The significance of aquatic lateral carbon (C) export in mangrove ecosystems highlights the extensive contribution of aquatic pathways to the net ecosystem carbon budget. However, few studies have investigated lateral fluxes of dissolved organic carbon (DOC) and inorganic carbon (DIC), partly due to methodological difficulty. Therefore, we evaluated area-based lateral C fluxes in a small mangrove estuary that only had one exit for water exchange to the coast. We sampled water from the mouth of the creek and integrated discharge and consecutive concentration of mangrove-derived C (ΔC). Then, we estimated the area-normalized C fluxes based on the inundated mangrove area. DIC and DOC concentrations at the river mouth increased during ebb tide during both summer and winter. We quantified the ΔC in the estuary using a two-component conservative mixing model of freshwater and seawater. DIC and DOC proportions of ΔC concentrations at the river mouth during ebb tide was between 34% and 56% in the winter and 26% and 42% in the summer, respectively. DIC and DOC fluxes from the estuary were estimated to be 1.36 g C m⁻² d⁻¹ and 0.20 g C m⁻² d⁻¹ in the winter and 3.35 g C m⁻² d⁻¹ and 0.86 g C m⁻² d⁻¹ in the summer, respectively. Based on our method, daily fluxes are mangrove area-based DIC and DOC lateral exports that can be directly incorporated into the mangrove carbon budget.

Mangrove ecosystems play an important role in carbon (C) accumulation in tropical and subtropical regions. Below-ground deep anoxic soil is especially important for C accumulation. However, quantitative data on below-ground soil C stocks in mangrove ecosystems are lacking compared with data on above-ground biomass. In addition, soil C accumulation processes in mangrove ecosystems have not been sufficiently clarified. In this study, we quantified soil C stocks and focused on the mass of fallen litter and below-ground roots, which are produced by tree and that may directly influence soil C stocks in a mature subtropical mangrove in the estuary of Fukido River, Ishigaki Island, southwestern Japan. The principal species in this study site were Bruguiera gymnorhiza and Rhizophora stylosa, and total above-ground biomass at the site was 80.7 ± 1.3 (mean ± SD) Mg C ha⁻¹ over the period from 2014 to 2016. Litter was collected in six litter traps from May 2013 to November 2016, it ranged from 7.8 to 11.5 Mg C ha⁻¹, with the major proportion of litter being from foliage (leaves and stipules). The root C density at 90-cm depth was 27.1 ± 11.3 Mg C ha⁻¹. The soil C stock in the mangrove forest at a depth of 90 cm at the study site was 251.0 ± 34.8 Mg C ha⁻¹, and it seems to be lower value in the tropical region but it to be higher in subtropical East Asian mangrove sites. Dead roots, especially dead fine roots, but not fallen litter, were significantly positively correlated with soil C stocks. The δ¹³C values obtained from soils ranged from −29.3‰ to −27.0‰; these values are consistent with those for below-ground fine roots. These results strongly suggest that dead fine roots could be a main factor controlling soil C stocks at this study site.

Mangrove forests are increasingly recognized as vital blue carbon ecosystems due to their high carbon sequestration capacity, primarily through the accumulation of soil organic carbon (SOC). Recent research highlights that, in addition to SOC, dissolved inorganic carbon (DIC), particularly in the form of bicarbonate (HCO₃⁻), plays a crucial role in carbon sequestration by being exported from these ecosystems to adjacent coastal waters. This study aims to investigate the previously unexamined mechanisms behind bicarbonate production in mangrove soils. We test the hypothesis that root exudates, specifically sugars and organic acids, induce a priming effect that enhances DIC production. We conducted a month-long incubation experiment using soil samples from an estuarine mangrove forest in Japan, under varying salinity levels and root exudate treatments. During the incubation period, the total amounts of DIC, HCO₃⁻, and carbonic acid (H₂CO₃) detected per vial in each treatment ranged from 30.0 to 43.9 mg C, 26.6 to 39.7 mg C, and 1.9 to 4.4 mg C, respectively, with HCO₃⁻ accounting for approximately 90% of the DIC. These values tended to be higher under high salinity conditions resembling seawater levels. In particular, the treatment with a complex solution of root exudates and high salinity (h-ASW + c-REC) showed the highest average values for all forms of DIC. Furthermore, in the h-ASW + c-REC treatment, HCO₃⁻ increased significantly (P < 0.005) by approximately 20% compared to the control without added root exudates. Additionally, changes in water quality parameters—including increased NH₄⁺ levels and decreased dissolved oxygen and oxidation-reduction potential—provided further support for the priming effect hypothesis. This study is the first to demonstrate that root exudates promote bicarbonate production in mangrove soils, offering new insights into the complex carbon and nutrient dynamics in these ecosystems.

The structural complexity, especially canopy and gap structure, of old‐growth forests affects the spatial variation of soil respiration (Rs). Without considering this variation, the upscaling of Rs from field measurements to the forest site will be biased. The present study examined responses of Rs to soil temperature (Ts) and water content (W) in canopy and gap areas, developed the best fit modelof Rs and used the unique spatial patterns of Rs and crown closure to upscale chamber measurements to the site scale in an old‐growth beech‐oak forest. Rs increased with an increase in Ts in both gap and canopy areas, but the effect of W on Rs was different between the two areas. The generalized linear model (GLM) analysis identified that an empirical model of Rs with thecoupling of Ts and W was better than an exponential model of Rs with only Ts. Moreover, because of different responses of Rs to W between canopy and gap areas, it was necessary to estimate Rs in these areas separately. Consequently, combining the spatial patterns of Rs and the crown closure could allow upscaling of Rs from chamber‐based measurements to the whole site in the present study.

The response of soil organic matter (SOM) to global warming is a crucial subject. However, the temperature sensitivity of SOM turnover remains largely uncertain. Changes in the mineralization of native SOM, i.e., priming effect (PE) may strongly affect the temperature sensitivity of SOM turnover in the presence of global warming. We investigated the direction and magnitude of the PE in a Japanese volcanic ash soil at different temperatures (15°C, 25°C, and 35°C) using a natural ¹³C tracer (C4-plant, maize leaf) in a short-term (25 days) incubation study. In addition, we evaluated the temperature sensitivity expressed as Q₁₀ value with and without the addition of maize to the soil and their relations to PE. We found that positive PE occurred at each temperature condition and tended to increase with decreased temperature, and these PE results were consistent with the microbial biomass at the end of the incubation period. CO₂ emission from control soil (without maize) increased with increasing temperature (Q₁₀ = 2.6), but CO₂ emission from the soil with added maize did not significantly change with increasing temperature (Q₁₀ = 1.0). This was caused by the suppression of CO₂ emission from the soil with increasing temperature (Q₁₀ = 0.9). On the other hand, soil-originated CO₂ emission clearly increased with increasing temperature (Q₁₀ = 3.4) when Q₁₀ values were calculated on the assumption that the temperature and substrate supply increase at the same time (from 25°C). These results suggest that not only the temperature increase but also the labile carbon supply may be important for the temperature sensitivity of Japanese volcanic ash soil.