Mangrove restoration is being increasingly employed to restore degraded coastal ecosystems and strengthen blue carbon (C) sequestration. Nevertheless, total ecosystem carbon (TEC) evaluations often omit deep soils, causing inaccurate estimates of C stored in restored mangroves. This study investigated a 10–14-year chronosequence restoration in Central Thailand using Avicennia alba to quantify TEC stocks across aboveground biomass, coarse roots and fine roots (≤2 mm in diameter), and soil organic carbon (SOC) at a depth of 3 m, as well as changes in total ecosystem carbon (ΔTEC). TEC stocks increased from 695.6 to 828.2 Mg C ha−1 with stand age, with SOC accounting for >80% of total stocks. The ΔTEC was substantial—21.06 Mg C ha−1 yr−1 at 1 m—and was largely associated with SOC, whereas the increase in biomass C was estimated at 10.11 Mg C ha−1 yr−1. Depth-integrated estimates indicated higher values when deeper soil layers were included. SOC stocks increased with stand age throughout the 0–3 m profile; increases up to 2 m were associated with vegetation-derived inputs, as indicated by δ13C shifts and fine root presence. However, soils at 2–3 m retained estuarine signatures that were not clearly associated with vegetation-derived inputs. This study highlights the importance of including deep soil layers in C assessments and emphasizes that interpretations of ecosystem C change should consider depth-dependent controls. These findings emphasize the importance of young mangrove restorations as emerging C sinks, particularly through substantial soil C storage.

This study evaluated the potential of biochar derived from the brown macroalga, Eisenia bicyclis, which is widely distributed along the Japanese coast, as a functional soil amendment and microbial attachment substrate. Marine macroalgae are characterized by rapid growth and high photosynthetic capacity, contributing to effective CO₂ fixation; however, stranded algae often accumulate as an environmental burden. Conversion of macroalgal biomass into biochar represents a promising strategy for the sustainable utilization of marine algal resources. Accordingly, biochars derived from E. bicyclis were produced at three pyrolysis temperatures (350, 450, and 550 °C), and their properties were evaluated in comparison with lignocellulosic biochars obtained from wood and rice husk, using elemental analysis and Fourier-transform infrared (FTIR)-based surface functional group characterization. Subsequently, to assess functional performance in soil, the biochars were added to agricultural field-collected soil and incubated for 28 days, after which microbial attachment was quantified using ATP-based biomass measurements and visualized by SEM. The results showed that algae-derived biochars contained higher concentrations of nitrogen and mineral elements than lignocellulosic biochars. Among the tested conditions, biochar produced at 450 ℃ exhibited recalcitrance comparable to that of wood- and rice husk–derived biochars. When biochars with comparable recalcitrance were applied, microbial attachment levels were similar regardless of feedstock type, and microbial aggregates were consistently observed on the biochar surfaces. These findings demonstrate that E. bicyclis–derived biochar is a nutrient-rich and persistent material that supports soil microbial colonization, highlighting the value of marine macroalgal biomass for agricultural and environmental applications.

Primary succession profoundly alters soil environments and microbial communities, but the mechanisms governing microbial stoichiometric regulation during ecosystem development remain unresolved. In this study, we investigated changes in soil resource stoichiometry, extracellular enzyme allocation, and microbial biomass C:N:P ratios as well as their relationships along a primary successional gradient in a volcanic desert on Mount Fuji, Japan. We quantified soil labile and total C, N, and P pools, extracellular enzyme activities associated with C, N, and P acquisition, and microbial biomass stoichiometry across six successional stages (bare ground to mature forest). As succession progressed, labile and total soil C:N and C:P ratios increased (labile C:N: 265–948; C:P: 2.4–66.5), indicating a shift toward relative N and P limitation. Correspondingly, ecoenzymatic C:N and C:P ratios decreased, reflecting increased microbial investment in N- and P-acquiring enzymes. However, microbial biomass C:N and C:P ratios increased during succession (from 5.1 to 9.8 and 2.5–27.2, respectively), demonstrating stoichiometric plasticity rather than homeostasis. Microbial C:N ratios were strongly correlated with soil labile C:N ratios, which was in contrast to the weak relationships with ecoenzymatic stoichiometry. These results indicate a decoupling between ecoenzymatic regulation and microbial biomass stoichiometry. Our findings suggest that in this C-limited volcanic system, extracellular enzyme allocation alone is insufficient for maintaining microbial stoichiometric homeostasis. Instead, microbial biomass stoichiometry during primary succession would reflect an integrated response to changing soil resource stoichiometry and community-level processes such as shift in community structure.

