to cause selective decomposition of recalcitrant compounds such as lignin. The content of acid-unhydrolyzable residues (AUR) was lower, and the contents of soluble carbohydrates and nutrients (N, P, K, Ca, Mg) and net mineralization rate of N were higher in bleached litter materials (BLM) than in adjacent nonbleached litter materials (NBL). Total hyphal length was 5.4 times greater in BLM than in NBL. A total of 49 fungal taxa were isolated, 30 from BLM and 42 from NBL. In pure culture decomposition tests, Clitocybe sp. caused greater mass loss in partly decomposed leaves than in freshly fallen leaves, which was attributed to the greater mass loss of AUR and more selective decomposition of AUR in partly decomposed leaves. Fourier transform infrared (FT-IR) spectroscopy showed that Clitocybe sp. was responsible for the selective transformation of the chemical structure of lignin. These results showed that Clitocybe sp. had a marked ability to remove AUR and lignin selectively from partly decomposed leaves and to enhance N mineralization, contributing to small-scale heterogeneity of the decomposition within the forest floor.

Disentangling the relative importance of host tree community, abiotic environment, and spatial factors on ectomycorrhizal fungal assemblages along an elevation gradient

Recent studies have shown that changes in community compositions of ectomycorrhizal (ECM) fungi along elevation gradients are mainly affected by changes in host tree communities and/or in abiotic environments. However, few studies have taken the effects of processes related to fungal dispersal (i.e. spatial processes) into account and distinguished the effects of host community, abiotic environment and spatial processes on community composition along elevation gradients. This has left unclear the relative importance of these factors in structuring the ECM community assemblages. To address this, we investigated the community composition of ECM fungi along an elevation gradient in northern Japan with 454 meta-barcoding. We found that the community composition of ECM fungi changed along the elevation and that all three factors jointly affected the compositional changes. We separated the magnitude of importance of the three factors in structuring ECM fungal communities and found that most of the spatial variation in ECM fungal community was explained by host communities and abiotic environments. Our results suggest that while biotic and/or abiotic environments can be important factors in determining the ECM fungal community composition along an elevation gradient, spatial processes may also be a primary determinant.

Behavior of Dissolved Organic Carbon in Larch Ecosystems

Dissolved organic carbon (DOC) has a significant contribution to carbon cycling in terrestrial ecosystems and links terrestrial and aquatic environments (McDowell and Likens 1988; Neff and Asner 2001; Dittmar and Kattner 2003; Sondergaard et al. 2003). Permafrost-affected ecosystems, which hold 25-33% of the world’s soil organic carbon (Hobbie et al. 2000), store significant part of this carbon temporarily in the surface layer of the ground as plant debris of different decomposition stages. This highly labile organic C may greatly exceed vegetation biomass (Prokushkin et al. 2006a, b; see also Chap. 8) and is most vulnerable to climate change (Schulze and Freibauer 2005). It is expected that along with climate-induced decomposition of this carbon pool, large amounts of dissolved organic matter will be released and transported to the oceans. Therefore, the transport of DOC from land to riverine systems and its chemical characteristics have received growing attention in the permafrost-dominated landscapes (Neff et al. 2006).

Interactive responses of dissolved sulfate and nitrate to disturbance associated with pine wilt disease in a temperate forest

Abstract To examine the effects of pine wilt disease on SO4 2− dynamics in a forested ecosystem, we analyzed the soil solution, groundwater, and streamwater in the Kiryu Experimental Basin, central Japan. The NO3 − concentrations in the soil surface layer showed a remarkable seasonality, with peak concentration in the fall. The SO4 2− concentrations in an area affected by pine wilt disease increased after NO3 − concentrations peaked. The delay between maximum SO4 2− to NO3 − concentrations may be explained by anion adsorption on variable charges in humus under low pH conditions resulting from nitrification. Concentrations of SO4 2− increased with groundwater depth, while the levels of NO3 − tended to decrease with groundwater depth. The vertical distribution of the SO4 2− and NO3 − concentrations in groundwater affected the seasonal changes in stream SO4 2− and NO3 − concentrations, as groundwater levels changed. It is reasonable to assume that nitrogen dynamics and hydrological processes play important roles in the retention and discharge of SO4 2− from disturbed forest soil systems.

Relationships Among pH, Minerals, and Carbon in Soils from Tundra to Boreal Forest Across Alaska

Tundra and boreal forests in northern high latitudes contain significant amounts of carbon (C) in the soil, indicating the importance of clarifying controls on soil C dynamics in the region and their feedback effects on climate systems. In northern Alaska, variations in soil C processes are closely associated with variations in soil acidity within ecosystems; however, the reason for this association remains unclear. In this study, we demonstrate that it results from weathering and subsequent changes in soil geochemical characteristics, including minerals and adsorptive organic C. We sampled soils from 12 sites in Alaska along a 600-km transect from the Arctic Ocean to interior Alaska, spanning the biomes of tundra, tundra–boreal forest ecotone, and boreal forest. Mineral soil analyses revealed that soils with low pH have fewer base cations, more aluminum/iron minerals, and lower base saturation, indicating that weathering is a major function of these geochemical characteristics in the broad area over northern Alaska. Adsorbed organic C in soil presented strong correlations with Al and Fe minerals, soil pH, and soil total C and represented approximately 30–55% of total soil C, suggesting that soil C accumulation in the Alaskan ecosystems is strongly controlled by weathering-related changes in geochemical characteristics. An adsorption test supported these observations and illustrated a greater capacity for acidic soil to adsorb organic C. These findings demonstrate that variations in weathering-associated characteristics have a strong influence on the regional variation in C dynamics and biogeochemical consequences in the Alaskan ecosystems.