黄土高原半干旱区封育草地土壤有机碳固定与稳定性机制

Global warming threatens humman existence and development. Sequestration of organic carbon in soils offers the possibility of adapting and mitigating climate change. Stabilization is suggested to determine the fate of soil organic carbon under changes in environmental situation and is crucial for carbon sequestration assessment. However, most of studies have concerned with quantifying soil carbon in upper layers, rather than considered the quality and stabilization of organic carbon in deeper soil layers. Additionally, most of research and assessment efforts focusing on the ecological effects of the conservation program on Loess Plateau are required. Therefore, our study will be carried out in semi-arid grasslands in Yunwu Mountain of Loess Plateau with restoration age in the chronosequence. We will define soil aggregate classes and isolate carbon fractions using aggregate hierarchy and fraction density methods. Soil organic carbon characteristics will be determined by XANES-spectroscopy. The objectives of this study are to evaluate soil organic carbon stocks and dynamics following grasland restoration and to explore the mechanisms of organic carbon stabilization. Our work will give reliable information about quantity, quality and the according stabilization mechanism of soil carbon of whole soil profiles(0-100 cm), and provide fundamental data to assess carbon sequestration potential and stabilization at regional scale.

气候持续变暖已经威胁到人类的生存和发展。增加土壤碳汇是应对和缓减气候变暖的重要举措之一。土壤碳固定及其稳定化直接关系到新积累碳在环境条件改变下的归宿，是衡量固碳效应的关键。但目前有关土壤有机碳动态的研究主要集中在表层土壤(0-30cm)的固碳容量上，对深层土壤和土壤固定碳质量的研究相对不足。黄土高原实施的大规模退牧还草生态工程对土壤有机碳固存的效果也亟需进行全面评估。因此，本项目针对土壤碳固定和稳定问题，利用黄土高原半干旱区云雾山自然保护区建立的年代序列封育草地，结合团聚体粒径分级-有机质密度分组和辐射光谱技术，研究封育草地恢复过程中0-100 cm土壤有机碳储量和质量的变化，探讨土壤固碳的稳定性机制。该项工作将对黄土高原半干旱区草地土壤固碳潜力的估算和固碳效应的评价提供科学依据。

该项目主要开展了样点尺度上退耕地自然恢复对土壤有机碳影响研究和区域水平上恢复植被与恢复模式固碳效应研究。研究结果表明，退耕地自然恢复25年0-60 cm土壤有机碳增加了3.6 kg C m-2，恢复到天然草地土壤碳水平的46.6%。0-30 cm和30-60 cm土壤固碳速率分别为88 g C m-2 yr-1 和55 g C m-2 yr-1。 0-30 cm 和30-60 cm土壤固碳均呈现渐进趋势，这主要是由与矿物结合组分有机碳含量增加和质量比例下降形成的。深层（60-100 cm）土壤有机碳没有显著变化，主要是由大团聚体有机碳密度降低而微团聚体有机碳密度增加形成的。因此土壤团聚过程控制着退耕还草地土壤有机碳的恢复过程。恢复植被对土壤固碳效应有显著影响。自然恢复模式下，林地和灌丛分别是草地土壤固碳的3.4和3.6倍；人为恢复模式下，林地和灌丛分别是人工草地土壤固碳的5.4和7.0倍。年限差异是导致恢复模式固碳效应不同的主要原因。土壤固碳效应随恢复年限增加而增大，因此延长人工恢复年限可以促进退耕地土壤固碳。