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题名: 黄土高原退耕还林多尺度土壤固碳效应
作者: 常瑞英
学位类别: 博士
答辩日期: 2012
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 傅伯杰
关键词: 深层土壤有机碳 ; subsoil soil organic carbon ; 土壤无机碳 ; soil inorganic carbon ; 样带 ; transect ; 区域尺度 ; the reginoal scale ; 碳氮关系 ; soil carbon and nitrogen interaction
其他题名: Soil Carbon Sequestration at multi-scales for ‘Grain for Green’ Project in the Loess Plateau, China
中文摘要:     退耕还林(造林)是当前主要的人类活动之一,对陆地生态系统碳、氮循环影响显著。然而目前退耕还林对土壤碳、氮的研究明显滞后,多数研究针对表层土壤有机碳变化,而缺少对深层土壤有机碳、无机碳和土壤氮素的探讨,且多集中在样地等小尺度上,缺少区域等大尺度的研究,尤其是对不同尺度上土壤碳氮相互作用的认识仍存在很大不足。 本文以我国大面积开展的退耕还林还草工程(退耕工程)为背景,从样点—样带—区域等多尺度上分析了土壤碳氮在退耕还林后的变化规律及尺度特征,并深入探讨了土壤碳氮的相互关系。主要研究区选择在退耕工程重点区域黄土高原陕北地区,以广泛种植的刺槐(Robinia pseudoacacia L.)为主要研究对象。 主要研究包括: 1. 利用配对样地对比方法,探讨了陕北地区退耕还林对土壤表层和深度土壤有机碳、无机碳和全氮的影响(样点研究); 2. 通过沿降水梯度设立样带,研究了土壤碳氮在退耕还林后的变化过程及其区域差异,探讨了降水对土壤碳氮变化的影响。同时分析了土壤碳、氮在退耕过程中以及降水梯度下的相互关系(样带—区域研究)。 3. 收集黄土高原地区退耕还林还草土壤固碳研究,结合陕北地区实测数据,利用Meta-分析方法估算了黄土高原退耕工程的土壤固碳量(区域研究)。 因此,本研究的开展不仅对深入认识造林(尤其是干旱-半干旱地区造林)对土壤碳、氮作用具有科学意义,而且对于我国陆地生态系统碳收支评估具有重要的实践意义。 本文主要结论: 1. 通过对比退耕30年的刺槐林和对照耕地土壤碳储量,发现在陕北地区,退耕还林在较短时期内不仅可以有效提高表层(0-20 cm)土壤有机碳储量,而且可以显著增加深层(30-60 cm)土壤有机碳储量。较高的地上凋落物输入和细根生物量是造林后土壤碳储量增加的一个主要原因。土壤全氮储量与土壤有机碳变化一致,说明土壤有机碳在土壤剖面上的变化可能受土壤氮素的控制。退耕还林影响土壤无机碳的垂直分布(增加了下层土壤无机碳比例),但对土壤无机碳储量影响很小。 2. 沿降水梯度(460 mm—580 mm—650 mm),表层10 cm土壤有机碳和全氮储量变化过程没有明显的区域差异,均随林龄增加而呈增加趋势;下层10-20 cm 土壤碳、氮在降水较低地区呈现线性增加趋势,但在降水较高地区表现为初期下降,之后随林龄增加而增加的二次函数关系。退耕过程中,不同降水带及土壤深度内,土壤碳、氮的变化趋势一致,土壤碳的增加速率(相对增加率,%)高于土壤氮,反映出土壤碳、氮变化的协同关系。 3. 沿降水梯度(380 mm—650 mm),林地生产力和土壤粘粒含量呈线性增加趋势。在降水梯度内(区域尺度上),表层10 cm土壤有机碳储量在退耕还林初期(<10年)变化不大,下层10-20 cm有机碳储量显著下降,且损失量(Mg C/ha)随降水增加而增大。随林龄增加(约30年),表层土壤碳储量显著增加,下层土壤碳储量恢复到退耕前耕地水平,且上下层碳储量的增幅(%)均随降水增加而下降。结果说明造林初期对降水较高地区的负作用较大,随林龄增加,对提高干旱、贫瘠耕地土壤有机碳作用更显著。区域尺度上,土壤氮在不同退耕阶段及土壤深度内变化速率(%)均低于土壤碳,但二者变化趋势一致,具有显著的线性关系。结果表明在整个降水带增加1 g土壤氮可增加4.7-9.5 g土壤有机碳。沿降水梯度,土壤碳、氮的变化与土壤粘粒含量无关。 4. 估算2000-2008年黄土高原退耕还林还草工程中三种主要退耕类型(退耕还草、退耕还灌木以及退耕还林)的表层(0-20 cm)土壤固碳量约14.18 Tg C,同期植被固碳约23.76 Tg C,生态系统固碳量约37.94 Tg C,表明黄土高原退耕还林还草工程具有巨大的固碳能力。三种退耕类型中,退耕还草的土壤固碳量最大,其次为退耕还灌木和退耕还林,与三者的退耕面积排序一致。在黄土高原尺度上,三种退耕类型土壤固碳能力接近,约为0.33 Mg C/ha/a,然而在黄土高原降水较高的南部地区,退耕还林的土壤固碳能力略高于退耕还草。此外,退耕还草的土壤固碳能力在黄土高原北部地区高于中部和南部地区,而其他退耕类型的土壤固碳能力在不同降水带间没有明显差异。结果表明,在黄土高原北部地区实施退耕还草,而在南部地区实施退耕还林有利于提高退耕工程土壤固碳量,但不能忽略退耕还林还草的环境适宜性和经济性。 总之,研究表明,黄土高原退耕还林对表层和深层土壤有机碳均有显著作用。考虑到深层土壤碳的变化,本文可能低估了黄土高原退耕还林还草工程的土壤固碳量。退耕还林后,土壤有机碳变化存在明显的时空变异性,在不同区域、退耕阶段及土壤深度(表层和深层),均受控于土壤氮素的变化。突出了造林过程中,固氮树种选择对提高土壤固碳量的重要性。
英文摘要:     Afforestation of cropland is one of main human activities, playing a significant role in affecting the carbon (C) and nitrogen (N) cycles of the terrestrial ecosystems. Recently, the interest in using soil as a C sink has been rapidly increasing, as it was proposed in Article 3.4 of the Kyoto Protocol of the United Nations Framework Convention on Climate Change and prescribed in later Marrakech accord to include C sequestration in soil to partially meet Quantified Emission Limitation or Reduction Commitments. However, the knowledge of the effects of afforestation on soil C is limited. Many studies have just considered the soil organic carbon (SOC) changes in the topsoil (e.g., 0-20 cm), but ignored the changes in the deep soil SOC and in the soil inorganic carbon (SIC) and soil total nitrogen (TN). Meanwhile, the effects of afforestation on SOC are mainly studied at the field scale, but the effects at the regional scale are still poorly understood, especially in arid and semiarid marginal lands. The ‘Grain for Green’ project, one of the most ambitious ecological programmes launched in modern China, aims at transforming the low-yield slope cropland into