落叶松人工林细根周转的纬度分异规律及其内在机制研究

Fine roots, which are short-lived, non-woody, small-diameter roots associated with mycorrhizae, are the most important part contributing to soil organic matter in forest ecosystems. Fine roots growth, death and subsequent decomposition of the dead roots , known as root turnover, is a critical component of ecosystem nutrient dynamics and carbon sequestration and is also an important sink for plant primary productivity. Root turnover has been shown to be sensitive to many of the factors considered in global change analyses. Although it is important to understand the mechanisms controlling root longevity and turnover for an individual root or plant, it is also desirable to identify broad scale patterns in root turnover among plant functional types and across large climatic gradients. Understanding carbon allocation processes belowground and predicting the response of roots to global change requires mechanistic information about broad scale patterns in root turnover and its controlling factors across large climatic gradients. However, fine root turnover is still poorly understood, especially lacking large-scale field research on allocation pattern of belowground carbon. Therefore, severely restricts us to recongnize and quantify belowground carbon flux..There is a clear need for in-depth investigations of root turnover pattern at large-scale. Our proposed research will address this need and answer the following questions: (1) what is the pattern of root turnover along an environmental conditions? (2) What is the relationship between root turnover and key variables which control and influence root turnover? (3) Does different branch order roots response the same to the changing environmental conditions? To answer these questions, we will conduct a field experiment using rhizotron technique in combination with soil coring methods. .Four Larch plantations with mean air tempreture from -1.5℃ to 4.6℃ and annual precipetation from 490mm to 800mm at different latitudes (N41 51′-N50 19′) in Northeastern China will be selected. Factors controlling root turnover will be measured and analyzed. The quantitative relationship between root turnover rate and environmental controlling varibles will also be obtained by statistical analysis. The results from the innovative experiment will lead to significant advances in our understanding of root turnover pattern and mechanism at large-scale. By generating empirical functions, the results also will be useful for large-scale models of ecosystem C flux, thereby impacting other areas of science.

根系，尤其细根是森林生态系统土壤有机碳库的重要来源，因此细根周转在调节森林生态系统碳和养分循环中具有重要作用。对大尺度细根周转规律及其内在机制的研究，是理解与量化地下碳分配格局的基础，尤其在全球气候变化背景下，对预测森林地下碳动态具有重要意义。然而，大尺度上的细根周转规律目前尚不清楚，采用同一种方法、针对同一树种的细根周转区域格局的研究还未见报道。因此，我们以我国东北地区广泛栽培的落叶松人工林为研究对象，选取分布在年均温从-1.5-4.6 ℃、年降水从490-800 mm的4个纬度梯度上的落叶松群落，采用原位的根窗法为主，并结合原状土柱法，探索森林细根周转的纬度分异规律，揭示细根周转与环境因子（温度、水分和光照等）和生物因子（根分枝等级和根化学成分等）之间的关系。为阐明森林细根周转的区域格局、预测气候变化条件下森林土壤碳动态及碳模型模拟森林生态系统地下过程提供理论依据和参数。

细根是地下生态过程中最活跃的组份，其处于不断的生长、衰老、死亡和分解动态之中（称之为细根周转）。据粗略估计，细根周转过程每年消耗全球净初级生产力（NPP）的30%以上。通过细根周转进入土壤中的碳和营养元素是地上凋落物的一至数倍。然而，由于细根观测和取样的困难，使得我们对细根周转过程的认知还十分有限。大量个体研究表明，细根周转几乎受研究者考察的所有因子的影响。全球尺度的综合分析表明，细根周转与年均气温和年降水量具有一定的关系。目前，我们对大尺度下细根周转格局及其影响因子的了解最薄弱，这严重限制了我们对地下碳过程的预测能力。.本项目在较大的时空尺度上（5个纬度梯度样地），针对细根周转格局这一重要科学问题，以落叶松人工林为研究对象，采用根窗法（Rhizotrons）对树木细根周转过程进行原位、连续和长期的观测，来阐明落叶松细根周转的纬度分异规律，并揭示细根周转与环境因子之间的关系。.结果表明：1）落叶松1~5级根，直径和根长随根序增加呈指数增加，比根长呈指数下降。然而，不同纬度间落叶松1~5级根，直径、长度未表现出明显的趋势，但在单个地点（如，伊春），细根直径和长度随土层加深而逐渐减小。2）落叶松细根生产力，未随纬度梯度表现出明显的趋势，与年均气温和年降水量不相关，但与土壤碳氮含量显著负相关，与林分细根现存量显著正相关。细根生产具有明显的季节格局，总体表现为双峰型，其中6-7月细根生产最多达到顶峰，9月左右还有一次小峰。3）落叶松细根中值寿命，随纬度下降而减小，即从北往南细根周转速率逐渐加快，并且细根中值寿命与各地年均气温显著负相关。落叶松1~3级根，中值寿命随根序增加而指数增加（如关帝山，y=136.6ex，R2=0.963）。此外，本研究发现落叶松高级根寿命很长（3级根>3年），接近同位素法的测值，因此，长期根窗观测可为调和不同方法间细根寿命测值的差距提供机会。.综上，本研究发现，落叶松细根形态或构型主要受树种遗传控制，外界环境在微立地条件下，对根系具有一定的可塑性。与地上相比，地下生产力格局更加复杂，受立地条件和林分因子影响较大，目前可用细根现存量进行预测。本研究首次用试验资料，揭示了树木细根周转的纬度格局，并建立了细根周转与年均气温的函数关系，结果加深对区域水平上细根周转机制的了解，为模型模拟地下碳过程提供宝贵的资料。