Sustainable management for Japan's cedar stands towards national carbon neutrality

Ota, Masakazu; Takagi, Kentaro*; Liang, N.*; Takahashi, Yoshiyuki*; Koarashi, Jun

Resources, Conservation & Recycling, 235, p.109089_1 - 109089_9, 2026/09

https://doi.org/10.1016/j.resconrec.2026.109089

To achieve carbon neutrality by 2050, Japan is revitalizing forestry. However, it is unclear what specific forest management method should be implemented. Using a numerical model, here we explore carbon balance and economy of currently feasible managements for Japanese cedar plantation, the Japan's most economically important stand. We show that a repetition of 45-year rotation cycle most sequesters carbon dioxide from the atmosphere, also generating highest revenue from wood production. Applying this management to domestic cedar stands contributes 35%-73% of Japan's 2030 interim target of carbon dioxide absorption by forests, offsetting 1.4%-3.0% of national net emission target. However, sustainability of the management depends on subsidies, requiring over two-thirds of forestry costs. These indicate continued and sufficient support for the forestry sector is necessary to enable Japan's cedar stands to significantly contribute to carbon neutrality.

Progressive release of long-stored carbon from tropical peatland disturbances

Koarashi, Jun; Ito, Masayuki*; Atarashi-Andoh, Mariko; Kokubu, Yoko; Matsueda, Makoto; Kusin, K.*; Jaya, A.*; Dohong, S.*; Hirano, Takashi*

Nature Communications (Internet), 17, p.4369_1 - 4369_9, 2026/05

https://doi.org/10.1038/s41467-026-72890-y

Tropical peatlands are globally important, millennia-old carbon sinks, yet unprecedented human-driven degradation is triggering alarming carbon emissions. Comprehensive quantification of carbon dynamics across the disturbance sequence, from peat swamp forests to drained and fire-impacted peatlands, remains a critical knowledge gap. Here we show that over 18 years (1996-2014), drainage and subsequent peat fires released approximately 30-41 kg C m from peatlands in Central Kalimantan, Indonesia, using radiocarbon dating of peat profiles and groundwater dissolved organic carbon. Drainage contributed 5-11 kg C m, primarily from centuries- to millennium-old, previously waterlogged peat. Fires released 23-32 kg C m from peat accumulated over the past 3,000 years, initiating progressive oxidative decomposition of older peat. Extrapolation to Indonesia's disturbed peatlands suggests a release of 0.81-3.70 Gt between 1996 and 2014, with ongoing decomposition releasing an additional 0.03-0.08 Gt C annually, accelerating the impact on the global carbon balance.

Microbial survivability during repeated extreme dry-wet cycles determines CO emissions after rewetting of dried soils in humid temperate forests

Nakayama, Masataka*; Suzuki, Yuri; Abe, Yukiko; Taniguchi, Takeshi*; Atarashi-Andoh, Mariko; Koarashi, Jun; Nagano, Hirohiko*

Biogeochemistry, 169(2), p.15_1 - 15_15, 2026/02

https://doi.org/10.1007/s10533-026-01310-6

Plastics as vectors of radiocesium in river environments of Fukushima, Japan

Battulga, B.*; Nakanishi, Takahiro; Ikenoue, Tsubasa; Atarashi-Andoh, Mariko; Koarashi, Jun

Journal of Hazardous Materials, 500, p.140593_1 - 140593_11, 2025/12

https://doi.org/10.1016/j.jhazmat.2025.140593

Radiocarbon-derived ages of carbon in live, dead, and newly emerged fine roots in a cool-temperate Japanese beech forest

Koarashi, Jun; Atarashi-Andoh, Mariko; Ishizuka, Shigehiro*; Noguchi, Kyotaro*; Kadono, Atsunobu*; Nakayama, Masataka*

Frontiers in Forests and Global Change (Internet), 8, p.1654883_1 - 1654883_10, 2025/11

https://doi.org/10.3389/ffgc.2025.1654883

Maintaining and enhancing the forest carbon (C) sinks are key to mitigating climate change; however, the forest C dynamics related to the production and turnover of fine roots (2 mm in diameter) have not been well captured. In this study, we examined ages of C in fine roots in a cool-temperate Japanese beech forest by measuring the natural abundance of radiocarbon (C). Root samples were collected and categorized by diameter size class and live/dead status. Newly emerged roots were also obtained using the ingrowth mesh bag method. The mean ages of fine root C were estimated to be 5-23 years for live roots and 1-34 years for dead roots, respectively. In contrast, the C signatures of newly emerged roots indicated the use of current-year photosynthetic products for new root production. Given the negligible time lag between photosynthetic C fixation and the use of the fixed C for new root production, the observed ages for live fine root C suggest that plants use current-year photosynthetic products to produce new roots but utilize older (C-enriched) internally stored C to support subsequent root growth, and/or that some fine roots live for many years. Our results provide a piece of knowledge to comprehensively understand belowground C allocation processes in plants.

