The soil Microbial Carbon Pump as a new concept for terrestrial carbon sequestration

Soil is a huge terrestrial carbon pool, which has higher carbon storage than the sum of atmospheric and terrestrial vegetation carbon. Small fluctuations in soil carbon pool can affect regional carbon flux and global climate change. As soil organic carbon plays key roles in soil carbon storage and sequestration, studying its composition, sources and stability mechanism is a key to deeply understand the functions of terrestrial ecosystem and how it will respond to climate changes. The recently-proposed concept of soil Microbial Carbon Pump(MCP) emphasizes the importance of soil microbial anabolism and its contributions to soil carbon formation and stabilization, which can be applied for elucidating the source, formation and sequestration of soil organic carbon. This article elaborates MCP-mediated soil carbon sequestration mechanism and its influencing factors, as well as representative scientific questions we may explore with the soil MCP conceptual framework.

Soil microbial carbon pump:Mechanism and appraisal

The soil microbial carbon pump(MCP)conceptualizes a sequestration mechanism based on the process of microbial production of a set of new organic compounds,which carry the carbon from plant,through microbial anabolism,and enter into soil where it can be stabilized by the entombing effect.Understanding soil MCP and its related entombing effect is essential to the stewardship of ecosystem services,provided by microbial necromass in the formation and stabilization of soil organic matter as well as its resilience and vulnerability to global change.The mechanism and appraisal of soil MCP,however,remain to be elucidated.This lack of knowledge hampers the improvement of climate models and the development of land use policies.Here,I overview available knowledge to provide insights on the nature of the soil MCP in the context of two main aspects,i.e.,internal features and external constraints that mechanistically influence the soil MCP operation and ultimately influence microbial necromass dynamics.The approach of biomarker amino sugars for investigation of microbial necromass and the methodological limitations are discussed.Finally,I am eager to call new investigations to obtain empirical data in soil microbial necromass research area,which urgently awaits synthesized quantitative and modeling studies to relate to soil carbon cycling and climate change.

Modeling the contribution of the microbial carbon pump to carbon sequestration in the South China Sea

The two key mechanisms for biologically driven carbon sequestration in oceans are the biological pump(BP) and the microbial carbon pump(MCP); the latter is scarcely simulated and quantified in the China seas. In this study, we developed a coupled physical-ecosystem model with major MCP processes in the South China Sea(SCS). The model estimated a SCSaveraged MCP rate of 1.55 mg C m^(-2) d^(-1), with an MCP-to-BP ratio of 1:6.08 when considering the BP at a depth of 1000 m.Moreover, the ecosystem responses were projected in two representative global warming scenarios where the sea surface temperature increased by 2 and 4°C. The projection suggested a declined productivity associated with the increased near-surface stratification and decreased nutrient supply, which leads to a reduction in diatom biomass and consequently the suppression of the BP. However, the relative ratio of picophytoplankton increased, inducing a higher microbial activity and a nonlinear response of MCP to the increase in temperature. On average, the ratio of MCP-to-BP at a 1000-m depth increased to 1:5.95 with surface warming of 4°C, indicating the higher impact of MCP in future ocean carbon sequestration.

Linking microbial carbon pump capacity and efficacy to soil organic carbon storage and stability under heavy metal pollution

Heavy metal pollution can lead to a great loss of soil organic carbon(SOC).However,the microbial mechanisms that link heavy metal pollution to SOC remain poorly understood.Here,we investigated five apple-orchard soils at different distances from a Pb-Zn smelter.After assessing the heavy metal pollution level based on Grade Ⅱ of the national soil environmental quality standard(China),we found SOC stocks and microbial carbon pump(MCP)capacity(i.e.,microbial residue carbon)under medium and heavy pollution levels were significantly lower than those under safe,cordon and light pollution levels.The structural equation model showed causality in the SOC variations linked to pollution level through MCP capacity,which could contribute 77.8% of the variance in SOC storage.This verified MCP capacity can serve as a key parameter for evaluation of SOC storage under heavy metal pollution.Soil MCP efficacy,i.e.,the proportion of microbial residue carbon to SOC,also decreased under medium and heavy pollution.This suggested that,with a heavier pollution level,there was a higher rate of reduction of microbial residue carbon in soil than the rate of reduction of SOC.As MCP efficacy can be a useful assessment of SOC stability,the significantly positive relationship between MCP efficacy and clay content in correlation analysis implied that lower MCP efficacy was correlated with SOC stability under the heavier pollution level.Our study provides valuable insights to identify the mechanisms of microbially mediated C transformation processes that are influenced by heavy metal pollution in agroecosystems.

