黄土高原天然草地深层土壤碳氮固持对封育的响应机制

Grassland fencing and establishing is an important measure to recovery and reclamation of degraded grassland vegetation, assessment of dynamics of soil carbon and nitrogen storage is the top issue in ecological assessment work with the changing of vegetation, and soil carbon and nitrogen storage changes affect the biogeochemical cycles directly and even the changes in the global environment. But almost of scientific researches, soil carbon and nitrogen dynamics after fencing, focused on the assessment of surface grassland soil (0-100cm), meanwhile, a few research focused on the deep grassland soil (0-500cm), maybe the value of grassland soil carbon and nitrogen storage become smaller in ecological assessment, more information of deeper soil（under 100cm）carbon and nitrogen accumulation and distribution are missing. Therefore, basing on field investigation and laboratory analysis, this study focuses on natural grassland that located in semi-arid region of Guyuan city, Ningxia Provence. The study contents are as follows: identifying dynamic of the deep soil organic carbon and total nitrogen storage, and calculating of soil organic carbon and total nitrogen fixed rate in different fencing years (fencing 0 years, 10 years, 20 years, 30 years); Making sure the soil layers where the net fixation of soil organic carbon and total nitrogen is happen and the vertical distribution of the net fixation of them, simulating of the dynamic relationship between the carbon and nitrogen process; Finally, studying on how the vegetation, weather and soil physical structure effect on stability and mechanism of the soil organic carbon and total nitrogen. This work will provide a new reference that related to fixed potential and effect evaluation of deep soil carbon and nitrogen sequestration, and provide a scientific basis of deep soil dynamic vertical accumulation of carbon and nitrogen in the semi-arid of loess plateau, China.

封育是恢复和改良退化草地的重要措施，评估植被变化过程中土壤碳、氮固持动态是目前生态评估的热点，土壤中碳、氮的变化直接影响生物地球化学循环，乃至全球变化。目前对于封育草地土壤碳、氮动态研究主要集中在表层土壤（0-100 cm），而对深层（0-500 cm）土壤碳氮研究较少，可能导致碳、氮固持评估偏小，100 cm以下土壤碳、氮积累信息缺失。基于此，本研究以宁夏固原天然草原为研究对象，基于野外调查和实验室分析，系统研究不同草地封育年限（0年、10年、20年、30年）深层土壤碳氮储量动态，准确计算土壤有机碳、全氮固定速率；描述封育过程中新增（或减）土壤有机碳、全氮发生土层及垂直分布规律，模拟碳氮之间的动态关系；研究植被、气象、土壤结构等对土壤有机碳、全氮稳定性及形成机理的影响。该工作为黄土高原半干旱区草地土壤碳、氮固持潜力与效应评价提供新的数据参考，对碳、氮在深层土壤中垂直积累动态提供科学依据。

封育是恢复和改良退化草地的重要措施，评估植被变化过程中土壤碳、氮储量动态是目前生态评估的热点，土壤中碳、氮的变化直接影响着生物地球化学循环，乃至全球环境变化。目前对于封育草地土壤碳、氮动态的研究主要集中在表层土壤（0-100 cm），而对深层（0-500 cm）土壤碳氮动态研究较少，可能导致生态评估中碳、氮储量偏小，100 cm以下土壤碳、氮积累分布信息缺失。基于此，本研究以黄土高原半干旱区宁夏固原天然草原为研究对象，基于野外调查和实验室分析，系统研究不同草地（柠条地）封育年限（0年、10年、20年、30年）深层土壤有机碳、全氮储量动态，准确计算土壤有机碳、全氮固定速率；描述封育过程中新增（或减）土壤有机碳、全氮发生土层及垂直分布规律，模拟碳氮之间的动态关系；研究植被、土壤水分、土壤结构等对土壤有机碳、全氮稳定性的影响。该项工作为黄土高原半干旱区草地土壤碳、氮固持潜力与效应评价提供新的数据参考，对碳、氮在深层土壤中垂直积累动态提供科学依据。.主要结论如下：.1.封育草地0-100 cm土壤碳氮含量占到0-500cm碳氮含量40%左右，研究黄土高原碳氮储量，需要充分考虑深层土壤碳氮信息。 .2.封育10-30年碳氮固持与固持速率均高于封育10年草地，封育前期土壤碳氮固持速率较低。.3.弃耕地SOC固持量及固持速率显著高于封育草地，切积累在表层（0-140cm）， STN固持及固持速率在土壤表层（0-140cm）低于弃耕地，封育和弃耕地两种土地利用形式均属于碳汇载体。.4.长期封育提升土壤水分含量，影响深度可深达380 cm。草灌结合（15年柠条）模式种植土壤干燥化程度显著高于草地，土壤水分匮缺主要受制于植被耗水和降水补水之间的矛盾。.5.封育有效改善土壤团结结构组成，减小土壤pH值。封育扰动较大为0-200cm，300-500cm土层扰动较小，大团聚体（2-3mm）是决定土壤pH值的主要团粒。.6.降水及pH值变化导致有机碳和无机碳之间的转变，可能是深层土壤有机碳积累的一个原因。