阴山北麓农牧交错区退耕地草地生态系统碳交换及水分利用效率

内蒙古阴山北麓农牧交错区由于长期不合理的开垦造成了荒漠草地生态系统碳交换等生态功能的显著丧失。我国20世纪末开始实施的退耕还林还草工程产生了大量退耕地，随着自然恢复演替，这些退耕地的生态功能得到了有效的修复，其巨大的碳汇潜力成为了荒漠草地生态系统碳循环研究的热点。研究通过空间代替时间的方法，对内蒙古阴山北麓典型区域——武川县周边无干扰的荒漠草原以及3个退耕恢复阶段草地的生态系统CO₂交换(NEE、GEP和ER)、水分利用效率(WUE)以及生物量等指标进行了实地测量。结果表明：(1)随着退耕恢复演替时间的推移，生态系统CO₂交换呈显著上升趋势，演替晚期植被NEE与未受干扰的荒漠草地无明显差异；(2)生态系统水分利用效率的变化趋势与生态系统CO₂交换基本一致，但已退耕17年后的退耕地WUE仍没有恢复至未受干扰荒漠草地的水平；(3)导致以上结果的原因主要与退耕地地上植被生物量的恢复以及一、二年生植物和多年生植物比例的演替变化有关。结果表明荒漠草地退耕地恢复过程中生态系统功能的恢复可能并非是同时的，而是分阶段有选择进行的。

Effects of natural restoration on the ecosystem carbon exchange and water use efficiency in the abandoned cropland in the Inner Mongolia

The changes of land-use and management practices have profound influences on ecosystem structure and functioning, such as carbon cycle, which may exaggerate climate change''s impacts. The terrestrial ecosystem carbon cycle is an important part of the global carbon cycle. With vast area and huge soil carbon pool, grasslands play an important role in the global carbon balance process, which is often disturbed and destroyed by reclamation easily. The long-term unreasonable reclamation resulted in a significant loss of ecological functions such as ecosystem carbon exchange in the desert grassland in the Inner Mongolia. A large number of croplands were abandoned with the gradual implementation of Grain for Green Project, which led to the grassland ecological function be restored effectively by natural restoration succession. These croplands, which have huge area and carbon sink potential, have become the focus of carbon cycle research in desert grassland ecosystem. However, the clearer understanding of the effects of cropland abandonment and its time effects on the ecosystem CO₂ exchange processes is still needed. In this study, the ecosystem CO₂ exchange (net ecosystem CO₂ exchange, NEE; gross ecosystem production, GEP; and ecosystem respiration, ER), water use efficiency (WUE), and vegetation biomass were measured in the undisturbed desert steppe and recovery steppe with succession stages of 6, 12 and 17 years. The results showed that the species richness of perennials was improved effectively with Grain for Green. Compared with the local native grassland, the aboveground biomass recovered to 74.2%, 75.9% and 94.4% in early stage of recovery (PR, 2013-2014), mid-term stage of recovery (MR, 2007-2008), and late stage of recovery (AR, 2002-2003) plots, respectively. The longer Grain for Green time also caused a strong increase in perennials biomass, and reducing in annuals biomass and annuals/perennials ratio. In humid years, as the time of succession restoration goes on, the ecosystem CO₂ exchange shows a significant upward trend, and there has been no significant difference on NEE between the late succession and the undisturbed desert grassland. The change trend of the ecosystem WUE is basically consistent with the ecosystem CO₂ exchange. However, the WUE in the returned grassland has not recovered to the level of undisturbed desert grassland even after 17 years of farmland conversion. These results may mainly relate to the succession changes of the vegetation biomass and the annuals/perennial ratio. The results also show that the restoration of ecosystem function is not simultaneous, but selective in desert grassland after abandoning cropland.