Organic Carbon Distribution,Function And Its Burial Processes In The Coastal Wetlands Of The Liaohe Delta,Northeast Of China

The distribution characteristics of organic carbon?Corg?in coastal wetlands were significant for researching carbon sequestration and responding to environmental evolution and global climate changes.While total soil carbon is an important indicator of carbon sequestration capacity,the form this carbon takes is perhaps more important.Glomalin-related soil protein?GRSP?,one existence form of Corg,could represent and perform the relevant influences of Corg to carbon sequestration in coastal wetlands.GRSP,the recently described glycoprotein formed by arbuscular mycorrhizal fungi?AMF?,was released to soil after hyphal death/turnover.The residence time of GRSP were estimated to be from 6 to 42 years,even thousands of years under the anaerobic environment in an coastal area.With about 30%-50% of carbon in GRSP,up to 30–60% of soil carbon may be contributed by GRSP under certain conditions in upland agricultural soils,whereas more typical values may be around 5%,These observations demonstrate that GRSP has the potential to accumulate and comprise a substantial proportion of soil carbon.In addition to being refractory,GRSP also helps bind soil particles to form soil aggregates.Such soil aggregation may further contribute to long-term carbon storage by protecting soil organic matter from oxygen and microbial decomposers.GRSP is therefore considered the key link between AMF and soil carbon storage,having scientific significance for coastal wetland carbon sequestration and subsequent long-term storage.Bradford-reactive GRSP?BR-GRSP?were analyzed for 150 surface sediment samples in Liaohe Delta?LHD?,showing a range of 0.11-11.31 mg g-1,which was among the scope of most temperate soil?2-14 mg g-1?.Significant differences were found for different vegetation types?p=0.0018?,and this was the synthetic influence of plant type,the colonization level of AMF,land-use type and the physical and geochemical properties of sediments.For the lowest BR-GRSP content in seablite,the plant-AMF interaction,salt stress,and the accelerated microbial organic matter mineralization and increased sulfate reduction following salt-water intrusion could be responsible.However,the second lower BR-GRSP content in maize field,the main reasons were increased tilling,crop monoculture and use of certain fungicides.The significant positive relation was recognized between BR-GRSP and Corg?r =0.511,p=0.000?,and 6.213%^-10.35% of sediment total Corg,4.901%–8.35% of sediment total carbon,and 8.147%^-13.219% of sediment total nitrogen could be bound directly to BR-GRSP,which indicated that BR-GRSP was an important constituent in sediment carbon/nitrogen sink of LHD.Moreover,significant relationships between BR-GRSP and sediment physical properties?bulk density,water content and clay content?suggested that clay particles could offer physical protection for AMF and GRSP,in return,AMF and GRSP could also improve the sediment/soil structure,both facilitating the accumulation of GRSP in coastal wetland.Various chemical properties,including nutrition elements?C,N,P?,stoichiometric ratio?TN/TP,TC/TP and TP/TK?,and mineral element?Cu,Fe,Zn?,had relation with BR-GRSP content,implying the stoichiometric ratio related P and sufficient nutrient in sediment could favour GRSP accumulation.Multi-heavy metal elements?As,Cd,Cr,Hg,and Pb?showed significant positive relation with BR-GRSP content,indicated the heavy metal fixed effect of GRSP.Nevertheless,the ability of GRSP to absorb heavy metal was weaker than humic substance,the increased BR-GRSP/Corg in farmland could reduce the Corg-heavy metal chelation degree.To proof the influence of decreased farmland Corg-heavy metal chelation to heavy metal migration,also to evaluate the spatial distribution,transportation environmental dynamics of metals,the provenance of metal pollution,and assess the filtration functions of wetlands in LHD,grain size,concentrations of Corg and particulate heavy metals?PHMs?As,Cd,Cr,Cu,Hg,Pb,Zn,Al,Fe,Mn of 373 surface sediment samples,salinities in 67 surface water samples,were analyzed in various environments,including the upper delta plain wetlands?UDPW?,its adjacent shallow sea wetland?SSW?in the Liaodong Bay,and river channels that are running through the LHD.The concentrations of PHMs for UDPW were generally higher