Takashi Osono

Ecology of ligninolytic fungi associated with leaf litter decomposition

Advances in our understanding of the decomposition processes in forest ecosystems over the past three decades have demonstrated the importance of lignin as a regulating factor in the decomposition of leaf litter. Consequently, increasingly more attention is being focused on the ecology of fungi associated with lignin decomposition. The aim of this review is to provide a critical summary of the ecology of ligninolytic fungi inhabiting leaf litter and forest floor materials. The review focuses on the following aspects of ligninolytic fungi: the taxonomic and functional diversity of ligninolytic fungi, the outcomes of interactions between ligninolytic fungi and other organisms, the activity and abundance of ligninolytic fungi measured by the production of bleached leaves and humus, the activity of ligninolytic enzymes in soil environments, the substratum and seral succession, spatial and temporal patterns in both mycelial abundance and species distribution, and the effect of environmental factors such as nitrogen deposition and global environmental changes on ligninolytic fungi. This review integrates the ecology, diversity, and activity of ligninolytic fungi into the context of an ecosystem in order to provide an understanding of the roles of ligninolytic fungi in decomposition processes.

Comparison of litter decomposing ability among diverse fungi in a cool temperate deciduous forest in Japan

The litter decomposing ability of 79 fungal isolates (41 genera, 60 species) was assessed with the pure culture decomposition test. The isolates were collected qualitatively in a cool temperate deciduous forest in Japan during a 21-mo period. Loss of original weight of sterilized litter ranged from 0.1% to 57.6%. Six isolates in the Basidiomycota caused high weight losses ranging from 15.1% to 57.6%. Fourteen isolates in Xylaria and Geniculosporium (the Xylariaceae and its anamorph) also caused high weight losses ranging from 4.0% to 14.4%. Other isolates in the Ascomycota and associated anamorphs and in the Zygomycota caused low weight losses on mean. Six fungi in the Basidiomycota, and all in the Xylariaceae showed a bleaching activity of the litter and caused lignin and carbohydrate decomposition. Mean lignin/weight loss ratios (L/W) and lignin/carbohydrate loss ratios (L/C), were 0.9 and 0.7 for the Basidiomycota and 0.7 and 0.4 for the Xylariaceae, respectively. Significant differences were found in L/W and L/C between the two groups when the result of Xylaria sp. that showed marked delignification was excluded. These differences in lignin and carbohydrate utilization patterns are discussed in relation to the structural and the chemical properties of the decomposed litter and to the implications for organic chemical changes during litter decomposition processes.

Organic chemical and nutrient dynamics in decomposing beech leaf litter in relation to fungal ingrowth and succession during 3-year decomposition processes in a cool temperate deciduous forest in Japan

Decomposition processes of beech leaf litter were studied over a 3-year period in a cool temperate deciduous forest in Japan. Organic chemical and nutrient dynamics, fungal biomass and succession were followed on upper (Moder) and lower (Mull) of a forest slope. Litter decomposition rates were similar between the sites. Nutrient dynamics of the decomposing litter was categorized into two types: weight changes in nitrogen and phosphorus showed two phases, the immobilization (0–21 months) and the mobilization phase (21–35 months), while those in potassium, calcium and magnesium showed only the mobilization phase. The rate of loss of organic chemical constituents was lignin < holocellulose < soluble carbohydrate < polyphenol in order. The changes in lignocellulose index (LCI), the ratio of holocellulose in lignin and holocellulose, were significantly correlated to the changes in nitrogen and phosphorus concentrations during the decomposition. During the immobilization phase, increase in total fungal biomass contributed to the immobilization of nitrogen and phosphorus. The percentage of clamp-bearing fungal biomass (biomass of the Basidiomycota) to total fungal biomass increased as the decomposition proceeded and was significantly correlated to LCI. Two species in the xylariaceous Ascomycota were dominantly isolated by the surface sterilization method from decomposing litter collected in the 11th month. The organic chemical, nitrogen and phosphorus dynamics during the decomposition were suggested to be related to the ingrowth, substrate utilization and succession of the Xylariaceae and the Basidiomycota. Twenty-one species in the other Ascomycota and the Zygomycota isolated by the washing method were classified into three groups based on their occurrence patterns: primary saprophytes, litter inhabitants and secondary sugar fungi. These species showed different responses to LCI and soluble carbohydrate concentration of the litter between the groups.

Decomposition of organic chemical components in relation to nitrogen dynamics in leaf litter of 14 tree species in a cool temperate forest

Decomposition of lignin, holocellulose, polyphenols and soluble carbohydrates was investigated in relation to nitrogen (N) dynamics in leaf litter of 14 tree species. The influence of organic chemical components and N on litter mass loss rate was then evaluated for 14 litter types. The study was carried out over a 3-year period on upper and lower parts of a forest slope in a cool temperate forest in Japan. The decomposition processes were divided into early and late phases based on N immobilization and mobilization. Mass loss rate of whole litter and organic chemical components was similar for the upper and lower sites. Litter mass loss was faster in the immobilization phase than in the mobilization phase in each of 14 litter types, which was ascribed to the decreased mass loss of holocellulose, polyphenols and soluble carbohydrates in the mobilization phase as compared to the immobilization phase. Mass loss rate of lignin was not different between the phases. Litter mass loss rate in the immobilization and mobilization phases was negatively correlated to lignin content and positively correlated to contents of polyphenols and soluble carbohydrates at the start of these phases, but was not correlated to holocellulose and N contents in either phase.

