Sediment Organic Carbon Sequestration Research Articles

Source and sequestration of sediment organic carbon from different elevation zones in estuarine wetland, North China

Estuarine wetland plays an important role in regulating global carbon cycle due to high terrestrial carbon input and burial. However, it is unclear how the source and sequestration of sediment organic carbon (SOC) in estuarine wetlands changes under the anthropogenic impact in the past century. In this study, combining parameters of TOC/TN ratios, δ13C, δ15N and 210Pb-chronology, temporal trends of SOC source and sequestration flux in Liaohe estuarine wetland were studied. The results showed that the source of organic carbon in Liaohe estuarine wetland was dominated by terrestrial input (contribution >60 %). Due to vegetation, TOC in shallow reed marsh was significantly higher than that of bare beach and subtidal flat. Affected by elevation, the sediment mass accumulation rate (MAR, kg·m−2·yr−1) showed differences in reed marsh (C1), bare beach (C2) and subtidal flat (C3), which were 6.57, 13.56 and 13.25 respectively in the past century. MAR fluctuated over time, it showed an overall increasing trend, especially since the 1980s. Correspondingly, the sequestration flux of SOC (SF-SOC, g·m−2·yr−1) showed an overall increasing trend with average of 82.84 (reed marsh), 151.93 (bare beach) and 123.71 (subtidal flat). Comparing to TOC, the higher MAR had a more distinct effect on carbon sequestration in Liaohe estuarine wetland. The difference in sedimentation rate and carbon sequestration are linked to the changes in sediment flux of riverine input and land utilization in the catchment area due to human activities in recent decades, including the construction of reservoirs, dams and local ditch wharf.

Seasonal flooding wetland expansion would strongly affect soil and sediment organic carbon storage and carbon-nutrient stoichiometry

In the past few decades, many non-flooding uplands (NF) and permanent flooding waters (PF) have been turned into seasonal flooding wetlands (SF) at the global scale. This trend could severely threaten global climate system by changing carbon cycling in terrestrial and aquatic ecosystems. However, the effects of SF expansion on soil and sediment organic carbon (SOC) storage and carbon-nutrient stoichiometry are far from clearly understood. Therefore, we explored SOC storage and carbon-nutrient stoichiometry among adjacent NF, SF and PF using 817 samples at 0–100 cm depth increment at Poyang Lake and Shengjin Lake in the middle-lower Yangtze River floodplain, China. The SFs of the two lakes were both Carex lakeshore wetlands. The NF of Shengjin Lake was a near-natural forest, while the NF of Poyang Lake was a disturbed grassland. The results showed that SOC storage at SFs of Poyang Lake and Shengjin Lake was 75.61 and 98.01 Mg C/ha at 0–100 cm depth increment. The difference in SOC storage among nearby NF, SF and PF depended on depth and disturbance. SOC storage at SF was equivalent to that at near-natural NF, but was much higher than that at disturbed NF. SOC storage at SF was 12.62%–24.50% higher than that at PF at 0–30 cm depth increment, but was 15.16%–25.87% lower than that at PF at 0–100 cm depth increment. Edaphic carbon and nutrients followed allometric relationships at most sites and C increased faster than N and P along the depth gradients. Carbon-nutrient stoichiometric relationships at SF and PF were similar, and were more coupled than those at near-natural NF. This research illustrates the strong effects of seasonal flooding on SOC sequestration in terrestrial and aquatic ecosystems, and expands our understanding of carbon cycling in these two ecosystems.

Sediment Organic Carbon Sequestration of Balkhash Lake in Central Asia

As an important part of the global carbon pool, lake carbon is of great significance in the global carbon cycle. Based on a study of the sedimentary proxies of Balkhash Lake, Central Asia’s largest lake, changes in the organic carbon sequestration in the lake sediments and their possible influence over the past 150 years were studied. The results suggested that the organic carbon in the sediments of Lake Balkhash comes mainly from aquatic plants. The organic carbon burial rate fluctuated from 8.16 to 30.04 g·m−2·a−1 and the minimum appeared at the top of the core. The organic carbon burial rate continues to decline as it has over the past 150 years. Global warming, higher hydrodynamic force, and low terrestrial input have not been conducive to the improvement of organic carbon sequestration in Balkhash Lake; the construction of a large reservoir had a greater impact on the sedimentary proxy of total organic carbon content, which could lead to a large deviation for environmental reconstruction. This is the first study to assess the sediment organic carbon sequestration using the modern sediments of Central Asia’s largest lake, which is of great scientific significance. The results contribute to an understanding of organic carbon sequestration in Central Asia and may provide a scientific basis for carbon balance assessment in regional and global scales.

