| The influences of carbon-based greenhouse gas emissions on global climate change have caused numerous attentions. Increasing or maintaining soil carbon holding capacity as carbon emissions reduction alternative strategic approaches is subject to the attentions of governments and scientists. The Kyoto Protocol proposed that we can increase carbon pools in ecosystem to compensate carbon emissions resulted from economic development. Different management practices have strong influences on the dynamics of soil carbon. Estuarine wetland, which has unique material circulation and energy budget, is an important part of terrestrial ecosystem carbon pools. It is extremely sensitive to human or natural disturbances.Phragmites community, which is typical vegetation in east Beach of Chongming Island, was taken as the research object. Mowing and straw returning of Phragmites, two main normal management practices in Phragmites production and wetland ecosystem, were carried out in the end of2008. Measurements of soil respiration,soil surface(0-5cm depth) temperature and humidity,together with growth and physiological characteristics and soil organic carbon content was conducted during the year of2009.The effects of management practices on soil respiration dynamics and related control factors were studied. At the same time, responses of growth and physiological characteristics of Phragmites to different management practices were also discussed. Futhermore, connecting with soil organic carbon content, the influences of different management practices on soil carbon holding capacity were compared. This study has great reference value for the management of wetlands with purpose of increasing soil carbon sequestration.The major results are summarized as follows:1)Under different management practices, annual average soil respiration showed that:late mowing+straw returning (LMR,0.44±0.054mol·m-2·-d-1)>control samples (CK,0.39±0.052mol·m-2·d-1)>late mowing+no straw returning(LMNR,O.35±0.051mol·m-2·d-1)> early mowing+straw returning (EMR,0.33±0.048mol·m-2·d-1)> early mowing+no straw returning (EMNR,0.26±0.036mol·m-2·d-1).LMR increased soil respiration significantly, suggesting that increased soil carbon emission; however, LMNR,EMR,EMNR reduced soil respiration to varying degree, suggesting that reduced soil carbon emission; soil respiration under management of straw returning is higher than that of no straw returning(EMR>EMNR, LMR>LMNR, p<0.05), soil respiration under management of late mowing is higher than that of early mowing (LMR>EMR, LMNR>EMNR, p<0.05).2) The soil surface temperature and water content can explain changes of soil respiration well. Single factor fitting results showed that soil surface (5cm) temperature can explain70.2%-87.3%of soil respiration variation, soil surface (5cm) water content can explain64.2%of soil respiration variation, two-factors fitting results can explain more than90%of soil respiration variation.3) Seasonal variation patterns of soil respiration and soil surface (5cm) temperature are consistent, showed a single-peak curve in the year of2009, maximum of respiration was achieved during August-September, minimum of respiration was achieved during January-February. Respective analysis of soil respiration and soil surface (5cm) temperature,soil surface (5cm) water content during growing season and non-growing season indicated that:during growing-season, soil surface (5cm) water content is the dominant factor that affecting soil respiration; during non-growing season, soil surface (5cm) temperature is the dominant factor that affecting soil respiration.4) In the initial growth period (April and May), management practices reduced plant height and increased stem diameter significantly, however, differences of individual biomass and leaf area were not significant. During late growth stage, EMR increased plant height and leaf area, but reduced total fresh weight and total dry weight, stem diameter had no significant difference; EMNR increased plant height, leaf area, total fresh weight and total dry weight. Stem diameter had no significant difference. The area of bottom leave did not change remarkably, but the area of top leave increased significantly. The indicator that height/diameter×10-2indicated that management practices may result in uncoordinated between radial-growth and cross-growth of Phragmites.5) Responses of photosynthetic physiological characteristics of Phragmites to management practices were different. Straw-returning could promote photosynthetic physiological processes:the net photosynthetic rate-light intensity (Pn-PAR) curves moved upward, showing positive effects; no straw-returning restrain photosynthetic physiological activities with Pn-PAR curves jumping down. Study on photosynthetic parameters showed that management practices reduced maximum photosynthetic rate (Pmax) and light saturation point (LSP). Under the management of straw returning, dark respiration rate (Rd), light compensation point (LCP) reduced significantly; under the management of no straw returning, Rd and LCP increased significantly. All management practices in this research reduced stomatal conductance (Gs) and transpiration rate (Tr) significantly, whereas increased intercellular CO2concentration (Ci) and water use efficiency (WUE).6) Management practices lead to soil bulk density to a certain degree, indicating that management practices could improve soil physical structure. Changes of soil organic carbon content and organic carbon storage were consistent, presented as follows:EMR>LMR>CK>LMNR>EMNR, which suggest that management practices could affect accumulation and decomposition of soil organic carbon significantly. EMR and LMR could increase soil organic carbon content significantly, LMNR and EMNR could reduced soil organic carbon content significantly.7) Combining analysis of soil respiration and soil organic carbon, we can infer that:EMR could increase soil carbon holding capacity, EMNR and LMNR could maintain soil carbon holding capacity, effect of LMR on soil carbon holding capacity was unsatisfactory. |
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