Mangroves are among the most carbon-rich tropical ecosystems; however, the long-term dynamics of their carbon storage in Thailand remain poorly understood. This study integrates palaeoecological records, sedimentological variables, and soil carbon to evaluate the drivers that influence millennial-scale carbon accumulation in two estuarine mangrove ecosystems from two case study sites (Bang Khun Thian and Klong Kone) in the upper Gulf of Thailand. Across the study sites, total soil carbon (Soil C) storage, determined from accumulating the organic carbon density throughout each sediment profile, ranged from approximately 3300 to 3970 Mg C ha−1. The carbon accumulation rates (CAR) varied substantially, ranging from 185 to 1256 g C m−2 yr−1 over the last millennium. The results showed that long-term soil carbon storage was not controlled by a single factor but by interactions among dry bulk density, CAR, sediment texture, mangrove occurrence, and past environmental conditions. Soil C was more closely linked to mangrove presence and fine-grained sediment retention, whereas CAR was more sensitive to hydrological connectivity. Palaeoecological records indicate that estuarine geomorphology was strongly influenced by sea-level change, sediment inputs from both allochthonous and autochthonous sources, and mangrove vegetation cover. During periods of rising sea levels and wet climatic conditions, mangrove expansion promoted high sediment accretion and carbon burial, whereas periods of marine regression led to mangrove retreat. Reduced tidal influence and land-use changes have lowered carbon input and storage. These patterns reflect the combined influences of sediment supply, hydrological connectivity, and vegetation change. Gathering data from deeper soil layers and conducting site-specific factor analyses are crucial for a more precise and comprehensive evaluation of coastal carbon storage. Therefore, long-term data are critical for designing climate adaptation policies that safeguard mangrove persistence and maximise the role of blue carbon in climate change mitigation.

In the UK, ~420,000 hectares of agricultural land is situated upon lowland peat, representing around 2.5 % of the total agricultural land area. These sites disproportionately contribute towards greenhouse gas emissions, accounting for ~3% of the UK’s annual reported CO₂-equivalent emissions. These emissions, in addition to rapid soil erosion in some lowland peatlands, highlight the need to implement sustainable land management practices that promote soil stability, enhance carbon retention, and reduce greenhouse gas emissions whilst supporting agricultural productivity in agriculturally managed lowland peatland. Few studies have explored the trade-offs between the environmental benefits of sustainable land practices and their potential effects upon farm businesses, and few studies have explored how such practices influence the biogeochemical processes driving greenhouse gas emissions, nutrient cycling, and soil carbon stability in these systems. To address these uncertainties, we are conducting a field-scale study at a farm in Tarleton, UK, formed of two adjacent fields: one managed under a ‘business-as-usual’ regime and the other undergoing rewetting. Here, we present an overview of our study and some preliminary results. Within each field, we will test the viability of various commercially available soil amendments within experimental plots. Over two growing seasons, we will monitor greenhouse gas fluxes (CO₂, CH₄, N₂O), soil and pore-water biogeochemistry and crop yields across the treatment plots. This will allow us to compare the environmental and economic outcomes of each treatment and to identify the biogeochemical processes underlying any observed changes. Our findings will inform UK land-use policy, offering evidence-based recommendations for reducing emissions from agricultural lowland peat whilst upholding soil integrity and food security. Our findings will also enhance our understanding of how different management changes affect biogeochemical processes within peatland soils. This study forms part of the Lowland Peat 3 Project, which will assess the environmental, economic, and social trade-offs of agricultural practices on lowland peatlands across the UK.