Organic carbon sequestration in sediments of subtropical Florida lakes.

Recent studies have shown that sediments of temperate and tropical lakes are sinks for organic carbon (OC), but little is known about OC burial in subtropical lakes. There are questions regarding the ability of subtropical lakes to store OC, given their relatively warmwater temperatures, lack of ice cover, frequent water-column mixing, and labile carbon forms. We used 210Pb-dated sediment cores from 11 shallow Florida (USA) lakes to estimate OC burial, i.e. net OC storage, over the last ~100 years. Shallow Florida water bodies average ~30% OC content in their sediments and displayed rates of net OC accumulation (63–177 g C m-2 a-1) that are similar to natural temperate lakes, but lower than temperate agricultural impoundments. We considered the influence of lake morphometry on OC storage in our study lakes, but did not observe an inverse relationship between lake size and OC burial rate, as has been seen in some temperate lake districts. We did, however, find an inverse relation between mean water depth and OC sequestration. Despite recent cultural eutrophication and the associated shift from macrophyte to phytoplankton dominance in the Florida study lakes, overall OC burial rate increased relative to historic (pre-1950 AD) values. Lakes cover >9000 km2 of the Florida landscape, suggesting that OC burial in sediments amounts to as much as 1.6 Mt a-1. The high rate of OC burial in Florida lake sediments indicates that subtropical lakes are important for carbon sequestration and should be included in models of global carbon cycling.

Macroalgae bloom decay decreases the sediment organic carbon sequestration potential in tropical seagrass meadows of the South China Sea

Seagrass meadows are experiencing worldwide declines mainly because of nutrient enrichment, which always result in macroalgae bloom and consequently periodic collapse and decomposition. However, effects of macroalgae decay on the sediment organic carbon (SOC) sequestration capacity remain unknown. Depending on the macroalgae biomass in eutrophic seagrass meadows of South China Sea, we carried out a laboratory chamber experiment to investigate the sediment labile organic carbon (OC) compositions and the influencing SOC transformation enzyme activity variations of seagrass meadows in response to common macroalgae bloom species (Cladophora spp.) decomposition. Although the dehydrogenase and β-glucosidase activities were not affected by macroalgae decomposition, the macroalgae decomposition significantly elevated the salt-extractable carbon (SEC) content, SEC/SOC, levels of invertase and polyphenol oxidase activities, and the CO2 release. Overall, this study indicates that macroalgae decomposition stimulates the SOC transformation, and therefore, it is not benefit for SOC sequestration within seagrass meadows of the South China Sea.

Labile and recalcitrant sediment organic carbon pools in the Pearl River Estuary, southern China

Little is known about the labile and recalcitrant sediment organic carbon (SOC) in estuarine ecosystem, and the effects of human activities on SOC sequestration also remain poorly understood. In this study, sediment cores in the Pearl River Estuary (PRE) and adjacent coastal areas in the South China Sea were collected to analyse variations in the sources of SOC and its labile and recalcitrant pools. Concentrations of SOC, microbial biomass carbon (MBC), and recalcitrant organic carbon (ROC) ranged from 4.37 to 10.4 g/kg, 0.522 to 1.53 g/kg, and 1.59 to 5.42 g/kg, respectively, with their corresponding mean values as 7.20 ± 1.43 g/kg, 0.896 ± 0.228 g/kg, and 3.71 ± 1.01 g/kg, respectively. ROC was the chief fraction of SOC, and SOC, MBC and ROC has a similar source. Higher SOC and MBC were observed in the upper sediments, which might be attributable to the enhancement of seawater nutrient and particulate organic carbon concentrations in recent decades. Higher concentrations of SOC, ROC, and SOC stock were found in the inner estuary relative to the outer estuary due to a higher terrigenous organic carbon contribution, while the ratio of water-soluble organic carbon, salt-extractable carbon, and MBC to SOC exhibited a contrasting trend caused by a higher autochthonous contribution. Sand excavation reduced SOC, ROC, ROC/SOC, and SOC stock. The estimated SOC stock of the top 75 cm of sediment in the PRE was 34.0 Mg C ha−1, and the reduction of SOC stock in the PRE due to sea reclamation from 1973 to 2015 was 1,171,159.6 Mg C. Therefore, measures should be taken to control sea reclamation and sand excavation activities in the PRE to enhance carbon sequestration capacity.