Accumulation and release of nitrogen and phosphorus in relation to lignin decomposition in leaf litter of 14 tree species

Immobilization and mobilization of nitrogen and phosphorus were investigated in relation to the nitrogen (L/N) ratio and lignin to the phosphorus (L/P) ratio as indicators of the nitrogen and phosphorus dynamics. The present study was carried out on upper and lower parts of a forest slope in a cool temperate forest in Japan. Net immobilization and net mobilization characterized the dynamics of nitrogen and phosphorus in 14 litter types and were related to the changes in the L/N and L/P ratio. The critical values of the L/N and L/P ratio at which the mobilization began were 23–25 and 500–620, respectively. In litter types with the L/N and L/P ratio higher than critical values, nitrogen and phosphorus were immobilized until the ratios reached at the critical values and then nitrogen and phosphorus began decreasing. In litter types with initial L/N and L/P ratios lower than or equal to the critical values, nitrogen and phosphorus were released from litter. The critical values of the L/N and L/P ratios showed convergent trends among litter types as compared to their initial values, and were approached to those of underlying humus layers. These results indicated the usefulness of L/N and L/P ratios as indicators of the nitrogen and phosphorus dynamics in the study site. The general validity of the L/N ratio as an indicator of nitrogen dynamics and the convergent trend of critical L/N ratio at 25–30 were demonstrated by a review of literature on lignin and nitrogen dynamics in 47 litter types in temperate and boreal forests.

Endophytic and epiphytic phyllosphere fungi of Camellia japonica: seasonal and leaf age-dependent variations

Seasonal and leaf age-dependent variations in the endophytic and epiphytic phyllosphere fungal assemblages of Camellia japonica were examined and compared. Live leaves of C. japonica were collected in four seasons (May, Aug, Nov, Feb), and fungi were isolated from healthy-looking leaves of 0, 1, 2 and 3 y old. The infection rate and total number of endophytic fungi increased May–Feb, and species richness of endophytes increased as leaves aged. In contrast the infection rate of epiphytic fungi was 100% for all leaf ages at every sampling date. The total number of epiphytic fungi isolated was greatest in May and lowest in Aug. The species richness of epiphytes did not differ significantly by season or leaf age. Eight fungal species were recorded as major phyllosphere fungi of C. japonica. Seasonal variations were detected for the frequencies of Colletotrichum gloeosporioides, C. acutatum, and epiphytes Pestalotiopsis sp.1, Aureobasidium pullulans, Phoma sp.1 and Ramichloridium sp., whereas the frequency of the endophyte Geniculosporium sp.1 varied with leaf age. The frequency of the epiphyte Cladosporium cladosporioides varied with both season and leaf age.

Roles of diverse fungi in larch needle-litter decomposition.

Functional biodiversity of fungi in larch (Larix leptolepis) forests needle-litter decomposition was examined by a pure-culture test. Weight loss of larch-needle litter, utilization pattern of lignocellulose and chemical composition of remaining litter were investigated and compared for 31 isolates in 27 species of basidiomycetes and ascomycetes. Weight loss (% original weight) of litter ranged from −2.0% to 14.2%. Mean weight loss of litter caused by the basidiomycetes was not significantly different from that caused by the ascomycetes. Basidiomycetes caused loss of lignin and carbohydrates in variable proportions, while ascomycetes exclusively attacked carbohydrates without delignification. The content of lignin and nitrogen in remaining litter was not significantly correlated when both basidiomycetes and ascomycetes were included. However, the correlation coefficient was significant when the relationship was examined separately for basidiomycetes, indicating that the degree of selective delignification determined the final nitrogen content in litter. Possible effects of fungal colonization on needle-litter decomposition in larch forests are discussed.

Decomposing ability of interior and surface fungal colonizers of beech leaves with reference to lignin decomposition

Abstract Fungi were isolated from interior and surface of beech ( Fagus crenata Blume) leaf litter by surface sterilization and washing methods. Species composition differed between the interior and surface of the leaves. Xylaria sp. (anamorph) was a major interior colonizer, while Pestalotiopsis spp. and Trichoderma spp. were predominantly surface colonizers. Ascochyta sp. was isolated from both the interior and surface of leaves. Leaf litter decomposing abilities of all isolated species were assessed by pureculture decomposition tests. Changes in lignin, carbohydrate and polyphenol amounts in the litter were investigated. Percent loss of original weight of sterilized beech leaf litter ranged from 0.24 to 11.16 % in the decomposition test. Interior fungi such as Xylaria sp. (anamorph) had the highest abilities to bleach leaf litter and decompose lignin. Most surface fungi had limited ability to decompose leaf litter and lignin. The difference in the decomposing abilities between the interior and surface fungi is discussed in relation to the difference in organic-chemical compositions between interior and surface of the litter.

Effects of organic chemical quality and mineral nitrogen addition on lignin and holocellulose decomposition of beech leaf litter by Xylaria sp.

Abstract Mycobiota and chemical composition of bleached and non-bleached portions were studied on leaf litter of beech ( Fagus crenata Blume). By surface sterilization method, two xylariaceous species Xylaria sp. and Geniculosporium sp.1 were dominantly isolated in both portions. Frequency of occurrence of Xylaria sp. was significantly higher in the bleached portion than in the non-bleached portion. In the bleached portion, lignin concentration was lower than in the non-bleached portion, indicating that Xylaria sp. and Geniculosporium sp.1 took part in lignocellulose decomposition in the study site. Effects of organic chemical quality of litters and exogenous mineral nitrogen (NH 4 and NO 3 ) addition were then investigated on in vitro lignin decomposition by Xylaria sp. Weight loss of lignin was significantly related to lignocellulose index (LCI) for four litter types tested. In NH 4 and NO 3 addition treatments, lignin decomposition was completely and partially suppressed, respectively. Xylaria sp. produced bleaching spots on beech leaf litter in vitro in which lignin concentration was lower than in the non-bleached portion. These results suggest that heterogeneous distribution of carbon and nitrogen resources may control lignin decomposition on the litter by the fungus.

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.