Saltmarshes are widely thought to sequester carbon at rates significantly exceeding those found in terrestrial environments. This ability arises from the in-situ production of plant biomass and the effective trapping and storage of both autochthonous and allochthonous organic carbon. The role saltmarshes play in climate change mitigation, through accumulating ‘blue’ carbon, depends on both the rate at which carbon accumulates within sediments and the rapidity with which carbon is remineralised. It has been hypothesised that carbon accumulation rates, in turn, depend on the local rate of relative sea-level rise, with faster sea-level rise providing more accommodation space for carbon storage. This relationship has been investigated over long (millennial) and short (decadal) timescales but without accounting for the impact of higher quantities of labile carbon in more recently deposited sediment. This study addresses these three key aspects in a saltmarsh sediment study from Lindisfarne National Nature Reserve (NNR), northern England, where there is a comparatively pristine marsh. We quantify rates of carbon accumulation by combining a Bayesian age-depth model based on 210Pb and 137Cs activities with centimetre-resolution organic carbon density measurements. We also use thermogravimetric analyses to determine the relative proportions of labile and recalcitrant organic matter and calculate the net recalcitrant organic matter accumulation rate. Results indicate that during the 20th century more carbon accumulated at the Lindisfarne NNR saltmarsh during decades with relatively high rates of sea-level rise. The post-depositional loss of labile carbon down the core results in a weaker though still significant relationship between recalcitrant organic matter accumulation and sea-level change. Thus, that increasing saltmarsh carbon accumulation is driven by higher rates of sea-level rise is demonstrated over recent multi-decadal timescales.

The near-coast environments where oysters occur are among the most impacted by humans globally, especially during the Late-Holocene. Yet, in West Africa, there is no documented historical record of change in these environments. We provide insight into the changing geochemical conditions of two oyster environments through high-resolution analysis of total organic carbon (C), total nitrogen (N), carbon and nitrogen isotope ratios (δ 13 C, δ 15 N), and trace elements, in two cores retrieved from the Densu estuary and the Anyanui (Keta) Creek in Ghana. Drastic shifts in sedimentation rate occurred in the Keta and Densu cores around 1996 CE and 960 CE respectively. At these times, comparatively, low levels of C and N were found in the Densu core. Increasing C and N levels and decreasing δ 13 C upcore aligned with the observed shift in sedimentation rate in the Keta core. The C/N ratios in the Keta core suggest allochthonous organic matter (OM) dominance in the creek. The Densu core showed periodic changes in C/N ratios from very high values (>20) between 1918 BCE and 1321 BCE, to values between 20 and 11 between 1321 BCE and 1977 CE and below 10 from the late 1970s CE to the present day, suggesting a varying degree of transformation in the catchment basin. Extremely high Sulfur (S) and moderate to significant Iron (Fe) increases suggest reducing conditions in the Keta sediments. Moderate Calcium (Ca), Zinc (Zn), and Strontium (Sr) concentrations in the upper part of the Densu core suggest a stronger influence of marine processes in the Densu in recent times. The findings reflect the impacts of catchment basin modification on the health of the two coastal environments, likely to impact the growth, productivity, and sustainability of the fishery of the West African mangrove Oyster.

<p>Salt marshes sequester carbon at rates significantly exceeding those found in terrestrial environments. This ability arises from the in-situ production of plant biomass and the effective trapping and storage of both autochthonous and allochthonous organic carbon. The importance of this blue carbon store for mitigating increasing atmospheric carbon dioxide depends on both the rate at which carbon is buried within sediments and the rapidity with which that carbon is remineralised. It has been hypothesized that carbon burial rates, in turn, depend on the local rate of sea-level rise, with faster sea-level rise providing more accommodation space for carbon storage. This study addresses these three key aspects in a salt-marsh sediment study from Lindisfarne, northern England. We quantify rates of carbon accumulation by combining a Bayesian age-depth model based on <sup>210</sup>Pb and <sup>137</sup>Cs activities with centimetre-resolution organic carbon density measurements. A Bayesian isotope mixing model pinpoints terrestrial sources as providing the majority of stored carbon. We compare two approaches for assessing the relative proportions of labile and recalcitrant carbon based on a two-pool modelling approach and thermogravimetric analysis. Preliminary results indicate that during the 20<sup>th</sup> century more carbon was stored at Lindisfarne salt marsh during decades with relatively high rates of sea-level rise.</p>