grassland and woodland. The Loess Plateau in China is the most important target of this project due to its serious soil erosion. The project in the Loess Plateau was initiated in 1999 in some local areas and expanded in 2000 to the whole plateau. The black locust (Robinia pseudoacacia L.), an N-fixing tree species, has been widely chosen as a plantation species under this project. The large-scale afforestation activities in the Loess Plateau provided a suitable site for studying the soil C and N changes during afforestation of cropland at the regional scale. The main topics of this study are, (1) to use a paired-site approach to determine the differences in SOC and SIC density in topsoil and subsoil, respectively, between a black locust forest site (converted from cropland 30 years ago) and a cropland site in the middle Loess Plateau; (2) to determine the temporal pattern of the SOC and TN changes following afforestaion in three different climate zones(precipitation gradient 1, from 460 to 580 to 650 mm); (3) to examine SOC and TN changes after the establishment of young (5 to 9 years old) and adult black locust forests (25 to 30 years old) on cropland along a precipitation gradient (precipitation gradient 2, from 380 to 650 mm) across the Loess Plateau; and (4) to estimate the rate of the SOC sequestration for this ‘Grain for Green’ project in Loess Plateau based on the widely collected data across the Plateau, and to compare the SOC sequestration among different restoration types, including grassland, shrub and forest, and compare the effect of restoration types on SOC accumulation among northern, middle and southern regions of the Loess Plateau. The mean results and conclusions as following: 1. In the middle Loess Plateau, we found that soil can accumulate organic C in both topsoil (0-20 cm) and subsoil (30-60 cm) following black locust plantation establishment on cropland. The higher litter input and fine root biomass may partly contribute to the greater SOC in the forest. In addition, the soil N content changed in synchronicity with SOC along the soil profile, which indicates that SOC accumulation in top/subsoil may be determined by the soil N increase in these layers. In contrast, the SIC density in the forest was significantly lower in the plow layer compared to the cropland. However, this decrease of SIC in the topsoil of the forest was offset by an increase in SIC in the subsoil (60-100 cm), suggested that afforestation of cropland can redistribute SIC along the soil profile without affecting the net SIC accumulation. 2. In all three different climate zones (precipitation gradient 1), the surface (0-10 cm) SOC and TN stocks increased linearly with stand age during afforestation. The SOC and TN stock of the lower soil layer (10-20 cm) in the two sites with higher precipitation showed initial decrease in the early 10 years, and then increased with stand age, whereas the subsurface SOC and TN stock in the drier site still increased linearly with stand age. The relative change rate (%) of TN was lower than that of SOC during afforestation, but the temporal pattern was similar between SOC and TN, suggesting a close interaction between them. 3. Across the precipitation gradient (precipitation gradient 2), SOC stock decreased in the 10-20 cm layer in the young forest compared to cropland. The loss of SOC in the 10-20 cm layer increased linearly with precipitation increased from the northern to southern area of the Loess Plateau (R2 =39.1, p<0.01), although the forest aboveground biomass and soil clay content were positively related