JAPANESE JOURNAL OF ECOLOGY

We measured the CO₂ flux from sediment surfaces (soil respiration in the pedosphere to the atmosphere) in a mangrove forest using an improved automatic open/close chamber (AOCC) method. Soil respiration rates and environmental factors were continuously measured from 4 to 8 July, 2013, in a mangrove dominated by Bruguiera gymnorrhiza. Variation in respiration rate did not exhibit a clear correlation with soil temperature. However, tidal effects were related to variation in soil temperature and may also have contributed to variation in respiration rate. High respiration rates were detected immediately before submergence or after exposure, due to the physical effects of tidal variation. Respiration rates during the period of exposure were lower than those in terrestrial ecosystems, likely due to three factors unique to mangrove forests: soil respiration measurements generally do not include root respiration, organic matter decomposition is restricted to a shallow anaerobic area, and some mineralized carbon is lost as dissolved inorganic carbon. Respiration rates during submergence were half of those measured during exposed conditions, suggesting that previous studies overestimated annual soil respiration. Therefore, measuring soil respiration rates during both exposed and submerged conditions using the AOCC method provides a much more accurate understanding of carbon dynamics in the pedosphere of mangrove forests.

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Mangrove forests are tropical forests with carbon-rich soil. Dissolved organic matter (DOM) is important for investigating the mangrove carbon cycle and ecological roles because the forests connect the river tributaries with the coast via rivers running through them. The ecological functions and turnover rate of DOM depend on its composition. Therefore, we characterized the DOM of the Fukido River in a mangrove forest by analyzing the percentage of humic substances (%HS) in DOM. Water sampling from the headwaters to the sea revealed that %HS declined going toward the sea due to mixing with low-%HS seawater and the possible selective precipitation of HS into mangrove soil. The headwaters had a higher %HS than most clear water rivers and may be an important supply of HS for mangrove and coastal areas. The sequential extraction of water-extractable organic matter from mangrove soil (sampling depth: 0–25 cm) using ultrapure water indicated a significant increase in HS concentration along with a decline in salinity. These results suggest that high salinity is responsible for the selective accumulation of HS in mangrove soil, which can be a key to elucidating the mechanism of organic matter accumulation in mangrove soils.

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Progress in ecological studies of soil fauna includes studies of the role and effects of soil fauna on decomposition and soil carbon dynamics in relation to global environmental changes, the introduction of molecular biology approaches to such studies, feeding habit analysis using stable isotopes, trait-based analysis of community ecology, and fine-scale experimental studies on the above/below ground relationship in ecosystems. In relation to this progress, six topics were reviewed: the function of soil fauna in the decomposition process; carbon sequestration in earthworms; the process of assemblage formation in earthworms; a trait-based approach to the collembolan community; food habit analysis using stable isotopes; and soil faunal impacts on plants.

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The clonal proliferation of neoplastic cells within a body and the clonal propagation of living organisms in a habitat are essential components of neoplastic and clonal population growth, respectively. In a favorable habitat, the number of clonal organisms increases exponentially up to the carrying capacity or to a niche size that reflects a specified sensitivity for environmental factors. This phenomenon corresponds to benign neoplasms in a body. For malignant neoplasms, neoplastic cells invade the surrounding tissue and metastasize to distant organs via the evolution of subclones that expand their niche size or carrying capacity by changing their genomes. This is an evolutionary mechanism that characterizes neoplastic cells in an organism, but has not yet been discovered for organisms other than bacteria. In humans, cancer cells treat medical treatments and the immune system as environmental disturbances within the organism. By accumulating genetic changes such as mutations, a neoplasm produces multiple subclones with differing genomic constitution. Such cell evolution can be represented as a phylogenetic tree. The spatial distribution of cancer cells in a tissue is a patchy structure of clones because of the genetic heterogeneity of neoplastic cells, environmental heterogeneity of the habitat tissues, and limited dispersal ability of subclones. In conclusion, a comparison of the concept of clonality between the cell and organism levels would give new insights into research in both cancer biology and ecology.

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