Peatlands are important sinks and stores of terrestrial carbon, but their future role in the global climate system under climate change is uncertain. Palaeoecological and experimental climate manipulation studies are commonly used to address these uncertainties, although their results often contradict each other. Furthermore, carbon accumulation rates estimated from both methods frequently fail to converge.We examine the effects of warming and episodic drought on carbon cycling from a long-term experimental site in an ombrotrophic peatland in Wales, simulating expected changes in temperature and drought frequency for the next c.70 years. We compare carbon accumulation rates modelled from the 10-year experiment (2010-2020) with those derived from a peat core covering the past c. 1500 years. We partition components of the annual carbon budget for each experimental treatment and compare them with core-derived carbon accumulation rates.Significant differences in carbon accumulation occurred between experimental treatments, resulting from cumulative non-significant changes in decay rates and productivity, driven by differences in air temperature and water table depth. We identify a positive time-lagged relationship between carbon accumulation and reconstructed temperature anomalies in the long-term carbon accumulation record. Changes in carbon accumulation also correspond to vegetation changes. We suggest that palaeoecological studies should not consider changes in plant productivity to be the sole determinant controlling carbon accumulation rates and should acknowledge the role of changing decay rates following plant succession upon long-term carbon accumulation. Carbon accumulation rates estimated from both methods may converge, but often differ due to unquantifiable losses such as by reduced productivity, decay, or anthropogenic disturbances like fire. We conclude that both methods are useful for discerning future peatland responses to climate change; however, core derived rates should be interpreted with consideration of all the contemporary mechanisms governing carbon accumulation. Based on these findings, we posit that the carbon sink function of existing peatlands will be weakened by increased warming and drought.

Abstract Testate amoebae are a frequently used palaeoecological proxy for reconstructing changes in palaeohydrological conditions, particularly in studies of Sphagnum-dominated peatlands. Their use in palaeoecological studies has increased following the development of transfer functions, allowing for the quantitative reconstruction of water-table depth changes through time. Increasingly, they are included in non-pollen palynomorph (NPP) studies alongside a wide range of other proxies, representing a valuable tool, particularly in multi-proxy studies. Testate amoebae have been used for qualitative assessment of palaeohydrology in NPP studies and may aid the verification of environmental interpretations of conditions inferred from curves of NPP with unknown ecology and taxonomy. Their usefulness in such studies is limited by the destruction of tests owing to harsh chemical treatments used in pollen preparation methods. This makes community distribution data of testate amoebae derived by these methods largely unsuitable for quantitative assessment of water-table depth. Furthermore, many palynological studies combine testate amoebae as one single curve, losing further ecological detail. Patterns of change of surviving species, most commonly of Assulina, Archerella, Arcella, Hyalosphenia and Archerella flavum, remain relatively unaffected and therefore can still be useful for interpreting qualitative changes in hydrological conditions through time, particularly when coupled with other proxies.

We test whether vegetation community composition from a 10-year climate manipulation experiment on a Welsh peat bog resembles vegetation communities during periods of climate change inferred from a peat core. Experimentally warmed and combined warmed and droughted treatments drove significant increases in ericaceous shrubs but Sphagnum was unaffected. Similarly, Calluna vulgaris seeds increase during inferred warmer periods in the palaeoecological record. Experimental short-term episodic drought (four 4-week drought treatments) did not affect vegetation. Plant community composition has undergone several abrupt changes throughout the past c. 1500 years, often in response to human disturbance. Only slight changes occurred during the Medieval Climate Anomaly (c. 950-1250 Common Era [CH) in vegetation and hydrology, while abrupt changes occurred during the Little Ice Age (c. 1300-1850 CE) when water tables were highest, suggesting that these shifts were driven by changes in water table, modulated by climate. A period of water table drawdown c. 1800, synchronous with historical records of increased drainage, corresponds with the development of the present-day vegetation community. Modern analogues for fossil material, characterized by abundant Rhynchospora alba and Sphagnum pulchrum, are more common after this event. Vegetation changes due to climate inferred from the palaeo record differ from those observed in the experiments, possibly relating to differences in the importance of drivers of vegetation change over varying timescales. Whereas temperature is frequently identified as the dominant driver of plant community change in experiments, sustained changes in water table appear to be more important in the long-term record. We find evidence that recent climate change and other anthropogenic stressors (e.g. drainage, heavy metal and nitrogen pollution) may promote the development of novel plant communities without analogues in the fossil record. These communities may be poorer at sequestering carbon and may respond differently to future climate change.