Evaluating sulfur-impurity removal and modern carbon contamination in different preparation methods for radiocarbon dating of soil samples by accelerator mass spectrometry

Koarashi, Jun; Takeuchi, Erina; Kokubu, Yoko; Atarashi-Andoh, Mariko

Radiocarbon, 67(2), p.307 - 317, 2025/04

https://doi.org/10.1017/RDC.2024.137

Radiocarbon (C) dating of soil samples by accelerator mass spectrometry has been proven useful for studying carbon (C) cycling in terrestrial ecosystems. There are, however, two main difficulties in sample preparation for this application: contamination of samples with modern C and inhibition of graphite formation due to sulfur (S)-containing impurities. Here we evaluated these effects from three different sample preparation methods, by conducting C measurements of C-dead sample and S-rich soil samples. The preparation methods were all successful in graphite formation and C measurement for soil samples with an organic S content 6.9%. The different methods showed different percent Modern Carbon (pMC) values ranging from 0.19% to 0.64% for C-dead sample. However, the three methods had little influence on the determination of C age for samples at least younger than 12,000 yr BP. The methods examined in the present study can be used for C dating with sufficient accuracy in the application to C cycle studies.

Unraveling the Earth's carbon cycle using isotope analysis

Koarashi, Jun

MEXT Nuclear Fellow News; The Nuclear Researchers Exchange Program (Internet), (11), P. 17, 2025/03

http://www.nsra.or.jp/int/iard/fellownews.html

no abstracts in English

Biofilm-mediated interactions between plastics and radiocesium in coastal environments

Battulga, B.; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Otosaka, Shigeyoshi*; Koarashi, Jun

Environmental Science and Pollution Research, 31, p.60080 - 60092, 2024/10

https://doi.org/10.1007/s11356-024-35164-y

A ubiquitous distribution of plastic debris has been reported in aquatic and terrestrial environments; however, the interactions between plastics and radionuclides and the radioactivity of environmental plastics remain largely unknown. Here, we characterize biofilms developing on the surface of plastic debris to explore the role of plastic-associated biofilms as an interaction medium between plastics and radiocesium (Cs) in the environment. Biofilm samples were extracted from plastics (1-50 mm in size) collected from two contrasting coastal areas in Japan. The radioactivity of plastics was estimated based on the Cs activity concentration of the biofilms and compared seasonally with surrounding environmental samples (i.e., sediment and sand). Cs traces were detected in biofilms with activity concentrations of 21-1300 Bq kg biofilm (dry weight), corresponding to 0.04-4.5 Bq kg plastic (dry weight). Our results reveal the interaction between Cs and plastics and provide evidence that organic and mineral components in biofilms are essential in Cs retention in environmental plastics.

Quantitative importance of subsoil nitrogen cycling processes in Andosols and Cambisols under temperate forests

Nakayama, Masataka; Abe, Yukiko; Atarashi-Andoh, Mariko; Tange, Takeshi*; Sawada, Haruo*; Liang, N.*; Koarashi, Jun

Applied Soil Ecology, 201, p.105485_1 - 105485_12, 2024/09

https://doi.org/10.1016/j.apsoil.2024.105485

Nitrogen often limits plant growth in forest ecosystems. Plants, including trees, change vertical root distribution when nutrient competition is strong within surface soil layer and take up nitrogen even from subsurface soil layers in addition to the surface soil. However, there is still limited knowledge about nitrogen cycles within deeper soil layers. In this study, we investigated the vertical profiles (0-60 cm) of the net nitrogen mineralization and nitrification rates at four Japanese forest sites with two different soil types (Andosols and Cambisols). The partial least square path modeling (PLS-PM) was used to determine factors affecting nitrogen-cycling processes. The net nitrogen mineralization and nitrification rates per unit soil weight were considerably higher in surface soil layer than in deeper soil layers in Andosols but not in Cambisols. PLS-PM analysis showed that microbial biomass and soil organic matter quantities were the main factors influencing the net nitrogen mineralization and nitrification rates, indicating that a similar mechanism creating the spatial variations of nitrogen-cycling processes in surface soil layer predominantly regulates the processes in subsoil layers. Moreover, it was estimated that the net nitrogen mineralization rate could be comparable at all soil types and depths when the rate was expressed per unit soil volume. Therefore, our results suggest that subsoil layers are a quantitatively important nitrogen source for plant nutrients in Andosols and Cambisols, supporting high forest productivity.

Sequential loss-on-ignition as a simple method for evaluating the stability of soil organic matter under actual environmental conditions

Sato, Yuhi*; Ishizuka, Shigehiro*; Hiradate, Shuntaro*; Atarashi-Andoh, Mariko; Nagano, Hirohiko*; Koarashi, Jun

Environmental Research, 239, Part 1, p.117224_1 - 117224_9, 2023/12

https://doi.org/10.1016/j.envres.2023.117224

The stability of soil organic matter (SOM) is important for improving our understanding of the global carbon cycle and ongoing climate change. This study examined the applicability of loss-on-ignition of soil with a stepwise increase in temperature (SIT-LOI) to evaluate the stability of the SOM using soil samples from Japan having different organic matter (OM) and mineral contents and different mean residence times (MRTs), estimated from radiocarbon analysis, for SOM. As the result of this examination, SIT-LOI data was strongly correlated with MRTs. This clearly suggests that SIT-LOI can be an indicator evaluating the stability of SOM in actual environments.