Eutrophication indirectly reduced carbon sequestration in a tropical seagrass bed

Seagrass beds occur globally and are substantial blue carbon sinks, yet factors that affect sediment organic carbon (SOC) sequestration are poorly characterized. We investigated the influences of eutrophication on SOC sequestration capacity in a mixed seagrass bed dominated by Thalassia hemprichii and Enhalus acoroides. We surveyed a tropical seagrass bed in the South China Sea along fish farming-induced gradients of nutrients to assess the variability of SOC content and composition in 30-cm-long sediment cores. No significant difference was observed for the SOC content and stock between the two species. Significantly higher microbial biomass carbon and SOC density were observed at the surface 3 cm layer under E. acoroides, and the longer roots of E. acoroides enhanced the seagrass contribution to the SOC near the root tip. SOC had a statistically lower concentration in the sediment layers where the seagrass roots primarily thrive close to fish farming, but greater exchangeable organic carbon was found in all the sediment layers as they approached fish farming. Nutrient enrichment markedly induced a larger ratio of labile C pools in almost all sediment layers. The mean SOC stock in the seagrass bed in Xincun Bay was 6.80 ± 1.03 Mg C/ha, and it showed insignificant variation along the nutrient gradient. Eutrophication may indirectly reduce the SOC sequestration capacity in the seagrass bed by enhancing labile organic carbon with a shorter resident time, and measures should be taken to reduce nutrient input into the seagrass bed to enhance its carbon sink.

Mechanisms of sediment carbon sequestration in seagrass meadows and its responses to eutrophication

Seagrass meadows are recognized as important and productive coastal ecosystems. They are globally-significant hotspots for sediment organic carbon (SOC) sequestration, storing about 4.2–8.4 Pg, and could up to 19.9 Pg SOC—an amount equivalent to 10-times that stored in the earth’s terrestrial soils. Sediment organic carbon (SOC) storage stocks in seagrasss meadows are affected by SOC sources, composition and transformations. Unfortunately, seagrass meadows are declining globally at a rate of 5% per year mainly due to eutrophication, which could influence the supply (source), composition, and transformation of seagrass SOC. We systematically reviewed the literature of seagrass SOC sources, composition, transformation, and storage, as well as summarized their response to eutrophication. Seagrasses develop organic-rich sediment composed of both autochthonous and allochthonous organic carbon. Seagrass, epiphyte, macroalgae, and suspended particulate organic matter (SPOM) trapped from the water column can be common sources of sediment organic carbon in seagrass meadows. The main sources of SOC, however, are retained within seagrass and SPOM. According to a worldwide database of δ 13C, the average contribution of seagrass to SOC is approximately 50%. The global average SOC content is 1.5%, with higher SOC concentrations have always been observed in the sediment of seagrass meadows compared to bare sediments. Additionally, there are significantly different of SOC contents between temperate regions and tropics. The SOC in seagrass meadows are composed of both labile organic carbon and recalcitrant organic carbon. Dissolved organic carbon and microbial biomass carbon are vital fractions of labile organic carbon, which are often served as an important indicator of change and future trends in SOC. Recalcitrant organic carbon is largely composed of belowground seagrass detritus. Sediment microorganisms determine the balance between SOC storage and remineralization processes within seagrass meadows, and high microbial activity and SOC transformation efficiencies are often observed in seagrass meadows. This is mainly due to excretion of amino acids, easily degradable sugars and oxygen, and suitable niches creation by the seagrass rhizosphere. However, the SOC transformation efficiencies decrease with increasing the sediment depth, which result in the high SOC sequestration potential. The main reasons for high SOC sequestration capacity of seagrasses are due to: high primary productivity, strong SPOM trapping capacity from the water column, and low decomposition rates. Although the sediment within seagrass meadows have high carbon sequestration capacity, carbon sequestration by seagrasses is spatially variable. The average SOC stock beneath seagrass meadows in the regions of East Asia, Southeast Asia and Australia are only quarter of the global average values. Eutrophication in coastal areas can trigger the overgrowth of algae, most commonly in the form of epiphytes and macroalgae within seagrass meadows. This cause algal material, which is more labile, to dominate SOC sources in seagrass meaodows, consequently elevating microbial activities/biomass and accelerating SOC breakdown. Meanwhile, the nutrient-induced loss of seagrass biomass can decrease the capacity of organic carbon trapped from seawater. Therefore, changes of SOC sources, composition and transformation induced by nutrient loading can ultimately weaken SOC storage potential. As is often the case, there are exceptions in nature, and some studies have found no impact of nutrient loading on seagrass SOC content. Possibly there is a non-linear and hysteretic response of seagrass SOC to eutrophication, such that nutrient thresholds must be reached before responses can be detected. For future research, we recommend: (1) conducting studies of SOC storage stock at global scales; (2) strengthening the research of the effects of environment changes on storage mechanisms; (3) developing carbon sink amplification technologies; and (4) exploiting new methods for recalcitrant organic carbon measurements. The research of carbon sequestration in seagrass meadows should be strengthened in China, which could provide scientific perspectives for climate negotiations and CO2 trading with other countries in the future.

