氮添加对内蒙古温带典型草原生态系统碳交换的影响

生态系统碳交换(NEE)是评估碳循环及平衡的重要指标,由生态系统总初级生产力(GPP)和生态系统呼吸(ER)共同决定。以往研究表明,N添加能显著促进草地生态系统植物的生长进而提高生态系统的生产力,但N添加如何影响生态系统碳交换的结论仍不明确。同时,对于不同剂量的N添加对生态系统碳交换影响有何差异也不清楚。于2012和2013年在内蒙古草原开展N添加控制实验,设置中等剂量(10 g N m~(-2)a~(-1),N10)和高等剂量(40 g N m~(-2)a~(-1),N40)两个N添加处理,并采用生态系统原位观测箱系统监测不同N处理条件下的NEE动态。结果表明:2年中等剂量N添加处理(N10)下GPP较对照分别增加了15.6%和20%,而ER的变化不显著,该处理下NEE较对照显著降低了230%和337%(即固碳能力增强)。与中等剂量N添加处理结果不同,高等剂量N添加处理下GPP和ER均有不显著的降低趋势,同时,尽管该处理下NEE有升高的趋势(即固碳能力降低),但并不显著。土壤水分改善、土壤温度下降以及叶片N浓度增加可能是中等剂量氮添加促进该生态系统固碳能力的重要机制,而土壤酸化和物种组成改变可能是导致高等剂量N添加下生态系统固碳能力低于中等剂量的重要原因。研究结果表明,不同剂量N添加对生态系统生产力与呼吸的作用机制存在差异,导致生态系统固碳能力有着明显区别。

Effects of nitrogen addition on carbon exchange in a typical steppe in Inner Mongolia

Net ecosystem carbon exchange (NEE), an important indicator of the carbon cycle and carbon balance, is determined by gross primary productivity (GPP) and ecosystem respiration (ER). With the increased use of nitrogen (N) in industry and agriculture, N deposition has significantly increased globally. It has been widely documented that N enrichment stimulates plant growth, and thereby enhances productivity in grassland ecosystems. However, little is known about how N addition affects NEE and what are the differences in the conditions between various N addition levels. To address these questions, we conducted a manipulative experiment of N addition in a typical steppe in Inner Mongolia, China, in 2012 and 2013. Two treatments and a control (0 g N m^-2a^-1) were set up: a moderate (10 g N m^-2a^-1) and a high (40 g N m^-2a^-1) level of N addition. NEE was observed continuously in high time-resolution using a dynamic chamber system. Our results showed that the GPP of moderate N addition increased by 15.6% and 20% in 2012 and 2013, respectively, whereas there was no significant effect on ER. As a result of GPP enhancement, NEE significantly decreased (i.e., enhanced carbon sequestration capacity) by 230% and 337% in 2012 and 2013, respectively. In contrast, high N addition did not significantly affect GPP, ER, or NEE. Our results also showed that the enhanced carbon sequestration capacity was mainly attributable to increased soil water content and N concentration of plant leaves, and a decrease in soil temperature in the moderate N addition treatment. However, enhanced soil acidity and changes in species composition could have resulted in lower carbon sequestration capacities of the high N addition treatment. Our results indicated that the effects of N addition on GPP and ER depend on the amount of N added, which leads to remarkably different carbon sequestration capacities. The findings of this study could be used to improve process models for grassland ecosystems.