Data for study measuring the effects of warming and drought upon greenhouse gas fluxes and annual carbon budgets from a long term field experiment on a raised peat bog in Wales, compared with long term carbon accumulation rates inferred from a peat core from the same site (Cors Fochno, Wales). Contents: 1 Cors Fochno measured gas fluxes from 2010 - 20202 Cors Fochno pore-water DOC concentrations from 2017 - 2020 3 Carbon content, bulk peat density and basal dates for LORCA calculation4 210Pb, 14C and other chronohorizon data for core BO175 Age-depth model and carbon accumulation rates for core BO176 Annual modelled gas fluxes and carbon budgets7 Tephra geochemical data from core BO178 Time series modelled 6 hourly gas fluxes and measured environmental data (averaged across treatments)9 Dupont Hydrological Index calculation sheet for core BO17

The effects of 21st century climate change are projected to be most severe in the northern hemisphere, where the majority of peatlands are located. Peatlands represent important long-term terrestrial stores of carbon (C), containing an estimated c.600-1055GT C, despite covering only 3% of total land area globally. In addition, pristine peatlands act as net sinks of atmospheric CO2, imparting a negative feedback mechanism cooling global climate, whilst simultaneously acting as sources of CO2 and CH4. Peatlands remain net sinks of C as long as the rate of carbon sequestration exceeds that of decomposition. Projected changes in temperature, precipitation and other environmental variables threaten to disrupt this precarious balance, however, and the future direction of carbon feedback mechanisms are poorly understood, due to the complex nature of the peatland carbon cycle. Two methods are used in order to help understand future the carbon dynamics of peat bogs under climate change. These are experimental studies, which measure greenhouse gas fluxes under manipulated climatic and environmental conditions (warmer, drier), and palaeoecological studies, which examine the effects of past climate change upon carbon sequestration throughout the peat profile. However, both methods fundamentally contradict each other. Palaeoecological studies suggest that carbon accumulation increases during warming periods, whereas warming experiments observe greater carbon loss with increased temperature. The aim of this project is to link contemporary experimental and palaeoecological approaches to explain this discrepancy. This will be achieved by comparing greenhouse gas fluxes between plots which have been subjected to 10 years of passive warming and drought simulation at an experimental climate manipulation site on Cors Fochno, Ceredigion, Wales. Long term rates of carbon accumulation will be compared with net ecosystem contemporary carbon budgets from each plot. Surface samples from each plot will be analysed by a range of palaeoenvironmental proxies to test how well the climate manipulations are represented by each proxy. Finally, a high-resolution multi-proxy palaeoenvironmental reconstruction spanning the past 1000 years will be compared with reconstructions derived from short-cores from each plot covering the duration of the experiment from each treatment, to see how faithfully climate manipulation mirrors real periods of climate change. Understanding the future role of peatlands in future carbon sequestration and storage is of vital importance for modelling future climate change, in terms of both quantifying the potential ecosystem services peatlands may offer in mitigating the effects of climate change, as well as enhancing the predictive capabilities of global climate models. Currently, the uncertainty associated with peatland carbon cycling is such that peatlands are rarely included in global climate models.