Effects Of Different Hydrological Conditions On Soil Organic Carbon Stability In Coastal Wetlands

Hydrological conditions are one of the influencing factors of soil carbon balance in wetland.As an important part of the carbon pool,soil organic carbon affects climate change through releasing greenhouse gases.Soil organic carbon stability affects the decomposition and sequestration of carbon,which in turn affects soil nutrient cycling and soil quality.Therefore,the investingation of the effects of hydrological conditions on the soil organic carbon stability are of great significance to the wetland carbon cycle and coping with climate fluctuations.This study area is located in the Yellow River Delta.And this study selected typical wetlands with different hydrological conditions as sampling sites including nonflooding Phragmites wetlands,freshwater Phragmites wetlands and tidal Phragmites wetlands.The soil active organic carbon components（dissolved organic carbon,DOC;readily oxidized organic carbon,ROC）and stable organic carbon components（Na₂S₂O₈ resistant oxidized organic carbon;H₂O₂ resistant oxidized organic carbon;HF-soluble organic carbon）were determined to explore the influence of hydrological conditions on the stability of organic carbon.In addition,properties and the different forms of cations（amorphous aluminum,iron,Al_o,Fe_o;free aluminum,iron,Al_d,Fe_d;complexed aluminum,iron,Al_p,Fe_p）,minerals Phase（poorly crystalline fraction of Fe oxides,Fe_o/Fe_d;crystalline Fe oxides,Fe_d-Fe_o;ferrihydrite,Fe_o-Fe_p;ratio organically-bound to total amorphous Al,relative amount of Al in organic complexes,Al_p/Al_o;sesquioxides,Al_o+0.5 Fe_o）,soil aggregates and their stability indices（mean weight diameter,MWD;geometric mean diameter,GMD）under different hydrological conditions to clarify the effects of hydrological conditions on the soil organic carbon stability from the perspects of chemical stability and physical stability.The results are as follows:（1）Soil organic matter,total carbon,total nitrogen,carbon to nitrogen ratio,electrical conductivity and base ion content of tidal Phragmites wetlands are higher than those of freshwater Phragmites wetlands and nonflooding Phragmites wetlands.The soil p H value of the three types of wetland are all greater than 7,and the soil texture is dominated by sand.（2）Soil organic carbon stability in freshwater Phragmites wetlands and tidal Phragmites wetland is higher than nonflooding Phragmites wetlands,which is affected by long-term flooding and intermittent flooding.The HF-soluble organic carbon content is highest in three samples.Along the 0～25 cm soil profile,the content of Na₂S₂O₈ resistant oxidized organic carbon and H₂O₂ resistant oxidized organic carbon is the highest in the 10～15 cm soil layer of nonflooding Phragmites wetlands and tidal Phragmites wetlands,the content of DOC and ROC in 0～5 cm is highest（except for the soil ROC in nonflooding Phragmites wetlands）.And the soil carbon stability of two types of wetlands in 5～20 cm is higher than 0～5 cm and 20～25 cm.The soil carbon stability of freshwater Phragmites wetlands in 0～5 cm is hhigher than other soil layers.RDA and correlation analysis show that the content of stable organic carbon component is significant positive correlations with organic matter,total carbon and total nitrogen,which is under different hydrological conditions.（3）Soil Al-Fe-Oxides and mineral phase indicators in freshwater Phragmites wetlands and tidal Phragmites wetland are higher than nonflooding Phragmites wetlands,except for Al_p,Fe_d-Fe_o and Al_p/Al_o.RDA and correlation analysis show that the variations of stable HF-soluble organic carbon is significant very high correlations with Al-Fe-Oxides and mineral phase indicators（expert for Al_p/Al_o）.Soil Al_o,Fe_o,Al_d,Fe_d,Al_p,Fe_d-Fe_o,Fe_o-Fe_p and Al_o+0.5Fe_o are the most important cations and mineral phases for stabilizing the H₂O₂ resistant oxidized organic carbon.The correlation coefficient showed that aluminum cations are more beneficial than iron cations to increase the content of Na₂S₂O₈ resistant oxidized organic carbon and HF-soluble organic carbon.（4）Soil aggregates under different hydrological conditions are mainly composed of micro-aggregates and the lowest in macro-aggregates.But the organic carbon content is the highest in macro-aggregates.The MWD and GMD indicators of soil aggregates in tidal Phragmites wetlands is better than that of the other two types of wetlands.