Effects of simulated nitrogen deposition on temperature sensitivity of soil respiration components in Populus L. plantations in a riparian zone of the Yangtze River
By the end of the 20th century, China has become one of the world's top three regions of nitrogen deposition. Continuous increase in production and deposition of nitrogen has a significant impact on the carbon cycle in the ecosystem. Therefore, the response of the carbon cycle in a forest ecosystem to nitrogen deposition has become a very important scientific issue. Soil respiration is an important component of the carbon cycle in terrestrial ecosystems, and it is also the only path of carbon output into the atmosphere from the soil and an important source of atmospheric CO_2. A study on the response of temperature sensitivity of soil respiration to nitrogen deposition is important for understanding the role of soil respiration in the mitigation of climate change. In this paper, based on the simulations of different quantities of nitrogen depositions, our research was focused on the changes in soil respiration and on the short-term responses in temperature sensitivity of each component of soil respiration to several gradients of nitrogen deposition in Populus L. plantations in a riparian zone of the Yangtze River. We found that: (1) Soil respiration and its components exhibited significant seasonal variations,in the form of bimodal curves,because of the seasonal flooding. The rate of soil respiration showed a downward trend because of the rise in groundwater level in June and July,which reached the maximum in August and the minimum in December and January. The correlation between soil temperature at different depths (5 cm, 10 cm, and 20 cm) and total soil respiration was significantly positive. The correlation between root respiration and soil temperature at different depths was lower than that between total soil respiration and soil microbial respiration. The correlation was the highest between the soil temperature at a depth of 5 cm and total soil temperature. The soil temperature at a depth of 5 cm explained 50.5%-71.0%, 51.5%-73.9%,and 35.7%-63.2% of total soil temperature, soil microbial respiration, and root respiration,respectively. (2) The Q_(10) values of total soil respiration, soil microbial respiration, and root respiration in the control group (CK,0gN m~(-2) a~(-1)) were 2.54, 2.72, and 1.94,respectively. (3) The treatment with the medium levels of nitrogen (MN, lOgN m~(-2) a~(-1)) enhanced the temperature sensitivity of total soil respiration, soil microbial respiration, and root respiration, whereas the treatment with high levels of nitrogen (HN, 20gN m~(-2) a~(-1)) reduced their temperature sensitivity. The treatment with low levels of nitrogen (LN, 5gN m~(-2) a~(-1)) also reduced the temperature sensitivity of total soil respiration and soil microbial respiration but enhanced root respiration.