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Effects of the changes in water content on root decomposition and nitrogen dynamics of Calamagrostis angustifolia in a natural wetland of Sanjiang Plain,northeastern China

LIU Jing-shuang~1;SUN Zhi-gao~2;QIN Sheng-jin~1.1 Northeast Institute of Geography and Agroecology,Chinese Academy of Sciences, Changchun,130012,P.R.China;2 Yantai Institute of Coastal Zone Research for Sustainable Development,Chinese Academy of Sciences,Shandong Province,264003,P.R.China.  
From May 2005 to September 2006,the effects of the changes in water content on root decomposition and nitrogen(N) dynamics of the typical meadow Calamagrostis angustifolia(TMC) and the marsh meadow C.angustifolia(MMC) were studied using the litterbag technique in the dish-shape wetland of the Sanjiang Plain,northeastern China.In this study,a natural water gradient was applied to simulate the changes in the hydrological condition in six decomposition sub-zones vegetated with six plant communities: Carex pseudocuraica,Carex lasiocarpa,Carex meyeriana,marsh meadow C.angustifolia,typical meadow C.angustifolia and island forest.Results showed that hydrological condition had significant effects on the decomposition rate of plant roots.The formation of a hydrous environment in C.angustifolia wetland due to the changes in future precipitation pattern would lead to the decreases in the root weightless rates of TMC and MMC by(29.12%——)48.54% and(21.81%——)42.22%,respectively.Meanwhile,the decomposition rates of plant roots would decrease by(36.36%——55.17%) and(14.10%——32.91%,) and the 95% decomposition time would increase by(2.95——)6.34 and(1.75——)5.24 years,respectively.The changes in N content and N accumulation coefficients of TMC and MMC roots were relatively consistent,but the patterns of change in different sub-zones were variable.Under the hydrous condition,the N content in TMC and MMC roots in the six sub-zones displayed a consistent pattern of net release,although the pattern of release differed among the sub-zones.The C/N ratios played an important role in regulating the changes in N content during the root decomposition process.The N standing crops of TMC and MMC roots were estimated as 10.24 and 25.80 g/m~2.The annual returning rate for N was estimated to be greater than 3.90 and 4.96 g/m~2 per year,respectively.This study indicated that temperature promoted while water condition inhibited the relative decomposition rates of roots system.Further analysis shows that the relative decomposition rate,to a great extent,depends on the substrate quality of roots if the nutrient status of decomposition environment does not change significantly.Conversely,if the nutrient status of decomposition environment changes significantly,the relative decomposition rate largely depends on the supply of nutrients in ambient environment.
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