Carbon Sink,Environmental Effect And Coupling Mechanisms Of Glomalin-related Soil Protein In Coastal Wetland

Coastal mangrove wetlands are one of the ecosystems with the highest carbon sink potential on Earth,and they play the role of a "buffer" for global climate change.Glomalin-related soil protein(GRSP)is a resistant organic component produced during the growth,reproduction,and degradation of arbuscular mycorrhizal fungi.GRSP plays a key ecological role in terrestrial ecosystems,but its carbon sink potential,carbon sequestration mechanism,and ecological functions(metal sequestration,water quality improvement,and particle aggregation)in coastal wetland ecosystems have not been revealed.Using the tropical and subtropical coastal mangrove wetland ecosystems in China as the research area,combining indoor control and in vitro simulation experiments and based on traditional chemical analysis methods and a comprehensive application of modern instrumental analysis techniques including solid state 13C nuclear magnetic resonance spectroscopy(13C-NMR),X-ray photoelectron spectroscopy(XPS)and thermogravimetric-infrared spectrometer(TG-IR),this study explored GRSP's contribution to blue carbon sinks,water quality improvement mechanisms,and ecological function coupling mechanisms.Key research findings are listed as follows:(1)Based on the combination of big data and small data,the GRSP reserves in the 10 cm surface layer of mangrove wetlands in China and the world are estimated,and the contribution of GRSP to soil organic carbon(SOC)is clarified.Regarding the determination method,GRSP was divided into GRSP total extract concentration(GRSPe)and total extract dry weight(GRSPs).GRSP is distributed in global ecosystems,including terrestrial,wetland and marine ecosystems.The average GRSPe concentration is 2.48±0.38 mg.g-1,and the GRSPe concentrations in different ecosystems have significant differences.The GRSPe concentration in a typical mangrove wetland in China is 3.29±0.08 mg.g-1,which is second only to tropical onshore forests,and is about the same as temperate forests,but is significantly higher than other ecosystems.In addition,the GRSPs content in mangrove wetlands was 30.66±1.28 mg.g-1,and the GRSPe and GRSPs reserves in the surface soils of typical mangrove wetlands in China were estimated to be 3.01 ±0.15 Mg.ha-1 and 29.04±2.08 Mg.ha-1.Considering the significant positive correlation between GRSP and SOC,it is estimated that the average reserve of GRSPs in surface soils is about 42.13± 1.33 Mg.ha-1 through global mangrove SOC data,of which GRSPe is 3.37-7.58 Mg.ha-1.GRSP is rich in C and N and is an important component of mangrove wetland organic matter.The contribution of GRSPe to SOC and TN is 3.44±0.56%and 3.15±0.68%,respectively.The contribution of GRSPs to the SOC was 30.76±0.61%,and the mean variation range of the contribution in different mangrove areas was 25.51-35.92%.(2)The inertia mechanism of GRSP and its carbon sink mechanism in mangrove wetland were revealed.Based on mangrove soil organic matter and GRSP content and soil characteristics such as particle size distribution,the large-scale typical mangrove area was divided into three groups,namely mud flats,mixed flats,and sand flats,by PCA and cluster analysis.The C:N:P stoichiometry of Chinese mangrove wetland ranges from 42:4:1 to 203:8:1,with an average value of 122:7:1,which is significantly lower than that of global mangroves.The comparison analysis showed that the N-limitation and P-enrichment characteristics of mangrove wetlands in China determined the lower eco-stoichiometry ratios,and the eco-stoichiometry ratios in different mangrove substrates were significantly different.C:N:P stoichiometry in GRSP was analyzed for the first time,and it differed depending on the soil substrate,ranging from 180:9:1 to 632:36:1,with an average value of 309:17:1,indicating that GRSP accumulation in the soil contributed to the enrichment of C,N,P elements.13C-NMR was used to analyze the molecular structure of GRSP and soil organic matter in different types of soil substrates.Comparative analysis showed that GRSP had a significantly higher degradation index than SOC.The stubborn structure of GRSP may have regulated the resistance of SOC retention in mangrove wetlands.X-ray diffraction and Fourier transform infrared spectroscopy revealed that the mineral composition and chemical functional groups of clay minerals in different mangrove soils are similar,indicating that clay minerals are not the main factor regulating SOC storage in different mangrove soil textures.Chemical analysis and XPS valence analysis of Fe showed that GRSP can be used as a Fe-rich organic ligand to enhance soil organic matter preservation.(3)The metal loading dynamics and sequestration potential of GRSP at the coastal sediment-suspended matter interface were clarified.The PCA analysis indicated that GRSP can be used as an indicator of mangrove wetland pollution levels.GRSP accumulates in sediments and suspended matter,and can absorb a large amount of metal elements,which directly improves the sequestration of metal elements in the mangrove aquatic ecosystem.This study reveals for the first time that GRSP can effectively bind arsenic,and the enrichment of Fe in GRSP is the main factor.As a carrier of GRSP,the transport and deposition of suspended matter regulated the dynamics of GRSP and related metals in the sediment,that is,the GRSP-bound metal is buried in the sediment profile along with the deposition of suspended matter,which indirectly controls the bioavailability of heavy metals.The direct and indirect effects of GRSP on metals at the sediment-suspended matter interface promote the improvement of coastal water quality.(4)The metal sequestration mechanism of GRSP and the redistribution mode of related metals were clarified.This study evaluated for the first time the metal loading dynamics and ecological functions of GRSP transported into the aquatic ecosystem as a metal carrier,characterized its compositional characteristics,and revealed the underlying mechanism of Cu and Cd chelation.Combining in situ sampling and in vitro culture,we found that the chemical composition characteristics of GRSP were significantly affected by sediment element and mineral composition.In coastal wetlands,Cu and Cd bound by GRSPs accounted for 18.91-22.03%and 2.27-6.37%of the total Cu and Cd,respectively.Functional group ligands and ion exchange are the main mechanisms for GRSP adsorption of Cu,while Cd adsorption is dominated by functional group ligands.In in vitro experiments,GRSP can effectively remove metal elements from water bodies and improve water quality.Water dialysis simulation experiments showed that Cu and Cd adsorbed by GRSP enter the water body and desorb at 25.74±3.85%and 33.53±3,62%,respectively,that is,surface adsorption may also be an important mechanism for GRSP to adsorb metals,and it is revealed that most metal elements form stable or irreversible complex with GRSP.(5)The "super" cementation mechanism of GRSP and particulate matter is analyzed,and the coupling mechanism of carbon sink and metal sequestration is clarified.It was discovered for the first time that GRSP prepared by the current extraction method is a new type of biological flocculant widely distributed in the global ecosystem,with a flocculation activity of more than 75%between pH 2.0 and 4.0.Since the flocculation activity of GRSP from different mangrove wetlands is different,and protein is the active component of flocculation.We used characterization techniques including 13C-NMR,XPS,TG-IR and Zeta potential,and found that the charge neutralization and the complex effects of adsorption and bridging are the main control mechanisms of GRSP flocculation.TG-IR analysis technology shows that GRSP has a high thermal stability.The volatile products of thermal degradation include water,CO2,CO,CH4,aliphatic ethers and amides.GRSP is rich in Fe.During the flocculation process,a red-brown "snow flake" precipitate is formed,that is,a "Fe-rich floc" is formed with suspended particles.The discovery of"Fe-rich flocs" links the ecological function of GRSP in the coastal environment,namely the coupling mechanism of carbon sequestration,metal adsorption and water quality improvement.