Nutrient availability, especially of nitrogen (N) and phosphorus (P), is of major importance for every organism and at a larger scale for ecosystem functioning and productivity. Changes in nutrient availability and potential stoichiometric imbalance due to anthropogenic nitrogen deposition might lead to nutrient deficiency or alter ecosystem functioning in various ways. In this study, we present 6 years (2014–2020) of flux‐, plant‐, and remote sensing data from a large‐scale nutrient manipulation experiment conducted in a Mediterranean savanna‐type ecosystem with an emphasis on the effects of N and P treatments on ecosystem‐scale water‐use efficiency (WUE) and related mechanisms. Two plots were fertilized with N (NT, 16.9 Ha) and N + P (NPT, 21.5 Ha), and a third unfertilized plot served as a control (CT). Fertilization had a strong impact on leaf nutrient stoichiometry only within the herbaceous layer with increased leaf N in both fertilized treatments and increased leaf P in NPT. Following fertilization, WUE in NT and NPT increased during the peak of growing season. While gross primary productivity similarly increased in NT and NPT, transpiration and surface conductance increased more in NT than in NPT. The results show that the NPT plot with higher nutrient availability, but more balanced N:P leaf stoichiometry had the highest WUE. On average, higher N availability resulted in a 40% increased leaf area index (LAI) in both fertilized treatments in the spring. Increased LAI reduced aerodynamic conductance and thus evaporation at both fertilized plots in the spring. Despite reduced evaporation, annual evapotranspiration increased by 10% (48.6 ± 28.3 kg H₂O m⁻²), in the NT plot, while NPT remained similar to CT (−1%, −6.7 ± 12.2 kgH₂O m⁻²). Potential causes for increased transpiration at NT could be increased root biomass and thus higher water uptake or rhizosphere priming to increase P‐mobilization through microbes. The annual net ecosystem exchange shifted from a carbon source in CT (75.0 ± 20.6 gC m⁻²) to carbon‐neutral in both fertilized treatments [−7.0 ± 18.5 gC m⁻² (NT) 0.4 ± 22.6 gC m⁻² (NPT)]. Our results show, that the N:P stoichiometric imbalance, resulting from N addition (without P), increases the WUE less than the addition of N + P, due to the strong increase in transpiration at NT, which indicates the importance of a balanced N and P content for WUE.

中文翻译：

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地中海稀树草原生态系统中氮和磷的可用性如何改变水的利用效率

营养物质的可用性，特别是氮（N）和磷（P）的可用性，对每种生物都至关重要，并且对生态系统功能和生产力具有更大的规模。由于人为氮沉积而导致的养分利用率和潜在的化学计量失衡的变化可能导致养分缺乏或以各种方式改变生态系统的功能。在本研究中，我们提供了来自地中海稀树草原型生态系统的大规模养分操纵实验的6年（2014-2020年）通量，植物和遥感数据，重点研究了氮和磷的影响。生态系统规模用水效率（WUE）和相关机制的处理方法。用N（NT，16.9 Ha）和N + P（NPT，21.5 Ha）施肥两个地块，第三个未施肥的地块作为对照（CT）。施肥仅在草本层内对叶片养分的化学计量有很大影响，在两种施肥处理中，叶片氮含量均增加，而在NPT中，叶片磷含量均增加。施肥后，NT和NPT的WUE在生长季节的高峰期增加。尽管NT和NPT的总初级生产力类似地增加，但NT上的蒸腾作用和表面电导比NPT更大。结果表明，NPT图具有较高的养分利用率，但N：P叶片化学计量比更为平衡，其WUE最高。平均而言，较高的氮利用率导致春季两种施肥处理的叶面积指数（LAI）增加40％。LAI的增加降低了空气传导率，从而降低了春季两个施肥地块的蒸发。尽管蒸发减少，但年蒸散量增加了10％（48.6±28。在NT图中为₂ O m ^-2），而NPT仍然与CT相似（-1％，-6.7±12.2 kgH ₂ O m ^-2）。在NT蒸腾作用增加的潜在原因可能是根系生物量增加，因此较高的吸水率或根际启动作用会增加通过微生物的P迁移。在两种施肥处理中，每年的生态系统净交换量都从CT的碳源（75.0±20.6 gC m ^-2）变为碳中性[-7.0±18.5 gC m ^-2（NT）0.4±22.6 gC m ^-2（NPT）]。我们的结果表明，由于NT处蒸腾量的强烈增加，由添加N（不含P）导致的N：P化学计量失衡增加的WUE小于添加N + P的WUE，这表明平衡的重要性WUE的N和P含量。