Nitrogen Cycling Dynamics in Sarracenia Purpurea at Harvard Forest 2004-2005

In nutrient poor systems, plants employ many strategies in order to acquire and recycle scarce nutrients, including nitrogen. Low leaf N content is associated with low photosynthetic rates, but carnivorous plants have unusually low photosynthetic rates given their N content. The northern pitcher plant Sarracenia purpurea readily uses any available nitrogen: NH4 and NO3 dissolved in precipitation; N mineralized from captured prey; the scant N in saturated peat; and N remobilized from storage. However, the dynamics of N cycling within S. purpurea are poorly understood. We conducted two greenhouse experiments to examine N-cycling dynamics of S. purpurea at the whole-plant and individual-leaf levels. In the first experiment we assessed assimilation, translocation, storage, and remobilization of 15N supplied to pitchers and roots. In the second experiment, we examined how 15N assimilated by the first pitcher produced at the start of the growing season contributed to the production and maintenance of subsequent pitchers, roots, and rhizomes. Patterns of N cycling were similar at the individual-leaf and whole-plant level. Pitchers assimilated 55 - 69% of available 15N and served both as the largest sink for newly assimilated N (more than 90% of the 15N assimilated during 2004) and the largest source of N remobilization the following spring. In contrast, N assimilated by roots was low and accounted for less than 2.5% of the overall S. purpurea N budget. S. purpurea uses both stored N and newly-acquired N throughout the growing season. The importance of stored N decreases throughout the growing season as newly assimilated N contributes more to later pitcher production.

在养分匮乏的生态系统中，植物会演化出多种策略以获取并循环利用包括氮素在内的稀缺营养元素。叶片低氮含量通常伴随光合速率降低，但食肉植物在自身氮含量水平下的光合速率却异常偏低。紫瓶子草（Sarracenia purpurea）可高效利用各类可获取氮源：降水中溶解的铵态氮（NH4）与硝态氮（NO3）、从捕获猎物中矿化得到的氮、饱和泥炭中极少量的氮，以及从储存组织中再动员的氮。然而，目前学界对紫瓶子草体内的氮循环动态仍知之甚少。
本研究开展两项温室实验，分别从全植株与单叶两个尺度探究紫瓶子草的氮循环动态。第一项实验中，我们评估了施加于捕虫瓶与根系的15N的同化、转运、储存及再动员过程。第二项实验则考察了生长季初期萌发的首个捕虫瓶所同化的15N，对后续萌发的捕虫瓶、根系及根状茎的生成与维持的贡献。
氮循环模式在单叶与全植株尺度下表现一致。捕虫瓶可同化55%~69%的可利用15N，既是新同化氮的最大储存库（2004年实验中，其吸收的15N占总同化量的90%以上），也是次年春季氮再动员的最大来源。与之形成鲜明对比的是，根系同化的氮占比极低，仅占紫瓶子草整体氮收支的2.5%以下。紫瓶子草在整个生长季同时依赖储存氮与新获取的氮；随着生长推进，新同化氮对后续捕虫瓶生成的贡献逐步提升，储存氮的重要性则随之降低。