Soil Organic Carbon Turnover In Response To Different Environmental Factors In Wetland Ecosystem

Wetlands are one of the most productive ecosystems on earth,which contain approximately 20%–30%of the terrestrial soil carbon（C）pool,only occupy disproportionately a mere 5%–8%of the land surface,thus acting"blue carbon"effect.With the aggravation of global warming and human activities,wetlands are largely disturbed by natural and human factors,which affect the carbon capture capacity and carbon source/sink function of wetlands.Therefore,biogeochemical C sequestration and turnover in wetlands have become the hot spot of current research.In this study,we have systematically studied the effects of different environmental factors on the distribution,source,stability and bury of soil organic carbon（SOC）in wetland ecosystems,including the supply of soil available silicon,different land use changes,Spartina alterniflora invasion,and sea-level rise.Combined with soil science,biogeochemistry and stable isotope ecology,from small-scale regional mechanism to large-scale macroscopic observation,we studied the response mechanism of SOC turnover to different environmental factors in wetland ecosystem.The main conclusions are as follows:（1）The supply level of soil available silicon regulates plant biogenic silicon accumulation,which can substitute lignin as a structural component,and reduce energy costs associated with lignin biosynthesis,thus increasing plant biomass of Phragmites australis.Lignin content had a negative relationship with litter decomposition rate.Hence,Si-mediated competing processes that increase decomposition rate and biomass C accumulation,may play an important role in the biogeochemical C cycles,especially in wetlands dominated by Si-accumulating plants.（2）Land conversion from wetland to cropland,agroforestry and urban areas decreased SOC contents by 26.97-52.81%,8.71-45.62%and 35.47-56.33%,decreased SOC stocks by 25.30-50.46%,6.98-40.76%and 30.34-51.62%,respectively,and C:nitrogen（N）ratios have been greatly depleted.Theδ¹³C values were overall depleted than the other three land use types and have evident differences with soil depth among four land use change types.We detected some differences in individual or three categories（physical,chemical and plant-derived lignin composition）of variables influencing SOC contents and isotopes in topsoil（0-40 cm）and subsoil（40-100 cm）,suggesting changes in organic C degradation processes and preservation mechanisms over time after buried under land use changes.The C:N ratios,δ¹³C,and lignin monomer composition are not completely consistent to comprehensively evaluate SOC stability in response to land use changes,which were related with litter inputs（i.e.,plant litter quality and biomass）,hydrological conditions（i.e.,drying-wetting cycles,unflooded conditions）and soil physicochemical properties（i.e.,bulk density and soil texture）,thus leading to large variance in SOC turnover rate.（3）SOC storage did not follow the trend in aboveground biomass from the native to invasive species,or with vegetation types and invasion duration（7–15 years）in three native vegetation communities following Spartina alterniflora invasion,highlighting the complexity of these systems.The changing trends of C:N ratio and lignin monomer composition（C/V,S/V,（Ad/Al）V,and（Ad/Al）S）were consistent with that of SOC content,which confirmed the reliability of SOC stability evaluation.The contribution of S.alterniflora-derived organic C accounts for 5.55%,23.07%and 12.22%in the P.australis,K.candel and A.marina communities,respectively,with a corresponding change in SOC storage of+3.43,-13.78 and-3.94 Mg C ha^-1.SOC storage decreased with increasing mean annual precipitation（1022–1871 mm）and temperature（15.3–23.4℃）.Edaphic variables in P.australis marshes remained stable after S.alterniflora invasion and so,their effects on SOC content was absent.In mangrove wetlands,however,electrical conductivity,total N and phosphorus,p H and reactive silicon were the main factors controlling SOC stocks.SOC stocks were regulated mainly by the joint effects of climate,plant biological traits and edaphic variables in the coastal wetlands that had different mechanisms in coastal herb marsh and mangrove wetlands.（4）Salinity was modelled to be the most important factor influencing vegetation landscape gradient in estuarine wetland.Organic C contents and storage decreased from upstream to downstream,and the terrestrial organic C input（60%–35%from C3vascular plants）gradually decreased,the marine organic C input gradually increased,and the non-woody angiosperms input gradually decreased,likely due to primarily changes in autochthonous sources（i.e.,decreased on-site plant biomass input）and allochthonous materials（i.e.,decreased fluvially transported upland river inputs,and increased sea-level induced phytoplankton）.Values ofδ¹³C increased with depth in aerobic soil layers（0–40 cm）but slightly decreased in anaerobic soil layers（40–100cm）,the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic layers,respectively.Stable isotopic(δ¹³C andδ¹⁵N),elemental（C and N）,and lignin composition（inherited（Ad/Al）_S and C/V）were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types,possibly due to differences in litter inputs,root distributions,substrate quality,water-table level,salinity,and microbial community composition/activity.（5）The large variabilities of organic C content/stocks in coastal wetlands of China are mainly reflected in the independent wetlands of each coastal city or Province,different sites of the same wetland,different vegetation types and wetland types.Mean annual temperature and annual precipitation have direct effects on plant primary production and thus potential C inputs to the soil,but SOC stocks in subtropical zones was higher than temperate zones,possibly due to fast decomposition of orgenic C under higher temperature.SOC stocks are not completely dependent on apparent plant biomass,emphasizing the importance of real quantity and quality of C inputs.SOC stabilization and stock capacity may be inherently determined by edaphic variables.SOC content,soil structure（i.e.,bulk density and clay content）,soil chemistry（i.e.,salinity and quality of organic C fractions,soil water content）and plant-derived lignin content and composition together control SOC stocks.To sum up,wetland types are affected by different main environmental factors,including dissolved silicon concentration in river wetland due to extreme precipitation or run-off change,land use changes in lake wetland,invasion of exotic plant Spartina alterniflora in coastal wetland,salinity gradient and vegetation composition change caused by sea-level rise in estuarine wetland.SOC stocks are regulated by the complex interplay of climate,edaphic and plant biological traits,and these influence mechanisms are different under various environmental factors,which provide important insights into blue C stabilization mechanism and sequestration capacity in coastal wetlands.