基于根窗法树木细根寿命的长期观测研究

Fine roots turnover is a crucial part of the terrestrial carbon and nutrient cycle. However, fine root longevity as a key parameter for estimating fine root turnover rate is still difficult to determine. There is currently an ongoing debate on the efficacy of methods that measure carbon residence time in fine roots using 13C-depleted atmospheric enrichment versus methods that observe roots directly by means of minirhizotrons. Published estimates of fine root longevity (or turnover time) differ more than tenfold. Since fine roots are increasingly recognized as a key to balancing whole-tree and ecosystem carbon budgets, such large discrepacies in root longevity and turnover estimates lead to uncertainty in assessing terrestrial carbon cycles. . There is a clear need for in-depth investigations of methodology for determining fine root longevity. Our proposed research will address this need and answer the following questions: (1) what is the relationship between fine root longevity and root branching order? Furthermore, dose this relationship change with different sites and species? (2) How to reconcile the widely discrepancies among methods? (3) What are the fine root longevity distribution patterns of different branching orders? . To answer these questions, we will conduct a field experiment using rhizotron technique in combination with 14C isotope methods. Fine root longevity will be measured based on branching order of four tree species (i.e. Larix spp., Pinus koraiensis, Fraxinus mandschurica, Quercus mongolica) in Northeastern China. In addition to field experiment, we also conduct a root boxes experiment in greenhouse using a unique high abundance 13C tracer method which will provide parameter and scientific basis for estimating root longevity in field experiment. The results from the innovative experiment will lead to significant advances in our understanding the relationship between root architecture and functions. By generating empirical functions, the results also will be useful for belowground carbon cycle modeling, thereby impacting other areas of science.

细根（通常直径<2mm）死亡和周转在陆地生态系统碳分配格局和养分循环过程中发挥重要作用。然而，估计周转的关键参数细根寿命却难以准确测定，不同方法间得出的细根寿命相差可达10倍以上。如此之大的误差范围，严重制约了我们对地下碳源碳汇功能的评估，以及对全球气候变化的预测能力。本项目针对树木细根寿命准确测定这一问题和难题，从根系复杂分枝入手，拟以东北地区的落叶松、红松、水曲柳和蒙古栎林分为研究对象，以根窗法为主并结合14C同位素法对野外固定样地进行长期连续观测，揭示树木不同根级尤其高级根的寿命及其主要影响因子。同时，首创性地采用室内高丰度13C示踪试验，为两种方法准确估计细根寿命提供支撑，最终阐明东北树木细根结构（根序）与功能（寿命）的关系。本项目将为准确量化根系向土壤的碳氮输入提供关键参数和理论依据。

树木的细根与叶片类似，处于不断的生长、衰老、死亡的周转过程之中，称之为细根周转。目前不同方法间得出的细根周转速率测值相差可达十倍以上，严重制约了我们对地下碳循环过程的评估以及模拟预测能力。本项目主要针对树木细根周转（寿命）准确测定这一问题和难题开展了研究。采用野外原位根窗法为主，并结合碳同位素法，从根系复杂分枝（根序）入手，深入探索了我国东北温带典型四个树种的细根构型、生产和周转。获得的主要结果如下：. 1）建立了东北四个主要树种（落叶松、水曲柳、蒙古栎和红松）根系属性与完整根序的函数关系。其中，根系直径和根长随根序（1~9级）的增加而呈指数函数增加，而比根长随根序的增加而呈指数函数下降。由此可见，根序是决定根系属性的重要因子。此外，初步揭示了南北两个地点（黑龙江帽儿山和辽宁清原）四个树种细根周转的空间格局。. 2）发现落叶松1级根的寿命（284天）与微根管的文献数据差别不大，而高级根的寿命可生存4年以上，与目前碳同位素法估计的结果接近。该数据为调和微根管法与同位素法细根周转测值之间的矛盾，提供了第一手资料。因此，原位大根窗法分根序来研究树木细根动态，可提高细根周转测值的准确性。. 3）基于长期的根窗观测数据，阐明了落叶松细根年际和季节的变异规律。其细根年生产量和年死亡量年际间差异分别达9.6倍和12.7倍。并且，细根生产呈单峰型（顶点，早夏），而死亡高峰发生在早春和早秋呈不对称的双峰型。这些结果例证了树木细根周转的时间格局，树木细根周转研究长期观测可减小细根周转估计的不确定性。. 综上，本项目应用改进的根窗法，通过长期观测从根系分枝（根序）的角度，系统研究了森林树木细根周转的时空格局。本项目结果为准确评估树木细根周转测值提供了重要的资料，还为提高东北森林的生产力方面具有重要的现实意义。