Stability Of Soil Organic Matter And Its Controlling Mechanism For Typical Areas Reclaimed In Different Ages In Eastern Chongming Island

The global soil organic carbon storage is three times that of the atmosphere or biosphere,which is the largest carbon storage in terrestrial ecosystem.Compared with other ecosystems on the land,salt marsh per unit area can store more carbon.Chongming Island is the largest and most developed wetland in the Yangtze River Estuary.In recent 60 years,the intensity of coastal reclamation has been increasing.The eastern part of Chongming Island has formed reclamation areas in different years.As an important ecosystem in the coastal area,the structure and function of ecosystem in salt marsh are directly and irreversibly affected by reclamation.Therefore,studying the changes of soil organic matter composition in salt marsh soil and revealing its stability mechanism will help to provide fundamental theories for the study of soil carbon cycle.The carbon cycle of wetland ecosystem,especially carbon storage variation,has an important impact on terrestrial ecosystem carbon cycle and global climate change.Taking the Eastern Chongming Island as study area,four different reclamation areas are defined based on the establishment time of the main seawalls in recent 60years.Four typical soil cores（CM2,CM4,CM5,CM6）and the surface soil samples under different land use types（farmland,grassland and forest）in reclamation areas were selected in this study.Solid state nuclear magnetic resonance(¹³C NMR)and pyrolysis gas chromatography-mass spectrometry（Py-GC-MS）were used to analyze the chemical structure and composition of soil organic matter,and to explore the variation characteristics of soil organic matter in salt marsh with time and burial depth.Combined with soil particle size,SOC（Soil organic carbon）,TN（Total nitrogen）,TC（Total carbon）,C/N（Ratio of carbon to nitrogen）,δ¹³C and land use type（farmland,grassland and forest）,the main factors affecting the structural composition and stability of soil organic matter in salt marsh were discussed.The main conclusions are as follows:（1）In typical reclamation areas of Eastern Chongming Island,the proportion of refractory carbon（alkyl carbon and aromatic carbon）in soil organic matter functional groups increased,while that of alkoxy carbon and carbonyl carbon decreased with depth.The alkylation degree,aromaticity and hydrophobicity of organic matter in the sample showed an increasing trend,while the degree of aliphatization decreased significantly with depth.This is in good agreement with which the gradual increase of the ratio of stable components（aliphatic and aromatic compounds）to unstable components（polysaccharides and phenolic compounds）in the pyrolysis products of organic matter.In different reclamation areas,the functional groups and pyrolysis products of soil organic matter have similar variation trend with depth.With the increase of burial depth,the refractory carbon in the chemical components of organic matter accumulated,and the stability of soil organic matter increased.（2）From east to west,with the increase of burial time（from CM6 core to CM2core）,the proportion of components of poor stability in the buried salt marsh samples decreased,and that of the stable components and refractory functional groups of pyrolysis products increased,and the chemical stability of organic matter increased gradually.The stability of soil organic matter in surface samples showed no obvious variation trend,and the stability of forest samples was slightly stronger than that of CM6 surface samples located at tidal flat.Therefore,burial time is an important factor in determining the stability of soil organic matter in buried salt marsh.（3）In different reclamation areas of Eastern Chongming Island,the composition and content of compounds in the pyrolysis products of surface samples under three land use types around one core showed the same variation trend.The compounds with high content were aliphatic compounds,phenolic compounds and nitrogen-containing compounds.This showed that in an area with specific reclamation time,land use type has no significant effect on the types of soil organic matter compounds and the stability of organic matter.（4）In different reclamation areas,the average particle size,clay content,SOC,TN and C/N ratio of the samples under the same land use type have no significant variations,the stability of soil organic matter has little difference.The stability of organic matter in surface samples（CM2,CM4,CM5）are poorer than that in CM6surface samples of tidal flat.For the same land use mode,the effect of reclamation time on the stability of organic matter in surface samples was not significant.The stability change of organic matter in surface samples may be attributed to local human activities for the same land use mode in different reclamation areas.（5）In different reclamation areas of Eastern Chongming Island,clay content,SOC content and TN content were significantly positively correlated with each other,and the three contents above were significantly negatively correlated with the stability of soil organic matter.δ¹³C and C/N ratio had no significant correlation with the stability of soil organic matter.With the increase of burial time and burial depth,the sediment particle size gradually increased,and the clay content,SOC content and TN content decreased.The amount of more stable functional groups in soil organic matter increased,and that of the unstable components in organic matter pyrolysis products decreased.Therefore,the stability of soil organic matter increased gradually.