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Despite the recognized organic carbon (OC) sequestration potential of mangrove forests, the ongoing climate change and anthropogenic disturbances pose a great threat to these ecosystems. However, we currently lack the ability to mechanically understand and predict the consequences of such impacts, primarily because mechanisms underlying OC stabilization in these ecosystems remain elusive. Research into OC stabilization has focused on terrestrial soils and marine sediments for decades, overlooking the vegetated coastal ecosystems including mangroves. In terrestrial soils and marine sediments, it is widely accepted that OC stabilization is the integrated consequence of OM’s inherent recalcitrance, physical protection, and interactions with minerals and metals. However, related discussion is rarely done in mangrove soils, and recalcitrance of roots and high net ecosystem production (high primary production and low heterotrophic respiration) have been considered as a primary OC sequestration mechanism in mangrove peat and mineral soils, respectively. This review presents the available information on the mechanisms underlying OC stabilization in mangrove soils and highlights research questions that warrant further investigation. Primary OC stabilization mechanisms differ between mangrove peat and mineral soils. In mangrove mineral soils, physico-chemical stabilization processes are important, yet grossly understudied OC stabilization mechanisms. In mangrove peat, recalcitrance of mangrove roots and the inhibition of phenoloxidase under the anoxic condition may be the primary OC stabilization mechanisms. Salinity-induced OC immobilization likely plays a role in both type of soils. Finally, this review argues that belowground production and allochthonous inputs in mangrove forests are likely underestimated. More studies are needed to constrain C budgets to explain the enigma that mangrove OC keeps accumulating despite much higher decomposition (especially by large lateral exports) than previously considered.

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 CO 2 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.

To clarify the effects of crab burrows on variation in sediment CO₂ flux in mangrove forest, we measured the traits of crab burrows (density and entrance area size) and the CO₂ flux rate from sediment surfaces, in areas with and without burrows, in a subtropical mangrove forest on Ishigaki Island, southwestern Japan. Burrow density and entrance area showed significant differences among seasons (warm, middle, and cool) and mangrove zones (upper-, middle-, and downstream), which may have depended on crab phenology, life cycle, and species composition. The sediment CO₂ flux rate was significantly higher at plots with crab burrows (B+) than at those without burrows (B-) in each zone and season. However, standardized sediment CO₂ flux rate by burrow surface area at B+ plots did not differ significantly from that at B- plots. In addition, there were no significant differences in sediment temperature and sediment water content between the two types of plots. Moreover, the level of microbial respiration differed significantly between sediments collected from the deep part and those collected from either the ground surface part or burrow walls. These results suggest that crab burrows increase sediment CO₂ flux from the mangrove forest floor by increasing the sediment–atmosphere interface area, thereby inducing a change to aerobic conditions in the sediments around burrows. Therefore, the seasonal and spatial effect of crab burrows on the forest floor should be considered when evaluating sediment CO₂ flux and examining the role of the mangrove ecosystem as a carbon sink.

The Selenga River contributes to 50% of the total inflow to Lake Baikal. Large tracts of the Selenga River Basin have been developed for industry, urbanization, mining, and agriculture, resulting in the release of suspended solids (SS) that affect downstream water quality and primary productivity. This study addressed SS as the main factor controlling pollutant transport and the primary indicator of land degradation in the Selenga River system. Tributaries with larger areas dedicated to agricultural use had higher SS concentrations, reaching 862 mg L −1 , especially during the high runoff and intensive cultivation season. Although the large SS flux was detected in the main river, the small tributaries were distinguished by high SS concentrations. The high SS concentration corresponded to widespread development in the watershed. Watersheds with high potential of SS release are sensitive to intensive land uses. SS in the river system had a constant elemental composition consisting mainly of Fe and Al oxides, indicating that surface soils were major constituents of the tributary SS. Three minor heavy metals (Zn, Cu, and Cr) appeared in high concentrations downstream of urban and mining areas (two- to sixfold increases), indicating that these contaminants are carried by SS. At two tributary junctions, the concentration of contaminants on the SS decreased due to a large influx of SS with low heavy metal contents. Changes in electric conductivity and pH at downstream of tributary junctions enhanced the sedimentation of SS and the removal of contaminants from the water phase after aggregation of the SS. Land use changes in the tributary watersheds are major controlling factors for the fate of contaminants in the river system.

Humic substances (HS) are the primary constituents of dissolved organic matter (DOM) and play pivotal roles in aquatic systems. Optical indices of DOM, such as specific UV absorbance (SUVA254), the fluorescence index (FI) and biological index (BIX), have gained wide interest because of their ease of use. In this study, we explored the relationship between HS and the indices in the Trat River Basin (eastern Thailand) from headwaters to the river mouth through the distinct dry and rainy seasons to examine whether changes in index values reflect variability in the relative contribution of HS to DOM, or %HS. The results show that %HS and the indices did not exhibit significant linear relationships (FI and BIX, P > 0.05), or the relationships changed seasonally (SUVA254). However, analyzing the indices versus %HS did show clear DOM composition changes by season with more humic-like or terrestrial material in the rainy season. Relationships between DOM and dissolved iron (dFe) concentrations were also explored. Separating the relationships of DOM versus dFe into HS versus dFe and non-HS versus dFe provides us the opportunity to better understand which fraction contributes more to dFe mobilization. The results indicate stronger positive linear relationships between HS and dFe concentrations independent of river tributary. Overall, this study highlights the importance of quantifying HS for the study of DOM dynamics or compositional changes along a river transect as well as for DOM-induced iron mobilization.