The increase in forest wood biomass is an important carbon sink in the biogeochemical carbon cycle. The effect of forest succession on carbon sequestration is essential for predicting the impact of future global warming. However, there are limited studies considering the temporal changes of net primary production (NPP) with secondary succession in cool-temperate forests based on long-term data. Therefore, we monitored changes in forest structures over 23 years in a cool-temperate secondary forest (Takayama Forest), which is dominated by Quercus crispula, Betula ermanii, and B. platyphylla var. japonica, to determine how individual stem growth and wood NPP changes with succession and thereby affect biomass accumulation. For Q. crispula and B. ermanii, the population size reduction (71

Plant fine roots are an important component of the carbon cycle in ecosystems. Nondestructive methods, such as tracing fine roots in soil cross‐section images, have become the mainstay of fine root dynamics analysis in recent years; however, manual tracing methods are slow and suffer from low consistency. To solve these problems, many artificial intelligence (AI)‐based image segmentation methods using deep learning have been developed. We aimed to verify the segmentation accuracy of RootPainter, an AI‐based software, in a real forest ecosystem and use it to estimate the production rate and turnover time. The images segmented by RootPainter contained many errors that led to overestimation. However, by manually eliminating the errors caused by incorrectly segmented roots in areas with no actual roots (a type of false positive, referred to as a “phantom error.”), segmentation accuracy was greatly improved. The AI‐segmented area was 2.34 times larger than that of the conventional method, but after removing the phantom errors, it decreased to 1.21 times the size. The correlation coefficient between these areas also increased. In addition, the time required was 43%–47% less than that required by the conventional method. Furthermore, this hybrid AI segmentation and manual correction method could estimate production rates and turnover times from soil cross‐sectional images of actual forests, and the results were comparable with those obtained using conventional methods. Thus, the AI‐based segmentation software was shown to be effective in analyzing fine root dynamics using soil cross‐section images in natural ecosystems, with appropriate human error‐correction assistance.

This study aimed to evaluate the suitability of alginate gels, specifically ferric-ion-cross-linked alginate (Fe-alginate) and calcium-ion-cross-linked alginate (Ca-alginate), as scaffolds for soil microbial attachment and biofilm formation in soil. Staining with crystal violet and observations with scanning electron microscopy showed that microorganisms formed biofilms on Fe-alginate surfaces in the soil. When the soil was incubated with Fe-alginate, microbial biomass, estimated by adenosine triphosphate content, increased not only in the Fe-alginate but also in the surrounding soil. The weight of Ca-alginate in the soil decreased with time owing to chemical dissolution. However, the weight of Fe-alginate in the soil did not decrease, likely because it was protected by the microbial biofilm that formed on its surface. These results demonstrate that the use of Fe-alginate, in contrast to Ca-alginate, as a scaffold may allow for more efficient use of soil microbial functions in agriculture and bioremediation.

Biochar application as a soil amendment is gaining attention as a stable, long-term carbon sequestration strategy for the mitigation of climate change. However, biochar applied to the soil may increase soil carbon efflux. This study aimed to determine the long-term (8 years) effects of biochar application to the forest floor on soil carbon effluxes (soil respiration [SR] and heterotrophic respiration [HR]) in a warm–temperate oak forest. Biochar was applied at the rate of 0, 5, or 10 Mg ha−1 to 20 m × 20 m plots (n = 4). The SR and HR rates were determined using the closed chamber method and the trenching method. The annual SR tended to increase over 8 years following biochar application, whereas a significant increase in the annual HR (+31%–37%) was observed in the short term (<3 years). The increased HR likely included CO2 emissions from the decomposition of the labile fraction of biochar carbon and from the microbial decomposition of the original soil organic matter stimulated through changes in the soil physicochemical environment, such as soil moisture and pH. The results suggest that a short-term increase in HR should be considered in the evaluation of carbon sequestration in response to biochar addition to forest ecosystems.

