Despite the important role of tropical forest ecosystems in the uptake and storage of atmospheric CO2, carbon (C) dynamics of tropical tree species remain poorly understood, especially regarding belowground roots. This study assessed the allocation of newly-assimilated carbon in the fast-growing pioneer tropical tree species Ceiba pentandra, with a special focus on different root categories. During a 5-day pulse labelling experiment, 9-month-old (~3.5 m tall) saplings were labeled with 13CO2 in a large-scale aeroponic facility, which allowed to trace the label in bulk biomass and in non-structural carbohydrates (sugars and starch) as well as respiratory CO2 from the canopy to the root system, including both woody and non-woody roots. A combined logistic and exponential model was used to evaluate 13C mean transfer time and mean residence time (MRT) to the root systems. We found 13C in the root phloem as early as 2 h after the labelling, indicating a mean C transfer velocity of 2.4 ± 0.1 m h-1. Five days after pulse-labelling, 27% of the tracer taken up by the trees were found in the leaves, 13% were recovered in the woody tissue of the trunk, 6% in the bark and 2% in the root systems, while 52% were lost, most likely by respiration and exudation. Larger amounts of 13C were found in root sugars than in starch, the former also demonstrating shorter MRT than starch. Of all investigated root categories, non-woody white roots (NRW) showed the largest 13C enrichment and peaked in the deepest NRW (2-3.5 m) as early as 24 ± 2 h after labelling. In contrast to coarse woody brown roots (WRC), sink strength of NRW increased with root depth. The findings of this study improve the understanding of C allocation in young tropical trees and provide unique insights into the changing contributions of woody and non-woody roots to carbon sink strengths with depth.

中文翻译：

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在航空设施中使用13C脉冲标记将碳分配给热带树木penta pentandra的根系。

尽管热带森林生态系统在吸收和储存大气CO2中具有重要作用，但对于热带树木物种的碳（C）动态仍然知之甚少，特别是对于地下根。这项研究评估了新近同化碳在快速生长的热带热带树种蓬蓬木（Ceiba pentandra）中的分配，特别侧重于不同的根类别。在为期5天的脉冲标记实验中，在大型航空设施中将9个月大（约3.5 m高）的树苗用13CO2进行了标记，从而可以追踪散装生物质和非结构性碳水化合物（糖）中的标记和淀粉）以及从冠层到根系的呼吸CO2，包括木本和非木本根。使用逻辑和指数组合模型评估13C到根系统的平均转移时间和平均停留时间（MRT）。我们早在标记后2 h就在根韧皮部中发现了13 C，表明C的平均转移速度为2.4±0.1 m h-1。脉冲标记后五天，在树叶中发现了27％被树木吸收的示踪剂，在树干的木质组织中回收了13％，在树皮中回收了6％，在根系中回收了2％，而52 ％的人丢失，最有可能是由于呼吸和渗出。根糖中的13 C含量比淀粉中的含量高，前者的MRT也比淀粉短。在所有调查的根类别中，非木本白根（NRW）表现出最大的13C富集，最早在标记后24±2 h出现在最深的NRW（2-3.5 m）。与粗木本棕根（WRC）相比，NRW的下沉强度随根深的增加而增加。这项研究的发现增进了对年轻热带树木碳分配的理解，并为深入了解木质和非木质根对碳汇强度的变化贡献提供了独特的见解。