• Nitrogen (N) loading and extreme drought strongly alter biomass production, species composition and carbon and water fluxes of temperate grasslands. Such changes at the community level are often attributed to species‐ and functional group‐specific responses in phenology and/or physiology.
• In a multifactorial field experiment, we studied the responses of three abundant grassland species (forb Centaurea jacea, grasses Arrhenatherum elatius and Dactylis glomerata) to N loading and extreme drought, focusing on responses of carbon and water relations at the leaf level. We analysed (1) changes in bulk leaf N (uptake efficiency of additional N), (2) adaptation of plant water status (leaf water potential) and (3) impact on leaf carbon and water fluxes.
• We observed more efficient N utilization in the two grasses compared to C. jacea. Naturally occurring summer drought significantly impacted the plant water status of all species, while extreme drought treatment only further affected water status during and after summer drought. C. jacea was able to maintain much lower leaf water potentials compared to the grasses during drought. Despite these clear species‐specific responses to N loading and drought, the species were able to maintain homeostasis of leaf carbon and water fluxes.
• Thus, strong declines in the (community) carbon sequestration observed at this site during the (natural) summer drought were not related to leaf physiological responses in assimilation, but were driven by phenological adaptions of the species community: the drought‐sensitive grasses, even though exhibiting higher N uptake efficiency, responded with a shortened life cycle to severe summer drought.

中文翻译：

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试验性干旱和氮素组合负荷：温带草原中碳和水交换的物种依赖性叶水平控制的作用

• 氮负荷和极端干旱强烈改变了温带草原的生物量生产，物种组成以及碳和水通量。社区一级的此类变化通常归因于物候和/或生理学中物种和功能群特定的响应。
• 在多因素的田间试验，研究了三种丰富的草原物种的响应（杂类草矢车菊jacea，草燕麦草和鸭茅），以氮负荷和极端干旱，重点叶级碳和水的关系的反应。我们分析了（1）散叶N的变化（附加N的吸收效率），（2）植物水分状况（叶水势）的适应性，以及（3）对叶片碳和水通量的影响。
• 与C相比，我们观察到两种草中的N利用率更高。jacea。自然发生的夏季干旱会严重影响所有物种的植物水分状况，而极端干旱处理只会进一步影响夏季干旱期间和之后的水分状况。Ç。与干旱期间的草相比，jacea能够保持低得多的叶片水势。尽管对氮负荷和干旱有明确的物种特异性反应，但该物种仍能够保持叶片碳和水通量的稳态。
• 因此，在夏季（自然）夏季干旱期间，在该地点观察到的（社区）碳固存的大幅下降与同化过程中的叶片生理反应无关，而是由物种群落的物候适应引起的：干旱敏感的草，甚至是尽管表现出较高的氮吸收效率，但对严重的夏季干旱却缩短了生命周期。