by a factor of ¹0–22% compared with its analogues in SSW,suggesting the accumulation of PHMs within the UDPW,indicate that the UDPW systems are efficiently physical and chemical traps for PHMs of anthropogenic sources by retaining and storing pollutants flowing into the sea.However,there was sever sewage-irrigation-induced-Cd-pollution in an area of ⁸6 km2 of the adjacent shallow sea wetland,where large amount wetlands were historical moved for agriculture in the UDPW.Remarkably,the distributions of PHMs were controlled by salinity induced desorption and re-adsorption mechanisms,and significantly dispersed the contamination coverage by the three-dimensional hydrodynamic and sedimentation processes that dominated by inputs of freshwater and ocean dynamics including NE-SW tidal currents and NE-E longshore drifts in the SSW of the Liaodong Bay.A high agreement between the UDPW and the SSW datasets in Principle Component Analysis essentially reflects that the characteristics of PHM sources in the SSW were actually inherited from that in the UDPW,with a much closer relationship among heavy metals,organic matter and fine particulates in SSW than that of UDPW,which was judged by their correlation coefficient range of 0.406–0.919 in SSW against the those of 0.042–0.654 in UDPW.By analyzing the correlationship between Corg and PHMs in different land-use types,the degree of farmland Corg-heavy metal chelation was indeed declined,accelerating heavy metal migration to rivers and SSW.In the case of coastal wetlands,glomalin is expected to have both autochthonous and allochthonous sources.The difference between GRSP ?¹³C of upland C3 plant and wetland C4 wetland may help distinguish between allochthonous and autochthonous glomalin in the wetland.By calculation,the autochthonous glomalin were major component with range of 45.87%-97.79% and mean value of 70.47±0.84%.The differences between isotope values of BR-GRSP and SED could rule out the influence terrestrial and wetland plants,directly reflecting the material input of marine environments.Spatial distribution of ?15NSED-?15NBR-GRSP and ?¹³C SED-?¹³C BR-GRSP could mirror the degree of mixing mechanism in estuary and coastal erosion,providing evidences for ocean-land interaction.Based on the significant correlation between BR-GRSP,Corg and multiple physical and geochemical properties of sediments,the associations between carbon sequestration and several physical and chemical index with the mediation of BR-GRSP in modern coastal wetlands were identified,which is the coupling nature of carbon cycle and other elements cycle.However,in geological evolution historical periods,the modern coastal wetlands have experienced different marine and terrestrial depositional environments because of sea-level changes,the carbon sequestration in sediments could revealed distinct characteristics.This paper studied the impact of Si and Fe to carbon burial in sedimentary environment evolution history,and highlight the more significant coupled relationship between carbon and Si and Fe.To explore these relationships,we present foraminifera data,physical and chemical parameters,and ages determined by accelerator mass spectrometry 14 C and optically stimulated luminescence from a core drilled at the southwestern edge of the Lower Liaohe Plain in May 2012.We divide the sedimentary environments since 33,000 cal yr BP into four depositional units,namely,a fluvial deposit?U1?,lacustrine deposit?U2?,marine-related deposit?U3?and upper delta plain deposit?U4?.Environmentally mediated differences in apparent mass accumulation rates?AMARs?of BSi and organic carbon?Corg?were significant.The BSi-AMAR in the later parts of U1 and U2 occurred mainly in the Pleistocene and averaged 11.34 ± 0.22 and 16.69 ± 0.91 g m^-2 yr^-1,which is lower than the analogues for U3(23.59 ± 2.89 – 41.74 ±6.37 g m^-2 yr^-1)and U4(37.25 ± 9.96 g m^-2 yr^-1)that occurred during the Holocene.The BSi record responded more coherently and was more sensitive than Corg to Northern Hemisphere paleoclimatic variations on a long timescale and to abrupt/periodic winter monsoon winds or warming forcings on a short timescale.There was a negative correlation between the concentration of bio-available Fe2O3 and the ratio of BSi/Corg,the implication being that Fe availability may have modulated silicic acid uptake on a very short timescale and in turn impacted the dynamics of carbon burial.