| Glomalin-related soil protein(GRSP),a key component in soil derived by arbuscular mycorrhizal fungi(AMF),is widely distributed in terrestrial ecosystems such as forests,grasslands,deserts,and agricultural lands.In land-sea interactions,a significant portion of GRSP in the soil can be eroded or leached and flow into aquatic ecosystems via atmosphere and runoff,and finally accumulates in marine sediments.The continental shelf area,an essential land-sea interface,is a key area for the burial of terrestrial organic matter.However,the deposition profile and ecological function of GRSP on the continental shelf at long time scales has not been revealed so far.In this study,we collected representative sediment core samples from the continental shelf adjacent to the Yangtze River estuary,the organic carbon(OC)and heavy metals content in sediments and GRSP were measured,137Cs and 210Pb isotope dating,513C and 815N analyses were performed to reconstruct the GRSP century-scale changes and its contribution to SOC fixation and mitigation of heavy metal pollution in the continental shelf sediments.The results showed that:1.The source variability in the continental shelf area adjacent to the Yangtze estuary was reconstructed based on the 813C and C/N of sediment SOC and GRSP.513C of SOC in the study area ranged from-23.97‰ to-22.25‰ and C/N from 6.5 to 14.5;while δ13C in GRSP ranged from-20.58‰ to-17.07‰ and C/N ratio from 15.9 to 35.7.It indicates that the SOC in this region is of mixed sea-land origin,while the GRSP is typical of terrestrial organic matter.2.The gravimetric mass of the extracted GRSP(GRSPs)and the total protein of the extract(GRSPe)in the sediments were 1.99 ± 0.08 mg·g-1 and 0.52±0.01 mg·g-1,respectively.The contents of SOC and GRSP gradually decreased with the increase of burial time,and the contribution of GRSPs and GRSPe to SOC was 13.81±4.14%,and 3.72 ± 0.52%,respectively.The contribution of GRSPs(13.81±4.14%and 3.72±0.52%)and GRSPe to organic carbon burial rates were 6.01～24.9%and 2.78～5.59%,respectively,and the contribution of GRSPe to SOC gradually increased with burial time,indicating that the protein fraction in GRSP presents more stable carbon storage potential than SOC.3.The IR spectra of GRSP from different layers showed that the proportion of hydrocarbon fraction in GRSP increased with burial time,protein fraction increased firstly and then decreased with burial time,while polysaccharide and nucleic acid fractions decreased with burial time,indicating that the recalcitrant fractions in GRSP are mainly hydrocarbon and protein.4.The Geoaccumulation Index(Igeo)and Enrichment Factor(EF)of heavy metals indicate that the area is free of contamination to lightly contaminated.The percentages of GRSP-bound heavy metals in the sediments were 0.01～0.87%Al,0.08～0.68%Fe,0.08～1.28%Mn,0.13～0.77%Pb,0.60～2.60%Zn,2.82～22.63%Cu,0.38～1.87%Cd,and 0.22～2.41%As.And GRSP shows strong affinity for Cu.GRSP binds and stabilizes heavy metals indicating its role in the mitigation of heavy metal contamination in sediments.5.The GRSP-bound heavy metals were significantly and positively correlated with the GRSP content(Cu,As,Fe,Mn:p<0.01;Al:p<0.05)and the corresponding heavy metal concentrations in the sediment(Al,Fe,Mn,Pb,Cu,As:p<0.01),indicating that the binding of GRSP to heavy metals was mainly influenced by the binding sites on GRSP and the abundance of heavy metals in the environment.Al,Fe,and Mn contents in GRSP and their percentages in the sediments decreased with burial time,and showed synchronous mutations with δ13C in the late 1970s,indicating that there was partial decomposition of GRSP during the burial process,which led to desorption and release of Al,Fe,and Mn.Hence GRSP was a source of heavy metal pollution in the shelf area to some extent.In conclusion,GRSP in the continental shelf is involved in the biogeochemical cycling of carbon and heavy metals,and plays a critical ecological role in the long-term sequestration of organic carbon and fixation of heavy metals in this region. |
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