How Nitrogen and Phosphorus Availability Change Water Use Efficiency in a Mediterranean Savanna Ecosystem
Creators: name: El‐Madany, Tarek S.
name: Reichstein, Markus
name: Carrara, Arnaud
name: Martín, M. Pilar
name: Moreno, Gerardo
name: Gonzalez‐Cascon, Rosario
name: Peñuelas, Josep
name: Ellsworth, David S.
name: Burchard‐Levine, Vicente
name: Hammer, Tiana W.
name: Knauer, Jürgen
name: Kolle, Olaf
name: Luo, Yunpeng
name: Pacheco‐Labrador, Javier
name: Nelson, Jacob A.
name: Perez‐Priego, Oscar
name: Rolo, Victor
name: Wutzler, Thomas
name: Migliavacca, Mirco
Publisher: name: FID GEO
Year: 2021

description: 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.
description: Plain Language Summary: The availability of nutrients like nitrogen (N) and phosphorus (P) is important for every living organism on Earth. Due to human activities, especially combustion processes large amounts of N are transported into the atmosphere and ecosystems. Therefore, ecosystems receive additional N but no other nutrients. We are investigating if the addition of N alone will lead to deficits in other nutrients and thus impact the functioning of ecosystems. Hence, we set up a large‐scale ecosystem experiment in a Mediterranean tree‐grass ecosystem where we fertilized two plots with N (16.9 ha) and N + P (21.5 ha). A third plot served as the control treatment. While the N‐only treatment created an imbalance between the available N and P, this imbalance was relieved in the N + P treatment where both N and P were provided. Our measurements showed that both fertilized treatments increased their carbon uptake and turned the ecosystem from a carbon source to carbon neutral. One of the main differences between the fertilized treatments which is associated with the imbalance of available N and P is the loss of water through the vegetation (transpiration). This increase in transpiration was only observed in the N‐only but not in the N + P treatment. Our results show, that the N:P stoichiometric imbalance, resulting from N‐only addition, increases the water‐use efficiency (i.e., the carbon gain per water loss) less than the addition of N + P, due to the strong increase in transpiration at the N‐only treatment.
description: Key Points: Stoichiometric N:P‐ratio imbalance increases ecosystem transpiration. High nitrogen availability increases carbon uptake and changed the ecosystem from a carbon source to carbon neutral. Ecosystem scale functional relationships are altered through nutrient availability and imbalance.
description: Ministerio de Economía y Competitividad http://dx.doi.org/10.13039/501100003329
description: Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946