Changes in soil nutrient dynamics after biochar application may affect indirect carbon sequestration through changes in plant productivity in forest ecosystems. In the present study, we examined the effects of woody biochar application on soil nitrogen (N) cycling over 8 months in a warm-temperate deciduous broad-leaved forest. Mineral soil samples were collected from the plots treated with different biochar applications (0, 5, and 10 Mg ha−1), and the soil inorganic N concentration was measured. Net mineralization and nitrification rates were determined in each plot using the resin–core method. Soil temperature and water content did not change significantly, but the pH increased significantly following biochar application. Soil inorganic N concentrations (NH4+ and NO3−) and net N transformation rates (mineralization and nitrification rates) were significantly reduced. Microbial biomass and the nitrification ratio (the ratio of nitrification rate to mineralization rate) were unchanged, indicating that the decrease in soil inorganic N concentration was due to the reduced mineralization rate. Adsorption of substrates (from organic matter) by the applied biochar is the most likely reason for the reduction in the N mineralization rate. The results indicate that biochar application does not necessarily stimulate N transformation, which will affect indirect carbon sequestration.

The organic (O) layer on top of forest soil is important in forest carbon (C) cycling, but not many studies have investigated integrated C dynamics, including dissolved organic C (DOC) fluxes. To clarify the DOC contribution and C dynamics in the O layer, this study quantified the C stock of the O layer, C input (litter fall (LF) and throughfall DOC (TF-DOC)), and C output (CO2 emission and litter leachate DOC (LL-DOC)) over the course of a year for three forest stands in the same region (middle slopes of Mt. Asama, Nagano, Japan) dominated separately by Quercus serrata (Q), Larix kaempferi (L), and Pinus densiflora (P). The TF-DOC contribution to C input was high in the rainy season. Additionally, TF-DOC was greater in the L forest than in the Q and P forests because of relatively small LF production in the L forest. The contribution of LL-DOC to C output also tended to be high in the rainy season, but the contribution increased in winter owing to the small emission of CO2 at low temperatures. Annual contribution of LL-DOC was much higher in the P forest than in the Q and L forests, reflecting differences in litter quality. The DOC contribution rate to C flux was generally < 20%, suggesting that DOC flux is a minor but non-negligible process in C cycling in the O layer depending on season and forest type. Our comprehensive C flux data provide better understanding of C cycling in the O layer of temperate forests.

Around one-third of the world’s most carbon-rich ecosystems, mangrove forests, have already been destroyed in Thailand owing to coastal development and aquaculture. Improving these degraded areas through mangrove plantations can restore various coastal ecosystem services, including CO2 absorption and protection against wave action. This study examines the biomass of three coastal mangrove plantations (Avicennia alba) of different ages in Samut Prakarn province, Central Thailand. Our aim was to understand the forest biomass recovery during the early stages of development, particularly fine root biomass expansion. In the chronosequence of the mangrove plantations, woody biomass increased by 40% over four years from 79.7 ± 11.2 Mg C ha-1 to 111.7 ± 12.3 Mg C ha−1. Fine root biomass up to a depth of 100 cm was 4.47 ± 0.33 Mg C ha−1, 4.24 ± 0.63 Mg C ha−1, and 6.92 ± 0.32 Mg C ha−1 at 10, 12, and 14 year-old sites, respectively. Remarkably, the fine root biomass of 14-year-old site was significantly higher than those of the younger sites due to increase of the biomass at 15–30 cm and 30–50 cm depths. Our findings reveal that the biomass recovery in developing mangrove plantations exhibit rapid expansion of fine roots in deeper soil layers.

The nitrogen (N) cycle, a major biogeochemical cycle in forest ecosystems, notably affects ecosystem multifunctionality. However, the magnitude and role of organic N and the snow season remain uncertain in this cycle. We assessed the N flux and pool data of a temperate deciduous broad-leaved forest to clarify N cycle processes. The results showed that the most important component of the N pool was the soil N pool. The N demand of the site amounted to 139.4 kg N ha−1 year−1 and was divided into tree production (83.8%) and bamboo production (16.2%). We clarified that retranslocation (37.4%), mineralization at a soil depth of 0–5 cm (15.3%), litter leachate (4.6%), throughfall (2.3%), and canopy uptake (0.5%) provided 60.1% of the N demand. In terms of soil at 0–5 cm in depth, the net mineralization rate during the snow season contributed to 30% of the annual mineralization. We concluded that the study site was not N-saturated as a result of a positive N input–output flux budget. More than half of the total N was accounted for by dissolved organic N flowing through several pathways, indicating that organic N plays a vital role in the cycle. The mineralization rate in the soil layer during the snow season is an important link in the N cycle.