Sediment microbes mediate the impact of nutrient loading on blue carbon sequestration by mixed seagrass meadows

Recent studies have reported significant variability in sediment organic carbon (SOC) storage capacity among seagrass species, but the factors driving this variability are poorly understood, limiting our ability to make informed decisions about which seagrass types are optimal for carbon offsetting and why. Here we show that differences in SOC storage capacity among species within the same geomorphic environment can be explained (in part) by below-ground processes in response to nutrient load; specifically, differences in the activity of microbes harboured by morphologically-different seagrass species. We found that increasing nutrient load enhanced the relative contribution of seagrass and algal sources to SOC pools, boosting sediment microbial biomass and extracellular enzyme activity within mixed seagrass meadows composed of Thalassia hemprichii and Enhalus acoroides, and thus possibly weaken the seagrass blue carbon sequestration capacity. The relative contribution of seagrass plant material to sediment bacterial organic carbon (BOC) and the influencing SOC-decomposing enzymes in E. acoroides meadows were half that of T. hemprichii meadows living side-by-side, even though the mixed seagrass meadows received SOC from the same sources. Overall this research suggests that microbial activity can vary significantly among seagrass species, thereby causing fine-scale (within-meadow) variability in SOC sequestration capacity in response to nutrient load.

Reviews and syntheses: Hidden forests, the role of vegetated coastal habitats in the ocean carbon budget

Abstract. Vegetated coastal habitats, including seagrass and macroalgal beds, mangrove forests and salt marshes, form highly productive ecosystems, but their contribution to the global carbon budget remains overlooked, and these forests remain hidden in representations of the global carbon budget. Despite being confined to a narrow belt around the shoreline of the world's oceans, where they cover less than 7 million km2, vegetated coastal habitats support about 1 to 10 % of the global marine net primary production and generate a large organic carbon surplus of about 40 % of their net primary production (NPP), which is either buried in sediments within these habitats or exported away. Large, 10-fold uncertainties in the area covered by vegetated coastal habitats, along with variability about carbon flux estimates, result in a 10-fold bracket around the estimates of their contribution to organic carbon sequestration in sediments and the deep sea from 73 to 866 Tg C yr−1, representing between 3 % and 1∕3 of oceanic CO2 uptake. Up to 1∕2 of this carbon sequestration occurs in sink reservoirs (sediments or the deep sea) beyond these habitats. The organic carbon exported that does not reach depositional sites subsidizes the metabolism of heterotrophic organisms. In addition to a significant contribution to organic carbon production and sequestration, vegetated coastal habitats contribute as much to carbonate accumulation as coral reefs do. While globally relevant, the magnitude of global carbon fluxes supported by salt-marsh, mangrove, seagrass and macroalgal habitats is declining due to rapid habitat loss, contributing to loss of CO2 sequestration, storage capacity and carbon subsidies. Incorporating the carbon fluxes' vegetated coastal habitats' support into depictions of the carbon budget of the global ocean and its perturbations will improve current representations of the carbon budget of the global ocean.