Abstract
AbstractNutrient 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.
Highlights
Nitrogen (N) and phosphorus (P) availability are key nutrient drivers of ecosystem carbon cycling (Coskun et al, 2016; Ellsworth et al, 2017; Fay et al, 2015; Fernández-Martínez et al, 2014; Jiang et al, 2019)
The comparison of N:P ratios between CT, NT, and N + P to another one (NPT) showed that fertilization modified the nutrient stoichiometry of the herbaceous vegetation as expected, with a higher average N:P ratio in the NT plot (9.3 ± 2.5; mean ± s.d.) than in the CT (7.3 ± 0.8) and NPT plots (7.5 ± 2.4) (Figure 3)
Systematic differences in ecosystem responses were detected between NT and NPT as a consequence of nutrient application
Summary
Nitrogen (N) and phosphorus (P) availability are key nutrient drivers of ecosystem carbon cycling (Coskun et al, 2016; Ellsworth et al, 2017; Fay et al, 2015; Fernández-Martínez et al, 2014; Jiang et al, 2019). Current and projected levels of anthropogenic N deposition are expected to cause stoichiometric imbalances in plant-available N and P in several terrestrial ecosystems (Du et al, 2020; Peñuelas et al, 2010, 2013). In such a scenario, plants must adapt to altered and potentially imbalanced nutrient availability (Elser et al, 2010; Güsewell, 2004; Oldroyd & Leyser, 2020; Zhu et al, 2016). It is especially important to understand and characterize how resource-use efficiencies such as WUE are changing with the stoichiometric imbalance and climate change. It is unclear to what degree an imbalance of the N:P ratio at ecosystem level will influence transpiration and WUE or how it is regulated by climate variability (Fernández-Martínez et al, 2014; Jiang et al, 2019; Luyssaert et al, 2014; Peñuelas et al, 2010, 2013)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
