| In the end of20th century, China has become one of the world’s top three nitrogen deposition regions. Continuous increasing production and deposition of nitrogen will have a significant impact on carbon cycle in ecosystem. Therefore, the response of carbon cycle in forest ecosystem to nitrogen deposition has become a very important scientific issue. Soil respiration is the important component of the carbon cycle in terrestrial ecosystem, and also is the only way to output carbon into atmosphere from soil carbon, and the important source of atmospheric CO2. Study on soil respiration is significant to confirm the relationship between carbon sources and sinks in terrestrial ecosystems as well as the global climate change. In this paper, based on the experimental method of positioning the simulated nitrogen deposition, my research was mainly focused on the variation of soil respiration, and the short-term response of each component of soil respiration to several gradient of nitrogen deposition, and revealed the relationship and impact between carbon and nitrogen cycle in the Populus L. plantations in a riparian zone of Yangtze River. The research provided theoretical basis and reference for predicting the variation of soil carbon fluxes in the Poplar plantations ecosystem in bottomland in case of increasing nitrogen deposition, and also provided reference for sustainable development and scientific management of the Poplar plantations. Four levels of N treatments including:control (0kgN·hm-2·a-1)、low-N (5kgN·hm-2·a-1)、medium-N (10kgN·hm-2·a-1) and high-N(20kgN·hm-2·a-1) were set in the current study. The main results are as follows.(1) There is obvious diurnal variation of soil respiration for Poplar plantation, showing a trend of single peak curve. Soil respiration rate reached the maximum at13:00～15:00and the minimum at2:00～5:00. Nitrogen addition enhanced diurnal variation of soil respiration.(2) Soil respiration and its components had significant seasonal variation, presenting bimodal curves because of seasonal flooding. Soil respiration rate showed a downward trend because of rising of groundwater level in June and July, reached the maximum in August and the minimum in December and January. The average of annual soil respiration in Poplar plantation was3.21μmol·m-2·s-1, the amount of soil respiration was11.47tC·hm-2·a-1, equivalent to42.06t·hm-2as annual CO2emissions. The average of annual contribution rates of soil microbial respiration and root respiration divided by total respiration were65.23%and34.77%, respectively.(3) The correlation between soil temperature at different depth (5cm,10cm and20cm) and total soil respiration was significantly positive. The correlation between root respiration and soil temperature at different depth was lower than the correlation between total soil respiration and soil microbial respiration. The correlation between soil temperature at5cm depth and total soil temperature was highest. Soil temperature at5cm depth explained50.5%～71.0%,51.5%～73.9%and35.7%～63.2%of total soil temperature, soil microbial respiration and root respiration, respectively.(4) The Q10values of total soil respiration, soil microbial respiration and root respiration in control treatment group were2.54,2.72and1.94, respectively. The treatment of medium nitrogen enhanced the temperature sensitivity of total soil respiration, soil microbial respiration and root respiration. The treatment of high nitrogen reduced the temperature sensitivity of them. The treatment low nitrogen also reduced the temperature sensitivity of total soil respiration and soil microbial respiration, but enhanced the root respiration.(5) The average of annual total soil respiration of control, low-nitrogen, medium-nitrogen and high-nitrogen treatment in Poplar plantation were3.21μmol·m-2·s-1,2.82μmol·m-2·s-1,2.82μmol·m-2·s-1and2.72μmol·m-2·s-1, respectively, equivalent to42.06t·hm-2,37.06t·hm-2,36.21t·hm-2and35.69t·hm-2as the annual amount of CO2released. The reduction rates of the annual amount of CO2released of low-nitrogen, medium-nitrogen and high-nitrogen treatment were12.35%,14.45%and15.73%, respectively. Simulated nitrogen deposition significantly restrained the function of total soil respiration.(6) The average of annual soil microbial respirations of control, low-nitrogen, medium-nitrogen and high-nitrogen treatment in Poplar plantation were2.12μmol·m-2·s-1, 2.05μmol·m-2·s-1,1.96μmol·m-2·s-1and1.99μmol·m-2·s-1, respectively, equivalent to27.85t·hm-2,26.98t·hm-2,25.24t·hm-2and25.95t·hm-2as the annual amount of CO2released. The reduction rates of the annual amount of CO2released of low-nitrogen, medium-nitrogen and high-nitrogen treatment were3.23%,9.70%and7.07%, respectively. The rate of decrease was significantly lower than the total soil respiration. Simulated nitrogen deposition restrained the function of soil microbial respiration, and this action is not significant.(7) The average of annual root respirations of control, low-nitrogen, medium-nitrogen and high-nitrogen treatment in Poplar plantation were1.09μmol·m-s·-1,0.77μmol·m-2·s-1,0.86μmol·m-2·s-1and0.75μmol·m-2·s-1, respectively, equivalent to14.22t·hm-2,10.08t·hm-2,10.96t·hm-2and9.97t·hm-2as the annual amount of CO2released. The reduction rates of the annual amount of CO2released of low-nitrogen, medium-nitrogen and high-nitrogen treatment were30.31%,23.81%and31.07%, respectively. The rate of decrease was significantly higher than the total soil respiration and the soil microbial respiration. Simulated nitrogen deposition significantly restrained the root respiration.(8) The inundation has the same effect on total soil respiration, soil microbial respiration and root respiration. In the season of abundant water, the level of underground water continued to rise, resulting in the soil respiration significantly decreased. During the flooding, soil respiration almost stopped. Under soil moisture stress conditions, the soil moisture became the dominant factor to effect total soil respiration, soil microbial respiration and root respiration. The correlation between underground water level and total soil respiration and its component was significantly negative. The correlation between soil microbial respiration and underground water level was highest, and the correlation between root respiration and underground water level was lowest. |
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