The research of typical microbial functional group reveals a new oceanic carbon sequestration mechanism——A case of innovative method promoting scientific discovery

Marine microbes are major drivers of marine biogeochemical cycles and play critical roles in the ecosystems. Aerobic anoxygenic phototrophic bacteria(AAPB) are an important bacterial functional group with capability of harvesting light energy and wide distribution, and appear to have a particular role in the ocean's carbon cycling. Yet the global pattern of AAPB distribution was controversial at the beginning of the 21 st century due to the defects of the AAPB enumeration methods. An advanced time-series observation-based infrared epifluorescence microscopy(TIREM) approach was established to amend the existing AAPB quantitative deviation and led to the accurate enumeration of AAPB in marine environments. The abundance of AAPB and AAPB% were higher in coastal and continental shelf waters than in oceanic waters, which does not support the idea that AAPB are specifically adapted to oligotrophic conditions due to photosynthesis in AAPB acting a supplement to their organic carbon respiration. Further investigation revealed that dependence of AAPB on dissolved organic carbon produced by phytoplankton(PDOC) may limit their competition and control AAPB distribution. So, the selection of carbon sources by AAPB indicated that they can effectively fractionate the carbon flow in the sea. Enlightened by these findings, the following studies on the interactions between marine microbes and DOC led to the discovery of a new mechanism of marine carbon sequestration—the Microbial Carbon Pump(MCP). The conceptual framework of MCP addresses the sources and mechanism of the vast DOC reservoir in the ocean and represents a breakthrough in the theory of ocean carbon sequestration.

Understanding the mechanisms and potential pathways of soil carbon sequestration from the biogeochemistry perspective

Soil carbon sequestration is listed by the United Nations Framework Convention on Climate Change as one of the key ways to achieve long-term “carbon neutrality” in the context of global warming. Soil carbon sequestration is a complex biogeochemical process that involves plants, microbes, and rock minerals at its core. Yet, its regulation mechanisms and promotion pathways remain unclear. This paper reviews recent progress in the related domestic and international research and provides an overview of the key processes and mechanisms of soil carbon sequestration. The main pathways for enhancing soil carbon sequestration(including plant inputs, mineral protection, microbial transformation, and rock weathering) are summarized. The paper also discusses and synthesizes how advanced biogeochemical methods and technologies may be employed to explore soil carbon sequestration mechanisms and potentials. The overall aim of this review is to improve our understanding of soil carbon sequestration as a nature-based solution to combatting climate change from the biogeochemistry perspective, and to highlight the role of fundamental research in Earth Sciences in helping to achieve China's carbon neutrality goals.

Carbon pools and fluxes in the China Seas and adjacent oceans

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km^2 from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semiclosed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea,Yellow Sea, East China Sea, and South China Sea, including different interfaces(land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems(mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO_2 sources, releasing about0.22 and 13.86–33.60 Tg C yr^(-1) into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92–23.30 Tg C yr^(-1) of atmospheric CO_2, respectively. Overall, if only the CO_2 exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO_2, with a net release of 6.01–9.33 Tg C yr^(-1), mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon (DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr^(-1),respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr^(-1), significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr^(-1), respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25–36.70 and 43.93 Tg C yr^(-1), respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36–1.75 Tg C yr^(-1), with a dissolved organic carbon(DOC) output from seagrass beds of up to 0.59 Tg C yr^(-1). Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr^(-1) and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr^(-1), respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72–104.56 Tg C yr^(-1). The DOC efflux from the East China Sea to the adjacent oceans is 15.00–35.00 Tg C yr^(-1). The DOC efflux from the South China Sea is 31.39 Tg C yr^(-1). Although the marginal China Seas seem to be a source of atmospheric CO_2 based on the CO_2 flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export,and microbial carbon pump(DOC conversion and output) indicate that the China Seas represent an important carbon storage area.

