【作者】 王晖；

【导师】 刘世荣；

【作者基本信息】 中国林业科学研究院 ， 生态学， 2010， 博士

【摘要】 通过造林、再造林和森林管理提高陆地生态系统的碳固持已被国际社会广泛地接受。造林对提高陆地生态系统碳汇的最直接影响是增加了植被的生物固碳量。而土壤中的碳储量非常大,即使是造林再造林对土壤碳产生较小的改变,也会对整个森林生态系统的净碳积累产生较大的影响。然而,目前有关研究结果表明森林经营管理对土壤碳固持的影响仍有相当大的不确定性。国内外人工林均存在树种单一,特别是人工针叶纯林所占比例较大,出现了生态稳定性较差和生态服务功能低等亟待解决的问题。不同人工林类型会由于自身根系统、冠层结构、凋落物组分和质量的差异而改变土壤化学性质,土壤温度和湿度,以及土壤生物/非生物过程。所以,选择不同的造林树种与造林地内土壤温室气体收支状况密切相关。凋落物叶和细根分解是过去几十年生态学领域的科研工作者始终关注的重要内容,然而人们对凋落物叶和细根分解之间关系的了解十分有限。本研究在广西壮族自治区中国林业科学研究院热带林业实验中心选取了位置邻近的具相似地形、土壤质地、林龄和经营历史的南亚热带四种人工林类型:马尾松人工林(Pinus massoniana)、红锥人工林(Castanopsis hystrix)、火力楠人工林(Michelia macclurei)和米老排人工林(Mytilaria laosensis)。主要采用常规理化实验分析方法、固体13C核磁共振波谱法、静态箱气相色谱法和凋落物分解袋法,研究了:(1)不同树种的凋落物叶、细根、土壤理化性质对土壤有机碳储量和土壤碳化学结构的影响;(2)不同树种土壤温湿度和凋落物叶、细根和土壤生物理化性质对土壤-大气间温室气体交换的影响;(3)马尾松、红锥、火力楠和米老排四个树种凋落物叶和细根(直径小于2 mm)在各自林分下的原位分解速率之间的相关关系,分解过程中氮固定速率之间存在相关关系,以及林下微环境和化学性质对凋落物叶和细根分解过程的影响。本研究的目的是阐明南亚热带人工林土壤碳固持潜力、温室气体排放和凋落物分解的驱动机制,为选择具有更高土壤碳固持潜力的人工造林树种提供科学依据。主要研究结果如下:(1)红锥、火力楠和米老排三种阔叶人工林的0 ~ 10 cm土壤有机碳储量比马尾松人工林高出了11 % ~ 19 %,四种林分间10 ~ 30 cm土壤有机碳储量差异不明显。不同人工林之间的土壤有机碳储量的差异主要归因于细根的输入而不是凋落物叶的输入。红锥、火力楠和米老排三种阔叶人工林的0 ~ 10 cm土壤比对应马尾松人工林土壤具有较低的烷基碳、较高的氧烷基碳和较低的烷基碳/氧烷基碳比值,尽管本研究尚不能完全确定三种阔叶人工林比马尾松人工林多出的土壤碳储量是否最终会通过呼吸作用释放进入大气,但是这表明了三种阔叶人工林的土壤有机碳与马尾松人工林比较而言不够稳定。因此,建议将来的造林树种的选择需要综合考虑土壤碳储量与碳化学结构的潜在变化。(2)马尾松人工林年平均土壤CO2和N2O的排放速率低于任何一种阔叶人工林(红锥、火力楠和米老排),并且马尾松人工林年平均土壤CH4的吸收速率高于任何一种阔叶人工林(红锥、火力楠和米老排)。树种间土壤N2O排放速率差异主要归因于树种间凋落物叶碳氮比和土壤氮储量的差异。树种间土壤CH4吸收速率差异主要归因于树种间土壤呼吸速率和土壤湿度的差异。树种间土壤CO2排放速率差异主要归因于树种间凋落物叶碳氮比的差异。本研究结果说明由马尾松人工林转化为红锥、火力楠和米老排阔叶人工林对减少该地区森林土壤温室气体排放的目标存在负作用。因此,在我国南亚热带地区,马尾松人工林应该与阔叶树人工林平衡发展,将来的造林树种的选择需要考虑不同树种对土壤-大气间温室气体交换的潜在影响。(3)马尾松、红锥、火力楠和米老排四种南亚热带树种中,树种对凋落物叶和细根的原位分解速率的影响非常相似,从而引起树种对整个地上和地下凋落物原位分解速率产生一致的影响。因此,凋落物叶的较快分解对应着细根的较快分解会导致树种之间在凋落物分解速率上的差异更为明显。四个南亚热带树种中凋落物叶和细根分解速率之间明显的正相关关系的主要有以下原因:1)土壤湿度对凋落物叶和细根的分解速率产生相似的影响;2)同时对凋落物叶和细根分解速率产生影响的化学性质表现出明显的相似性;3)其它的初始化学性质各自对凋落物叶和细根产生不同的专一性的影响,由此导致凋落物叶的分解速率与细根的分解速率正相关。四个南亚热带树种中凋落物叶和细根之间的最大N固定速率无明显的相关关系。本结果有助于为南亚热带树种对凋落物原位分解过程中C、N循环产生影响的某些重要机制提供解释。更多还原

