Effects of simulated nitrogen deposition on the each component of soil respiration in the Populus L. plantations in a riparian zone of the mid-lower Yangtze River
Continuous increasing production and deposition of nitrogen will have a significant impact on carbon cycling of ecosystems. Soil respiration is the important component of carbon cycling in terrestrial ecosystems,the only way to release carbon into atmosphere from soil carbon, and an important source of atmospheric CO_2. Study on the response of soil respiration to nitrogen deposition is significant for understanding the role of soil respiration in the mitigation of climate change. By using the experimental method of positioning the simulated nitrogen deposition, this research was focused mainly on the variations in soil respiration, and the short-term response of each component of soil respiration to different levels of nitrogen deposition, and revealed the relationship between carbon and nitrogen cycling of Populus L. plantations in a riparian zone of Yangtze River. We found that: (1) Soil respiration and its components had significant seasonal variations, 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. (2) The average of annual total soil respiration of control, low-nitrogen, medium-nitrogen and high-nitrogen treatments in the poplar plantations were 3.21, 2.82, 2.82 mumol m~(-2) s~(-1) and 2.72 mumol m~(-2) s~(-1) of CO_2, respectively, equivalent to 42.06, 37.06,36.21 t/hm~2 and 35.69 t/hm~2 of = CO_2released annually. The annual amounts of CO_2released from low-nitrogen, medium-nitrogen and high-nitrogen treatments were reduced by 12.35%, 14.45% and 15. 73%, respectively. Nitrogen deposition significantly restrained the function of total soil respiration. (3) The average of annual soil microbial respirations of control, low-nitrogen, medium-nitrogen and high-nitrogen treatments in the poplar plantations were 2.12, 2.05, 1.96 mumol m~(-2) s~(-1) and 1.99 mumol m~(-2) s~(-1), respectively, equivalent to 27.85, 26.98, 25.24 t/hm~2 and 25.95 t/hm~2 of CO_2released per annual. The annual amount of CO_2 released from low-nitrogen, medium-nitrogen and high-nitrogen treatments were reduced by 3.23%, 9.70% and 7.07%, respectively. The magnitude of the decreases was significantly lower than that for the total soil respiration. Nitrogen deposition restrained the function of soil microbial respiration, and this action is not significant. (4) The average of annual root respirations of control, low-nitrogen, medium-nitrogen and high-nitrogen treatments in the poplar plantations were 1.09, 0. 77, 0.86 mumol m~(-2) s~(-1) and 0. 75 mumol m~(-2) s~(-1), respectively, equivalent to 14.22, 10. 08,10. 96 t/hm~2 and 9.97 t hm~(-2) of CO_2released annually. The annual amount of CO_2released from low-nitrogen, medium-nitrogen and high-nitrogen treatments were reduced by 30. 31%, 23.81% and 31.07%, respectively. Such levels of decreases were significantly higher than those for the total soil respiration and the soil microbial respiration. Nitrogen deposition significantly restrained the root respiration.