Lucidophyllous (evergreen broad-leaved) forests are the dominant forests in human-dominated subtropical/warm-temperate regions in East Asia. Biometric-based estimates of net primary production (NPP) were conducted in a secondary lucidophyllous forest on Mt. Kinka (35°26′ N, 136°47′ E) near the northern limit of their distribution in central Japan for three years, including the masting event. The forest stand mainly consists of Castanopsis cuspidata (Thunb.) Schottky and Cleyera japonica Thunb. in the canopy and subtree layers, respectively. In 2018, the total NPP of the masting year was 14.53 ± 2.03 ton ha−1 yr−1, including woody NPP (above: 2.63 ± 0.35 ton ha−1 yr−1; below: 0.57 ± 0.08 ton ha−1 yr−1), foliage NPP (4.07 ± 0.23 ton ha−1 yr−1), reproductive NPP (4.81 ± 0.77 ton ha−1 yr−1), and fine root production (Pfr) (2.46 ± 1.84 ton ha−1 yr−1). Pfr and belowground production comprised 16.9% and 20.9%, respectively, of the total NPP. The nut production of C. cuspidata in 2018 (4.31 ± 0.75 ton ha−1 yr−1) was significantly higher than that in 2017 (0.77 ± 0.13 ton ha−1 yr−1) and 2019 (0.23 ± 0.06 ton ha−1 yr−1). No significant change was observed for the three years of foliage NPP and total NPP without Pfr. However, the woody NPP in 2018 (3.20 ± 0.43) was lower than in 2017 (5.37 ± 0.33 ton ha−1 yr−1) and 2019 (4.71 ± 0.38 ton ha−1 yr−1). This suggests that nut production in the masting years compensated by decreasing woody production in the Castanopsis forest.

Seasonal variabilities in hydrological fluxes of dissolved organic carbon (DOC) and their driving factors in the evergreen broad-leaved forest are inadequately understood. To aid this understanding, we conducted a three-year study to examine seasonal changes in DOC concentration and flux in throughfall, stemflow, and litter leachate in an evergreen broad-leaved subtropical forest in central Japan. We specifically addressed (1) how DOC in different hydrological fluxes vary on a monthly to seasonal basis, and (2) how canopy phenology and meteorology shape the DOC concentration and flux of throughfall, stemflow, and litter leachate trends in this evergreen forest. Clear seasonal changes were found in throughfall and stemflow DOC concentration but not in litter leachate DOC concentration; the highest throughfall DOC concentrations were observed in spring (10.03 mg L−1 in 2017 and 9.59 mg L−1 in 2018, respectively) and the highest stemflow DOC concentrations were observed in summer (13.95 mg L−1 in 2017 and 16.50 mg L−1 in 2018, respectively). Correlation analysis revealed the monthly throughfall DOC concentration to be positively related to the dry weight of fallen leaves (r = 0.72, p < 0.05) and flowers (r = 0.91, p < 0.05). In addition, Random Forest models predicted that the dry weight of flowers was a primary driver of throughfall DOC concentration and that the DOC concentrations of stemflow and litter leachate were constrained by the throughfall DOC concentration. DOC fluxes in different hydrological flux were significantly positive related to bulk precipitation amounts and temperature. Moreover, the throughfall DOC concentration had a considerable effect on throughfall and litter leachate DOC fluxes. Over 75% of annual net tree-DOC (throughfall + stemflow) fluxes and more than 70% of the annual litter leachate DOC fluxes were produced in the flowering season. Thus, we speculated that the seasonal phenological canopy changes (leaf emergence, fallen leaves, flowering, and pollen) and the sufficient rainfall had great impacts on the amount and quality of DOC concentrations in the evergreen forest; and, furthermore, that the DOC from different forest hydrological fluxes was a significant fraction of the carbon that accumulates in soils.