with precipitation. With increasing plantation age, SOC increased more in the top layer and increased to the SOC level of cropland in the 10-20 cm soil layer. The changes in SOC (in unit of Mg C/ha) were weakly related to precipitation during adult forest establishment. However, the relative SOC changes ((SOC stock forest –SOC stock cropland) / SOC stock cropland, in unit of %) decreased with a precipitation increase (R2 =57.5, p<0.001 and R2 =35.0, p=0.02 for top 10 and 10-20 cm layers, respectively), indicating a greater effect of afforestation on improving soil quality in drier, infertile land. In contrast, the changes in TN (regardless of Mg N/ha or %) were not correlated with precipitation. The relative change rate (%) in TN was low than that of SOC across the precipitation. However, the accumulation rate of SOC was also found to be positively correlated with the accretion rate of soil N, indicating that SOC sequestration may be controlled by soil N accretion at the regional scale, as suggested at the field scale. The soil clay content increased from the northern to southern area of the Loess Plateau and the SOC and TN stock of cropland and forest increased as predicted, but the changes in SOC and TN stock following afforestation did not. This study suggests a weak effect of soil clay content on SOC and TN accumulation at the regional scale. 4. From 2000 to 2008, SOC was found to increase by 14.18 Tg C in the top 20 cm soil layer under this project across the entire Loess Plateau, while the vegetation C (including grassland, shrub and forest) increased by 23.76 Tg C, and the total ecosystem C (soil C and vegetation C) increased by 37.94 Tg C. At the whole Loess Plateau scale, grassland, shrub and forest had a similar SOC accumulation rate of about 0.33 Mg C/ha/a. However, forest had a significantly greater effect on SOC accumulation in the southern Loess Plateau compared with grassland, but had a similar effect in the middle and northern Loess Plateau. There were no differences found in SOC sequestration between shrub and grassland across the entire Loess Plateau. Grassland had a stronger effect on SOC sequestration in the northern Loess Plateau than in the middle and southern regions, whereas forest and shrub respectively had a similar effect on SOC sequestration across the Loess Plateau. Our results suggest that the ‘Grain for Green’ project can significantly increase the SOC storage in Loess Plateau, and it is recommended to expand grassland areas in the northern Loess Plateau and forest in the southern Loess Plateau to enhance the SOC sequestration under this project. In summary, afforestation of cropland can increase significantly the SOC and TN stocks in both topsoil and subsoil in the Loess Plateau. When considering the increase in deep soil SOC following afforestation, the SOC sequestration of the ‘Grain for Green’ project in the Loess Plateau may have be underestimated in present study. The changes in SOC during afforestation are temporal and spatial scale dependent, and the accumulation of SOC is controlled by the accretion of soil TN associated with the large-area N-fixed tree (black locust in this case) plantation in the project, highlighting the importance of species choice in rehabilitation projects when considering soil C sequestration. rate
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/35113
Appears in Collections:城市与区域生态国家重点实验室_学位论文

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Recommended Citation:
常瑞英. 黄土高原退耕还林多尺度土壤固碳效应[D]. 北京. 中国科学院研究生院. 2012.
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