The variation in CO2 flux from the forest floor is important in understanding the role of mangrove forests as a carbon sink. To clarify the effects of soil temperature and tidal conditions on variation in CO2 flux, sediment–atmosphere CO2 fluxes were measured between June 2012 and May 2013. We used the closed chamber method for two plots, with a 0.5 m difference in elevation (B, high elevation; R-B, low elevation), in a mangrove forest in south-western Japan. CO2 fluxes were highest in the warm season and showed a weak positive correlation with soil temperature in both forests. Estimated monthly CO2 flux showed moderate seasonal variation in accordance with the exposure duration of the soil surface under tidal fluctuation. Additionally, measured CO2 flux and soil temperature were slightly higher in the R-B plot than the B plot, although estimated annual CO2 flux was higher in the B plot than the R-B plot due to different exposure durations. These results suggest that variation in the exposure duration of the forest floor, which changes seasonally and microgeographically, is important in evaluating the annual CO2 flux at a local scale and understanding the role of mangrove ecosystems as regulators of atmospheric CO2.

The organic-Fe association in Scottish freshwater rivers has received little attention compared with in the estuarine mixing zone. We collected 201 water samples from rivers and lakes in Scotland across different sampling years and seasons. Relationships among the hydrophobic (HPO) fraction of dissolved organic matter (DOM), specific UV absorbance (SUVA254), and dissolved metals (Al and Fe) were examined to better understand their co-transportation in Scottish waters. The average DOM, HPO fraction, Fe, and Al concentrations for all the samples co-varied and were lower during winter than during summer. There was a strong positive correlation between DOM and HPO fraction concentrations (R2 = 0.99, p < 0.0001). A significant positive correlation was also found between the HPO fraction and Fe and Al concentrations. The regression slope indicating the overall relationships between the HPO fraction and Fe concentrations differed by as much as 12 times depending on both the sampling period and the river. These slope differences were not significantly determined by the chemical structures of DOM, SUVA254, or Al and Cu concentrations. These results suggest that the Fe transport capacities vary among the Scottish rivers because of other factors such as seasonal effects (temperature and the level of water table) and a suspended solid concentration in the water column.

An anion exchange resin, diethylaminoethyl (DEAE) Sepharose®, was utilized for the isolation of dissolved organic matter (DOM) from fresh waters as an alternative to the discontinued DEAE cellulose. We used the following two chemically distinct model DOM samples to determine the optimized adsorption conditions onto DEAE Sepharose: the International Humic Substances Society’s standard samples, Suwannee River Fulvic Acid (FA) and Pony Lake FA. The optimized conditions consisted of the following: a contact time of 1 h (with shaking), a resin dosage of 1 ml mgC−1, and a dissolved organic carbon (DOC) concentration range of 1–100 mgC l−1. In addition, we examined the distribution of the DOM fractions extracted with DEAE Sepharose and DAX-8 from Lake Biwa (Japan) and Scottish river DOM samples. The majority of DOM (70% and 65%) was extracted by both of the resins. As indicated by 1H NMR, the evapo-concentrate (bulk DOM), the DEAE Sepharose fraction and the DAX-8 fraction from the Scottish DOM sample had substantial similarity in their proton distributions, while those of a clear-colored, low DOC sample (Lake Biwa) showed different NMR spectra. These findings highlight a need to pay more attention to the extraction selectivity of resins for experimentally ‘challenging’ samples.

Aquatic humic substances (AHSs) are major constituents of dissolved organic matter (DOM) in freshwater bodies. We performed quantitative analyses of AHSs in Japanese lake and wetland waters, focusing mainly on clear waters with low carbon contents, by using a resin isolation technique based on the carbon concentration in the AHSs of each sample. The proportion of AHS to DOM in the clear waters ranged from 38.4 to 64.1 %; these proportions are lower than those widely assumed for freshwater of 20–80 %. Moreover, the proportions of AHSs in DOM were not constant, so regression analysis cannot be used to predict the AHS concentration from the DOM concentration. Thus, AHS and DOM concentrations must be determined separately for each water sample.

Despite the recognized organic carbon (OC) sequestration potential of mangrove forests, the ongoing climate change and anthropogenic disturbances pose a great threat to these ecosystems. However, we currently lack the ability to mechanically understand and predict the consequences of such impacts, primarily because mechanisms underlying OC stabilization in these ecosystems remain elusive. Research into OC stabilization has focused on terrestrial soils and marine sediments for decades, overlooking the vegetated coastal ecosystems including mangroves. In terrestrial soils and marine sediments, it is widely accepted that OC stabilization is the integrated consequence of OM's inherent recalcitrance, physical protection, and interactions with minerals and metals. However, related discussion is rarely done in mangrove soils, and recalcitrance of roots and high net ecosystem production (high primary production and low heterotrophic respiration) have been considered as a primary OC sequestration mechanism in mangrove peat and mineral soils, respectively. This review presents the available information on the mechanisms underlying OC stabilization in mangrove soils and highlights research questions that warrant further investigation. Primary OC stabilization mechanisms differ between mangrove peat and mineral soils. In mangrove mineral soils, physico-chemical stabilization processes are important, yet grossly understudied OC stabilization mechanisms. In mangrove peat, recalcitrance of mangrove roots and the inhibition of phenoloxidase under the anoxic condition may be the primary OC stabilization mechanisms. Salinity-induced OC immobilization likely plays a role in both type of soils. Finally, this review argues that belowground production and allochthonous inputs in mangrove forests are likely underestimated. More studies are needed to constrain C budgets to explain the enigma that mangrove OC keeps accumulating despite much higher decomposition (especially by large lateral exports) than previously considered.