Seasonal variabilities in throughfall and stemflow dissolved organic carbon (DOC) and their driving factors in the evergreen broad-leaved forest are inadequately understood. To aid this understanding, we conducted a 3-year study to examine seasonal changes of DOC concentration and flux in throughfall and stemflow in an evergreen broad-leaved subtropical forest in central Japan. Throughfall and stemflow DOC concentration showed a clear seasonal change, with the highest throughfall DOC concentrations observed in spring (10.03 mg L −1 in 2017 and 9.59 mg L −1 in 2018, respectively) and the highest stemflow DOC concentrations observed in summer (13.95 mg L −1 in 2017 and 16.50 mg L −1 in 2018, respectively). Correlation analysis revealed the monthly throughfall DOC concentration to be positively related to the dry weight of fallen leaves ( r = 0.72, p < 0.05) and flowers ( r = 0.91, p < 0.05). In addition, Random Forest models predicted that the dry weight of flowers was a primary driver of throughfall DOC concentration and the stemflow DOC concentration was constrained by the throughfall DOC concentration. For DOC fluxes, the relationships between monthly precipitation amounts and DOC fluxes in throughfall and stemflow were significantly positive. Moreover, the throughfall DOC concentration had a considerable effect on its DOC fluxes, and over 75% of annual net tree-DOC (throughfall + stemflow) fluxes were from the flowering season. Thus, we speculated that phenological changes to the canopy (leaf emergency, fallen leaves, flowering, and pollen) had a great impact on the amount and quality of DOC concentrations, although there are no clearly demarcated leafed and leafless seasons in the evergreen forest.

Differences in dominant tree species affect ecosystem carbon budgets in forests, but their independent impacts have not been fully recognized. We aimed to clarify the interannual variation in net primary production (NPP) in three different forest types and the effects of dominant tree species on this. We calculated NPP excluding belowground litter fall using a biometric-based approach for 7 years (2012 - 2018) in three different forests in the same region dominated by deciduous oak (Quercus serrata), Japanese larch (Larix kaempferi) and Japanese red pine (Pinus densiflora). Aboveground litter fall (LF) and stand biomass increment (SI) were determined using the litter trap method and allometric relationships of tree biomass with diameter, respectively. Although inter-annual variation in LF seemed to be affected by the strong typhoons that passed through the study area, the sensitivity to the typhoons differed among the forest types: L. kaempferi and P. densiflora forests were more susceptible than the Q. serrata forest. Inter-annual variation in NPP (LF + SI) differed greatly among the three forests; this was primarily explained by the inter-annual variation in SI. Mean NPP excluding belowground litter fall for 7 years was 6.0 +/- 0.50, 5.2 +/- 0.44, and 6.7 +/- 0.40 MgC ha(-1) year(-1) in Q. serrata, L. kaempferi, and P. densiflora forests, respectively. Our results suggest that differences in dominant tree species have non-negligible effects on inter-annual variations in NPP and its components (SI and LF), even in the same region.

Chronic atmospheric nitrogen (N) deposition could influence the functioning of ecosystems as well as their biodiversity. However, N deposition in urban forest ecosystems, especially natural evergreen broad-leaved forests, is not well known. In this study, the concentrations and fluxes of dissolved inorganic N (DIN) and dissolved organic N (DON) in bulk deposition, throughfall, and stemflow were assessed in an urban evergreen broad-leaved forest site over three years, in order to clarify the characteristics of N deposition. At the study site, bulk DIN deposition was 3.7 kg N ha(-1) year(-1) (1.5 kg N ha(-1) yea(r-1) for NH4-N and 2.2 kg N ha(-1) year(-1) for NO3 + NO2-N), which is the same level as that found in rural areas. In contrast, 6.5 kg N ha(-1) year(-1) for bulk DON deposition contributed to 66% of the bulk N deposition, which suggests the importance of bulk DON deposition in Japanese forest ecosystems. Passing through the tree canopy, DIN was enriched by 8.8 kg N ha(-1) year(-1) (3.7 kg N ha(-1) year(-1) for NH4-N and 5.1 kg N ha(-1) year(-1) for NO3 + NO2-N) and DON was enriched by 1.5 kg N ha(-1) year(-1) as net throughfall in the evergreen broad-leaved forest. This reveals that dry deposition of DIN dominates the total DIN deposition onto the urban forest floor, compared to that found in the rural areas, due to the non-negligible N emissions from outside and possibly because of the evergreen broad-leaved forest's greater ability to capture N.

