Four decades of data indicate that planted mangroves stored up to 75% of the carbon stocks found in intact mature stands.

Mangroves' ability to store carbon (C) has long been recognized, but little is known about whether planted mangroves can store C as efficiently as naturally established (i.e., intact) stands and in which time frame. Through Bayesian logistic models compiled from 40 years of data and built from 684 planted mangrove stands worldwide, we found that biomass C stock culminated at 71 to 73% to that of intact stands ~20 years after planting. Furthermore, prioritizing mixed-species planting including Rhizophora spp. would maximize C accumulation within the biomass compared to monospecific planting. Despite a 25% increase in the first 5 years following planting, no notable change was observed in the soil C stocks thereafter, which remains at a constant value of 75% to that of intact soil C stock, suggesting that planting effectively prevents further C losses due to land use change. These results have strong implications for mangrove restoration planning and serve as a baseline for future C buildup assessments.

Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes.

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m-2 d-1 in mangroves and 57 ± 104 mmol m-2 d-1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.

State of rare earth elements in the sediment and their bioaccumulation by mangroves: a case study in pristine islands of Indian Sundarban.

The mangrove ecosystems are known to efficiently sequester trace metals both in sediments and plant biomass. However, less is known about the chemistry of rare earth elements (REE) in the coastal environments, especially in the world's largest mangrove province, the Sundarban. Here, the concentration of REE in the sediment and plant organs of eight dominant mangrove species (mainly Avicennia sp.) in the Indian Sundarban was measured to assess REE sources, distribution, and bioaccumulation state. Results revealed that light REE (LREE) were more concentrated than the heavy REE (HREE) (128-144 mg kg-1 and 12-15 mg kg-1, respectively) in the mangrove sediments, with a relatively weak positive europium anomaly (Eu/Eu* = 1.03-1.14) with respect to North American shale composite. The primary source of REE was most likely linked to aluminosilicate weathering of crustal materials, and the resultant increase in LREE in the detritus. Vertical distribution of REE in one of the long cores from Lothian Island was altered by mangrove root activity and dependent on various physicochemical properties in the sediment (e.g., Eh, pH, organic carbon, and phosphate). REE uptake by plants was higher in the below-ground parts than in the above-ground plant tissues (root = 3.3 mg kg-1, leaf + wood = 1.7 mg kg-1); however, their total concentration was much lower than in the sediment (149.5 mg kg-1). Species-specific variability in bioaccumulation factor and translocation factor was observed indicating different REE partitioning and varying degree of mangrove uptake efficiency. Total REE stock in plant (above + live below ground) was estimated to be 168 g ha-1 with LREE contributing ~ 90% of the stock. This study highlighted the efficiency of using REE as a biological proxy in determining the degree of bioaccumulation within the mangrove environment.

Control of "blue carbon" storage by mangrove ageing: Evidence from a 66-year chronosequence in French Guiana.

The role of mangroves in the blue carbon stock is critical and requires special focus. Mangroves are carbon-rich forests that are not in steady-state equilibrium at the decadal time scale. Over the last decades, the structure and zonation of mangroves have been largely disturbed by coastal changes and land use conversions. The amount of time since the last disturbance is a key parameter determining forest structure, but it has so far been overlooked in mangrove carbon stock projections. In particular, the carbon sequestration rates among mangrove successional ages after (re)establishment are poorly quantified and not used in large-scale estimations of the blue carbon stock. Here, it is hypothesized that ecosystem age structure significantly modulates mangrove carbon stocks. We analysed a 66-year chronosequence of the aboveground and belowground biomass and soil carbon stock of mangroves in French Guiana, and we found that in the year after forest establishment on newly formed mud banks, the aboveground, belowground and soil carbon stocks averaged 23.56 ± 7.71, 13.04 ± 3.37 and 84.26 ± 64.14 (to a depth of 1 m) Mg C/ha, respectively. The mean annual increment (MAI) in the aboveground and belowground reservoirs was 23.56 × Age-0.52 and 13.20 × Age-0.64  Mg C ha-1  year-1 , respectively, and the MAI in the soil carbon reservoir was 3.00 ± 1.80 Mg C ha-1  year-1 . Our results show that the plant carbon sink capacity declines with ecosystem age, while the soil carbon sequestration rate remains constant over many years. We suggest that global projections of the above- and belowground reservoirs of the carbon stock need to account for mangrove age structures, which result from historical changes in coastal morphology. Our work anticipates joint international efforts to globally quantify the multidecadal mangrove carbon balance based on the combined use of age-based parametric equations and time series of mangrove age maps at regional scales.