2011年的5月至2012年的12月,对兴安落叶松(Larix gmelinii)人工林进行了模拟氮沉降实验,设置了4个氮沉降水平处理,分别为:对照(0 g N m~(-2) a~(-1))、低氮(5 g N m~(-2) a~(-1))、中氮(5 g N m~(-2) a~(-1))和高氮(15 g N m~(-2) a~(-1)),每个处理设置3个重复样地。所施氮肥为NH_4NO_3,以溶液方式在生长季内每月喷施1次。采用红外气体分析法于2012年4月底10月份,每隔15d左右测定1次树干呼吸,共测定12次。结果表明:各个氮处理下的树干呼吸速率基本与树干温度的变化一致,均呈单峰型季节变化模式,其中7月底最高、10月底最低。氮处理均提高了落叶松的树干呼吸速率,且随着氮输入水平的增加,平均树干呼吸速率呈现逐渐增强的趋势。生长季内,对照、低氮、中氮、和高氮处理条件下的树干表面释放的CO_2通量分别为67.3、72.5、78.9 g C/m~2和86.5 g C/m~2。树干温度与树干呼吸速率存在显著的指数函数关系,而且温度敏感性(Q_(10))随着氮输入的逐渐升高亦随之增强,对照、低氮、中氮、和高氮处理下的Q_(10)值分别为1.67、1.80、2.01和2.54。另外,伴随氮输入的逐渐增加,树干的氮含量也逐渐升高,树干氮含量与树干呼吸速率之间也具有一定的相关性,能够解释树干呼吸变化的38.3%。
英文摘要:
Anthropogenic activities such as fossil fuel emissions and ammonium from production and application of fertilizers have elevated atmospheric nitrogen (N) deposition by three- to five-fold over the past century. Nitrogen deposition also has been accounted as one major component of global change and is predicted to increase in the future. Nitrogen additions in the atmosphere and soil have been demonstrated to alter regional and global environments and affect forest ecosystem carbon cycle, which is projected to alter the trajectory of future climate change through climate-carbon cycle feedback. Concern about the ecological effects of elevated nitrogen deposition on terrestrial ecosystems is currently increasing. The stem surface carbon flux is one major component of tree autotrophic respiration,and may represent tree growth activities and metabolism. Despite the importance of stem respiration,few studies have been conducted to examine the effects of simulated nitrogen deposition on stem respiration in forest ecosystem. In this study, response of simulated nitrogen deposition on stem respiration in a 40-year-old Larix gmelinii plantation in Northeastern China was studied. The objective was to test the hypothesis that N addition would increase stem respiration in N limited temperate forests. Nitrogen deposition experiments were initiated in April 2011. Four nitrogen deposition treatments (in three replicates) were established: Control (without N added),Low-N (5 g N m~(-2) a~(-1)), Medium-N (10 g N m~(-2) a~(-1)),High-N (15 gN m~(-2) a~(-1)). Twelve plots, each with 10 mx 20 m dimensions were set up, surrounded by an approximately 15 m wide buffer strip. All plots and treatments were installed randomly. Ammonium nitrate solution was sprayed monthly as 6 equal applications from May 2011 to October 2011,and May 2012 to October 2012. During each application, fertilizer was weighed, mixed with 50 L of water, and applied to each plot below the canopy using a backpack sprayer. Two passes were made across each plot in order to ensure an even distribution of fertilizer. The Control plot received 50 L water without N. A polyvinyl chloride collar (10.2 cm inside diameter*5 cm height) was cut and polished to fit the stem shape of each sampled tree, and set up on the northward side with silicon adhesive at breast height. During the growing season between April 2012 and October 2012, stem respiration was measured once a week from 10:00 to 12:00 with a Li-6400 portable C0_2 infrared gas analyzer equipped with a Li-6400-09 chamber. Stem temperature at 1 cm depth beneath the bark was simultaneously measured with a digital thermometer. The measured stem respiration was calibrated with the polyvinyl chloride collar volume and stem surface area of each sampled tree. Our results showed that stem respiration exhibited a strong seasonal pattern, with the highest rates found in July and the lowest rates in October. The estimated stem respiration during the measuring period varied from 1.33 to 4.71 mumol m~(-2) s~(-1) in the Control treatment, 1.60 to 5.01 mumol m~(-2) s~(-1) in the Low-N treatment, 1.85 to 5.38 mumol m~(-2) s~(-1) in the Medium-N and 1.73 to 5.96 mumol m~(-2) s~(-1) in the High-N treatment, respectively. Stem respiration rates showed a significant positive exponential relationship with stem temperature within all treatments. We fitted site-based models and used continuous measurements of stem temperature to estimate cumulative stem respiration for the growing season of 2012. Cumulative stem respiration in the growing season was estimated to 67.3 g C/m~2,72.5 g C/m~2,78.9 g C/m~2 and 86.5 g C/m~2 in the Control, Low-N, Medium-N and High-N treatment, respectively.