Amendment by biochar made by thermal degradation of biomass is expected to enhance carbon sequestration through stimulating carbon assimilation by plants. We clarified the effect of biochar amendment on the photosynthesis of trees in forest ecosystems. Biochar was applied to young oak trees (Quercus serrata) in temperate deciduous forest at rates of 0, 5, 10 and 20 Mg ha(-1) in four plots (C0, C5, C10, and C20). The variation in photosynthetic parameters (the maximum photosynthetic rate: P-max, maximum carboxylation rate: V-cmax and the potential rate of electron transport: J(max)) and leaf traits (the stomatal conductance: g(c), leaf mass per area (LMA) and leaf nutrient concentrations) were examined every month during the growing seasons for 3 years. P-max generally increased in C5 and C10 and did not increase in C20. Similarly, V-cmax and J(max) increased in C5 and C10 and correlated significantly positively with P-max, suggesting that biochar amendment basically increased the photosynthetic rate through improvements in physiological activities but that there was a maximum useful dosage. We also found that g(c), LMA and leaf nutrient (N, Mg, and S) showed significant positive correlations with P-max, indicating that an increase in photosynthetic rates would be supported by these leaf traits. However, stimulation of photosynthesis became smaller year by year, indicating that the effects of biochar amendment faded gradually. We concluded that biochar amendment basically improved the photosynthesis of oak trees in the forest through the change of all g(c), LMA and leaf nutrient concentrations but declined yearly.

Few studies have evaluated the application of biochar to forest ecosystems and their responses under field conditions. We manually spread grounded biochar on the forest floor, at rates of 0 (control), 5, and 10 Mg ha−1 (C0, C5 and C10, respectively), of an oak forest in central Japan to test the effects of biochar on tree growth and productivity. The relative growth rate of the diameter at breast height (dbh) of canopy oak trees (dbh > 20 cm) significantly increased in C10 compared with that of the control (C0), but not in C5, in the second to third years after application. Despite the increasing growth rate of canopy trees, foliage production (NPPF) and woody production (NPPW) did not respond to biochar application. Conversely, the production of reproductive organs (NPPR, mainly oak acorns) increased in line with the biochar application rate gradients (1.04 ± 0.09 Mg ha−1 yr−1 in C0, 1.30 ± 0.08 Mg ha−1 yr−1 in C5, and 1.47 ± 0.13 Mg ha−1 yr−1 in C10). Since the contribution of NPPR to total NPP was fairly small, there were no significant differences in total NPP (=NPPW + NPPF + NPPR) for C5 (14.57 ± 0.20 Mg ha−1 yr−1) or C10 (16.11 ± 0.73 Mg ha−1 yr−1) compared with the control (15.07 ± 0.48 Mg ha−1 yr−1).

Mangroves are increasingly recognized as an important component of regional and global carbon cycles especially for their high carbon storage capacity. Global estimation of mangrove soil organic carbon (SOC) storage requires detailed regional studies, but estimates of SOC data in deep soils are currently missing in many countries. Furthermore, little is explored on the molecular composition of mangrove SOC. Here, we assessed the SOC stock in a Trat mangrove forest (Thailand) by collecting deep soils (3.5 m) and analyzed the SOC composition for better understanding its potential sources and influencing factors. The Trat mangrove forest had four times higher SOC stock than has been considered for Thai mangrove forests, with the per-area SOC stock of nearly 1000 Mg C ha-1 which rivals that of Indo-Pacific mangrove forests. The SOC composition analyzed by C/N ratios and spectroscopic techniques differed by tree species and depth. Compositional data principal component analysis revealed that a biological factor (root abundance) had stronger influences than the soil texture (sand versus clay) on the abundance and composition of mangrove SOC. Although surface soil (~1 m) C density was largely controlled by the recent vegetation, deep soil C density reflected other historical processes. This study contributed to a refined estimate of Thailand mangrove SOC stock and revealed that factors influencing SOC abundance and composition differ by tree species and depth.