【Abstract】 Subtropical China has more than 60 % of the total plantation area in China and over 70 % of these subtropical plantations are composed of pure coniferous species. In view of low ecosystem services and ecological instability of pure coniferous plantations, indigenous broadleaf plantations are being advocated as a prospective silvicultural management for substituting in place of large coniferous plantations in subtropical China. However, little information is known about the effects of tree species conversion on stock and stability of soil organic carbon(SOC). Also, little information is known about the effects of tree species conversion on soil-atmosphere greenhouse gas(GHG)exchanges. Elucidating the processes of leaf litter and fine root decomposition has been a major research focus, while the underlying correlation between leaf litter and fine root decomposition is unclear.The four adjacent monospecific plantations were selected to examine the effects of tree species on the stock and chemical composition of SOC, the effects of tree species on soil-atmosphere exchanges of N2O, CH4 and CO2, and the in situ decomposition and N dynamics of leaf litter and fine root of four subtropical tree species to determine whether leaf litter and fine root decomposition is correlated across species as well as to identify key factors influencing decomposition above versus below ground, by using elemental analysis, solid-state 13C nuclear magnetic resonance ( NMR ) spectroscopy, the static chamber and gas chromatography techniques, and litter/root decomposition bags methods. One coniferous plantation was composed of Pinus massoniana(PM), and the three broadleaf plantations were Castanopsis hystrix(CH), Michelia macclurei(MM), and Mytilaria laosensis(ML). The main results are as follows:(1)SOC stock differed significantly among the four plantations in the upper(0 ~ 10 cm)layer, but not in the underneath(10 ~ 30 cm)layer. SOC stocks in the upper(0 ~ 10 cm)layer were 11 %, 19 % and 18 % higher in the CH, MM and ML plantations than in the PM plantation. The differences in SOC stock among the four plantations were largely attributed to fine root input rather than aboveground litterfall input. However, the soils in the broadleaf plantations contained more decomposable C proportion, indicated by lower percentage of alkyl C, higher percentage of O-alkyl C and lower alkyl C/O-alkyl C ratio compared to those in the PM plantation. Our findings highlight that future strategy of tree species selection for substituting in place of large coniferous plantations in south subtropical China needs to consider the potential effects of tree species on the chemical composition of SOC in addition to the quantity of SOC stock.(2)The mean soil N2O and CO2 emissions in the PM plantation were 4.3μg N m-2 h-1 and 43.3 mg C m-2 h-1, respectively, lower than those in the broadleaf plantations. The mean CH4 uptake in the PM plantation soil was higher than that in the broadleaf plantation. Variations in soil N2O emissions among tree species could be primarily explained by the differences in litter C:N ratio and soil total N stock. Differences in soil CH4 uptake among tree species could be mostly attributed to the differences in mean soil CO2 flux and water filled pore space(WFPS). Litter C:N ratio could largely accounted for variations in soil CO2 emissions among tree species. This study indicated that there was no GHG benefit of converting PM plantation to broadleaf plantations in south subtropical China. Therefore, the future strategy of tree species selection for substituting in place of large coniferous plantations in south subtropical China needs to consider the potential effects of tree species on soil-atmosphere GHG exchanges.(3)Decomposition rate of leaf litter was related to that of fine root across species. The strong correlation between leaf litter and fine root decomposition rates accounted largely for the following several reasons. First, soil moisture had the similar influences on both leaf litter and fine root decomposition rates. Second, traits(i.e., initial Ca concentration)important to both leaf litter and fine root decomposition rates showed significant similarity among species. Third, initial P, N and aromatic C concentrations, and C/N ratio were uniquely important for fine root decomposition rate, while no unique traits for leaf litter decomposition rate. This also could account for the strong correlation between leaf litter and fine root decomposition rates. Our study suggested that among these south subtropical trees, species effects on in situ decomposition rates of leaf litter and fine root were very similar. Thus, species differences in decomposition rates may be as large as they would be if faster decomposition of leaf litter was correlated with faster decomposition of fine root. N immobilization rate of leaf litter was not related to that of fine root across species. Our results contributed to the understanding of some important mechanisms by which tree species influence litter in situ decomposition.更多还原