| The temporal and spatial distributions and stoichiometry of carbon?C?,nitrogen?N?,and phosphorus?P?of the wetlands are important links in the studies of biogeochemical cycle,which is one of the crucial issues in wetland ecosystem.As a unique wetland type in the west of Songnen Plain,the inland soda saline-alkali reed wetland plays an important role in maintaining regional ecological balance.However,similar to other wetland types,the natural wetland in this area is facing the problems as shrinking and functional degradation due to water shortage.Until now,large area of wetland has been restored through the wetland ecological water replenishment,however,this restoration process will result in the change in biogenic elements biogeochemical cycle.Hence,we selected Zhalong wetland,Momoge wetland and Niuxintaobao wetland in the west of Songnen Plain as study objects to study the spatial and temporal distributions of soil C,N,and P concentrations,stocks,and stoichiometries,Phragmites australis aboveground biomass and C,N,and P accumulations,and water characteristics.Furthermore,we have evaluated the restoration effects of the inland soda saline-alkali reed wetland.The main conclusions were as follows:?1?Soil C,N,and P concentrations in the inland soda saline-alkali wetland showed a significantly spatial heterogeneity.The variation coefficients of SOC in Zhalong wetland is 62.3%,TN is 68.36%,and TP is 27.74%.In Momoge wetland,the variation coefficients of SOC,TN,and TP are 26.4%,28.87%,and 14.7%,respectively.The Niuxintaobao SOC,TN,and TP variation coefficients were 48%,27.74%,and 33.18%,respectively.SOC increases by 39.1%,TN 52.1%,and TP 14.9%in the wetland soil in the end of growing season than those in the beginning of growing season.The accumulations of C and N are much higher than that of P.The average value of SOC storage was 8.34 kg·m-2 in 1 m depth of the inland soda saline–alkali reed wetland soil,lower than those in the inland freshwater wetlands and higher than that in the coastal wetlansd.The TN storage in the study area(0.83 kg·m-2)is higher than that in the coastal/estuarine wetlands,and the TP storage(0.38 kg·m-2)is lower than that in the inland freshwater and coastal/estuarine wetlands.The inland soda saline-alkali reed wetland is characterized by high N and low P.Soil C:N,C:P,and N:P ratios are 12.1,127.1,and 10.4,respectively.The C:N ratio is lower than the average value of wetland soil in China,inland freshwater wetlands and coastal/estuarine wetlands,but the C:P and N:P ratios are higher than those in the coastal/estuarine wetlands.Different from the general coastal/estuarine wetlands,inland soda saline-alkali reed wetland is limited by P.?2?The storages of C,N and P in the aboveground parts of Phragmites australis have obvious seasonal characteristics.Organic carbon?OC?concent increases during the whole growing period,while the contents of TN and TP decrease.The highest OC value(883.02 g·m-2)is observed in September.The highest TN and TP values are 17.27g·m-2?in July?and 1.63 g·m-2?in August?,respectively.The C storage and N accumulation capacity of P.australis are lower,while the P accumulation capacity is higher than those in the coastal/estuarine wetlands and the inland freshwater wetlands.The average value of N:P in the aboveground part of P.australis is 22.8,and the growth of reed is characterized by P limitation.?3?In the study area,the concentrations of TN and TP in the water showed obvious seasonal characteristics,which has relatively lower values in the deeper water and the exuberant growth season,and has no significant difference in other months?p>0.05?.In the years with sufficient water,the concentrations of N and P are relatively lower than that in the years without water.According to the calculation of wetland nutrient state index,the saline-alkali reed wetland in the west of Songnen Plain is in the stage of light eutrophication.There is a significant negative correlation between the N and P pools of plants and the N and P concentrations of water?p<0.05?.The N and P concentrations in the water outlet are far lower than that in the water inlet,indicating the wetland has a certain capacity of N and P retention in the water,which plays an important role in controlling the eutrophication of the water.?4?The change of hydrological conditions influenced SOC,TN,and TP stocks and stoichiometries directly.Regular drying-wetting action is more beneficial to the accumulations of soil C,N,and P.In addition,salinity is another important factor that restricts the accumulations of soil and plant C,N,and P in the soda saline-alkali wetland in the west of Songnen Plain.In this study,we compared the soil physical and chemical properties,C,N,and P concentrations and stoichiometris among the degraded wetlands,restored wetlands and natural wetlands,to evaluate the effects of wetland restoration.The implementation of restored measures could contribute to higher soil C,N,and P concentrations and stoichiometry,resulting in continuous nutrients storage with wetland restoration time.The increases of C,N,and P concentrations and stoichiometries indicated positive changes in wetland ecosystem function.Under the current restoration,we inferred that it would take at least 30 years for P.australis OC,TN,and TP reserves and 34 years for soil SOC,TN and TP to reach the natural wetland.Compared to other wetlands,the inland soda saline-alkali reed wetland is more vulnerable and the wetland protection is more important.?5?Hydrological restoration and salinity control are effective methods for restoring the soda saline-alkali wetlands in the west of Songnen Plain.In the process of restoration,the C,N,and P stocks and ecological stoichiometris of the wetland are affected by many factors,such as water depth,flooding time and salinity,which should be fully considered.We found that 0.3–0.5 m water depth and waterlogging time from May to June are favorable for P.australis growth.Furthermore,reasonable irrigation and drainage are effective measures to reduce the salinity.Moreover,adding appropriate proportion of organic fertilizer could effectively shorten the restoration time.The results of this study provided some references for the protection and restoration of soda saline-alkali reed wetlands in the west of Songnen Plain. |
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