毛乌素沙地油蒿灌丛演替土壤固碳能力及有机碳积累机制

Shrubland is a widely distributed ecosystem type in China and covers about 20% of the land surface area. The latest estimates show that shrubland is an important carbon sink in Chinese terrestrial ecosystems, and the largest soil carbon sink has been found in shrublands.In deserts, shrubland is the most important vegetation type. Many studies suggest that shrubland is the basis of vegetation restoration in deserts and plays a critical role in desertified land rehabilitation. In the past 30 years, precipitation increase and effective protection measures have promoted the vegetation recovery and shrubland succession in desert ecosystems of northwest China. The recovery and succession of a large area of shrublands will significantly increase the ability and capacity of soil carbon sequestration in desert ecosystems. However, the capacity and mechanism of soil carbon sequestration in the desert shrublands are less understood. Mu Us sand land is one of the most important sand lands in China. Shrubland is the dominant biome type and the semi-shrub, Artemisia ordosica, is the most popular vegetation of this area. During 1950s-1970s, desert vegetation was seriously destroyed by overgrazing, reclamation and harvesting. Since 1980s, precipitation increase plus vegetation enclosure, grazing exclusion and grain for green project significantly promote the shrubland recovery, which provides us an opportunity to study the potential and mechanism of soil carbon sequestration in desert shrubland ecosystems. In this study, the experiment sites are planned to set in Mu Us sand land and a series of succession communities of Artemisia ordosica are selected. In the research project, the carbon stores at a depth of 0-3 m in the Artemisia ordosica desert shrub communities will be investigated. Moreover, the spatial heterogeneity and profile distribution of soil organic carbon will be analyzed. In order to better understand the mechanisms of soil carbon sequestration, the changes in soil particle-size carbon fractions along the succession series will also be analyzed.Stable carbon isotope ratio analysis is a useful method to trace the dynamics of soil organic carbon. In this study, the ratios of stable carbon isotope(δ13C) will be determined for the soil bulk samples and particle size samples, which are helpful to explain the mechanisms of soil carbon sequestration.The research project is expected to provide a large amount of information on the capacity and mechanism of soil carbon sequestration in the Artemisia ordosica desert shrub ecosystems.

灌丛土壤的固碳功能是陆地生态系统固碳功能研究的薄弱环节。油蒿灌丛是毛乌素沙地分布面积最大的植被类群。近30年来，由于气候的好转加之生态保护措施的加强，毛乌素沙地植被覆盖率明显提高。植被恢复将促进裸露沙地向以油蒿为主的固定沙地演替，提高沙地土壤的碳固持能力。然而，油蒿灌丛的发育演替能够产生多大的土壤固碳能力，有机碳如何在沙地土壤中积累和固定，尚缺乏充分的理解。基于此，本项目以毛乌素沙地油蒿灌丛的自然演替序列为研究对象，从较深层（0-3 m）土壤碳库出发，研究油蒿灌丛自然演替过程中土壤有机碳的累积量，分析其固碳能力；在此基础上进一步分析土壤有机碳的空间异质性变化、剖面分布变化和颗粒组分变化，同时结合稳定碳同位素比值的测定，初步探讨与揭示油蒿灌丛土壤有机碳的积累机制。研究结果对于预测沙漠化逆转背景下油蒿灌丛的土壤固碳潜力具有重要的科学意义，同时对评价沙地灌丛的土壤碳汇功能也具有重要的实践意义。

本项目按照计划任务书的要求，对毛乌素沙地不同演替阶段油蒿灌丛群落进行了大范围的野外调查和样品采集。研究发现，不同演替阶段油蒿灌丛群落碳源汇效应不同。本氏针茅群落作为油蒿灌丛演替的顶极群落，表现为一个较强的碳“源”，年均释放碳量约为47.47 g C m-2，而典型的油蒿灌丛群落和油蒿＋牛心朴子灌丛群落表现为一个较强的碳“汇”，年均固碳量分别为58.57和38.54 g C m-2。在整个植被演替过程中，植被碳储量呈“n”字型变化曲线，即油蒿+牛心朴子群落演替形成油蒿灌丛群落，植被碳储量增长至最高；如果进一步演替为本氏针茅群落，植被碳储量下降。植被恢复演替过程中，土壤有机碳含量逐渐增加。对不同演替阶段油蒿灌丛群落进行土壤取样和分析表明（2 m深度），植被恢复的初始阶段，流动沙丘土壤有机碳库为15.12 Mg ha-1，其中1米深度土壤有机碳库为8.84 Mg ha-1；植被恢复到半固定沙丘阶段，土壤有机碳库为23.76 Mg ha-1，1米深度土壤有机碳库为14.98 Mg ha-1；植被恢复至固定沙丘阶段，土壤有机碳为30.03 Mg ha-1，1米深度土壤有机碳库为19.11 Mg ha-1。从土壤有机碳的积累机制来看，浅层土壤有机碳积累较多，与植被生产力增大和有机质输入增加密切相关。土壤粒径组分分析表明，植被恢复过程中土壤有机碳的增加，主要归功于土壤细颗粒组分有机碳的增加。土壤全碳稳定同位素分析表明，固定沙丘土壤全碳稳定同位素比值（δ 13C）平均为-17.73‰，半固定沙丘土壤全碳δ 13C值平均为-13.173‰，流动沙丘土壤全碳δ 13C值平均为-6.05‰。该结果表明，随着植被的恢复，植被向土壤输入的有机质越来越多，土壤有机碳的积累逐步增加，使得全碳δ 13C值逐步偏负。本项目通过5 m深度环境钻探分析发现，毛乌素沙地深部可能存在一个大的硝态氮库，但不同类型沙丘，硝态氮的淋溶特征和来源不同。固定沙丘的硝态氮主要来源于土壤的硝化作用；而流动沙丘硝态氮可能主要来自大气沉降。该研究系国内首次明确提出我国沙漠深部可能存在硝态氮积累的现象，对我国沙漠地区地下水资源管理具有指导意义。