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.

在养分贫瘠的生态系统中，植物会采取多种策略以获取并循环利用包括氮（nitrogen）在内的稀缺养分。叶片低氮含量通常与低光合速率相关，但食虫植物相对于其氮含量而言，光合速率异常偏低。紫瓶子草（Sarracenia purpurea）可高效利用各类可获取的氮源：溶解于降水中的铵态氮（NH4）与硝态氮（NO3）、从捕获猎物中矿化得到的氮、饱和泥炭中的微量氮，以及从储存组织中再活化的氮。然而，目前学界对紫瓶子草体内的氮循环动态仍知之甚少。

我们开展了两项温室试验，以探究紫瓶子草在整株与单叶尺度下的氮循环动态。第一项试验中，我们评估了供给至瓶状体与根系的氮15（15N）的同化、转运、储存与再活化过程。第二项试验则探究了生长季初期首个形成的瓶状体所同化的氮15，如何对后续瓶状体、根系与根状茎的生成与维持产生贡献。

氮循环模式在单叶与整株尺度下表现一致。瓶状体可同化55%~69%的可获取氮15，同时既是新同化氮的最大储存库（占2004年期间同化氮15总量的90%以上），也是次年春季氮再活化的最大来源。与之相对，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中同时利用储存氮与新获取的氮。随着新同化氮对后续瓶状体生成的贡献逐步提升，储存氮的重要性在整个生长季中逐渐下降。