| Global climate change aggravates regional imbalance of drought and precipitation,triggering changes in precipitation patterns.The effects are particularly pronounced in temperate regions.Temperate forest swamps play a critical character in the global carbon cycle.In recent years,water levels of temperate forest swamps face more frequent and sharper fluctuations under the the interaction of climate change and human disturbance.As a result,the uncertainty and complexity of the response of soil organic carbon decomposition to water level change are enhanced.Because of short-term precipitation,seasonal alternation,ecosystem succession,the variation in water level of temperate forest swamps have significant differences in the action time and intensity.Therefore,the results and influence mechanisms of decomposition of wetland soil organic carbon may be quite different.These are the important contents that cannot be ignored in the study of terrestrial ecosystem carbon cycle.Based on this,this study investigated the characteristics and mechanisms of the effects of water level changes at different time scales（short-term,medium-term and long-term）on the soil organic carbon decomposition rate in forest swamps.A typical study region was set up in the Gushantun swamp-forest transition zone at the northern foot of Changbai Mountain in the middle temperate zone.The 49 day short-term water level changes were simulated through field in-situ sampling and indoor control experiments.The seasonal water level variation were monitored by the method of fixed sample plots and field detection.The water level changes in different succession stages were monitored by using spatio-temporal alternative method during the same period by selecting the"forest swamp-drought forest swamp-forest"sequence sampling sites.Comprehensive analysis of soil physicochemical characteristics（Moisture content,p H,Soil organic carbon,Dissolved organic carbon,Total nitrogen）,iron content of different forms（Fe（Ⅱ）、Fe（Ⅲ）,et,al）,activities of soil enzyme（β-glucosidase,Polyphenol oxidase,et,al）,the research results are as follows:（1）Short-term scale.At constant water level（10cm,2cm）,the constant high water level significantly inhibited the decomposition rate of soil organic carbon.At the end of the culture period（49th day）,soil at a depth of 0-10 cm,the decomposition rate of soil organic carbon at constant high water level decreased by 37.96%compared with the rate of soil organic carbon at constant low water level.After the natural drying treatment,the decomposition rate of soil organic carbon at the depth of0-10 cm showed a trend of first increasing and then decreasing,and the peak appeared on the 7th day of natural drying(4.43μg C g-1h-1).The lowest value appeared on the49th day of natural drying(0.19μg C g-1h-1).At the end of the culture period,the decomposition rate of soil organic carbon decreased by 93.75%and 96.12%respectively compared with constant high water level and constant low water level.Both high water level and drought stress inhibits the decomposition of soil organic carbon.Soil moisture content,β-glucosidase activity and Fe（II）content were positively correlated with the decomposition rate of soil organic carbon significantly.（2）Medium-term scale.The response of forest swamps soil organic carbon decomposition rate to the seasonal variation of water level showed an unimodal trend,the highest in summer(0.94μmol m-2s-1)and the lowest in winter(0.47μmol m-2s-1).The contents of Fe（II）and Dissolved organic in soil are important influencing factors,and are significantly positively correlated with the decomposition rate of organic carbon（P<0.05）.（3）Long-term scale.The decline of water level significantly promoted the decomposition of soil organic carbon in forest swamp.The decomposition rate of soil organic carbon in low water level was the highest(3.03μmol m-2s-1),which is 0.36times higher than the medium water level and 2.22 times higher than the high water level,and the long-term water level decline mainly regulates the decomposition rate of soil organic carbon by significantly affecting soil physicochemical properties,enzyme activities and different forms of iron content.The soil organic carbon decomposition rate was negatively correlated with the soil moisture content,Fe（II）content,amorphous iron oxide and complex iron oxide content,while positively correlated with soil p H significantly（P<0.05）.（4）The three scale water level change conditions conducted in this paper,the physicochemical properties of the soil,the enzyme activity and the change of Fe（II）in the vertical direction of 0-30 cm depth gradually decreased with the increase of depth.（5）Analysis of the response mechanism of forest swamp soil organic carbon decomposition to water level variations through the"enzyme latch"theory and the"iron gate"mechanism.The results of this paper are different from the main viewpoints of the"enzyme latch"theory,but some viewpoints of the"iron gate"mechanism can be mutually confirmed with the results of this paper.Thus,the change of water level drives the redox of iron in the soil,which plays an important role in regulating the decomposition of soil organic carbon.In conclusion,the decline of forest swamps water level（within a certain range）will enhance the soil organic carbon decomposition rate and aggravate carbon emission.The wetland soil organic carbon decomposition has regional characteristics and influenced by the interaction of different time scales and environmental factors.This study provides a scientific basis for strengthening the function of wetland“Carbon Sink”and reinforcing the wetland protection and management. |