Eco-engineering approaches for ocean negative carbon emission

The goal of achieving carbon neutrality in the next 30-40 years is approaching worldwide consensus and requires coordinated efforts to combat the increasing threat of climate change.Two main sets of actions have been proposed to address this grand goal.One is to reduce anthropogenic CO2emissions to the atmosphere,and the other is to increase carbon sinks or negative emissions,i.e.,removing CO2from the atmosphere.Here we advocate eco-engineering approaches for ocean negative carbon emission(ONCE),aiming to enhance carbon sinks in the marine environment.An international program is being established to promote coordinated efforts in developing ONCE-relevant strategies and methodologies,taking into consideration ecological/biogeochemical processes and mechanisms related to different forms of carbon(inorganic/organic,biotic/abiotic,particulate/dissolved) for sequestration.We focus on marine ecosystem-based approaches and pay special attention to mechanisms that require transformative research,including those elucidating interactions between the biological pump(BP),the microbial carbon pump(MCP),and microbially induced carbonate precipitation(MICP).Eutrophic estuaries,hypoxic and anoxic waters,coral reef ecosystems,as well as aquaculture areas are particularly considered in the context of efforts to increase their capacity as carbon sinks.ONCE approaches are thus expected to be beneficial for both carbon sequestration and alleviation of environmental stresses.

Coastal blue carbon： Concept, study method, and the application to ecological restoration

Coastal blue carbon refers to the carbon taken from atmospheric CO2; fixed by advanced plants(including salt marsh,mangrove, and seagrass), phytoplankton, macroalgae, and marine calcifiers via the interaction of plants and microbes; and stored in nearshore sediments and soils; as well as the carbon transported from the coast to the ocean and ocean floor. The carbon sequestration capacity per unit area of coastal blue carbon is far greater than that of the terrestrial carbon pool. The mechanisms and controls of the carbon sink from salt marshes, mangroves, seagrasses, the aquaculture of shellfish and macroalgae, and the microbial carbon pump need to be further studied. The methods to quantify coastal blue carbon include carbon flux measurements, carbon pool measurements, manipulative experiments, and modeling. Restoring, conserving, and enhancing blue carbon will increase carbon sinks and produce carbon credits, which could be traded on the carbon market. The need to tackle climate change and implement China's commitment to cut carbon emissions requires us to improve studies on coastal blue carbon science and policy. The knowledge learned from coastal blue carbon improves the conservation and restoration of salt marshes,mangroves, and seagrasses; enhances the function of the microbial carbon pump; and promotes sustainable aquaculture, such as ocean ranching.

Production of dissolved organic carbon in the South China Sea:A modeling study

The South China Sea(SCS)is the largest semi-enclosed marginal sea in the western Pacific.The alternation of East Asian monsoon causes a significant seasonal pattern of chlorophyll,primary productivity,and export flux of sinking particles.However,the source and sink of dissolved organic carbon(DOC)pools with different bioavailability are less studied.Here we evaluated the seasonal production of DOC in labile,semi-labile and refractory forms using a coupled physical-biogeochemical model.This study aims to understand the dynamics and budgets of organic matters in the SCS.Model results show that the production of labile,semi-labile and refractory DOC is highly correlated with the net primary productivity(NPP)which is higher in winter and lower in summer,reflecting a dependence of DOC on the NPP.The seasonal variation in Pearl River discharge dominates the DOC production in the northern coastal region.In the northeast,the Kuroshio intrusion associated frontal system is attributed to cause high winter production.The DOC production in the southwest is controlled by both winter mixing and summer upwelling.The production of refractory DOC with the least bioavailability favors carbon sequestration.Its annual mean production is 1.8±0.5 mg C m^(−2) d^(−1),equivalent to 26%of the export flux of particulate organic carbon at 1000 m.