How Nitrogen and Phosphorus Availability Change Water Use Efficiency in a Mediterranean Savanna Ecosystem

Abstract

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.

Predicting Characteristics of the Water Cycle From Scaling Relationships

“…Our interpretation is that precipitation that arrives out‐of‐phase with the maximum in irradiance is not conducive to an optimal use of water by plants, but with increased water waste, here noted as evaporative losses. If this is correct, it is a result that is not in accord with current thinking (Berghuijs et al., 2014) regarding the role of seasonality of precipitation, though it is consistent in a more general sense with a recent study (Madany et al., 2021; discussed in Eos by Thompson [2021]) that showed a diminution of water use efficiency (WUE) when resources are scarce. In further support of this diagnosis, we find that non‐seasonal drainages fall equally on both sides of the 3D prediction.…”

Ideas and perspectives: Beyond model evaluation – combining experiments and models to advance terrestrial ecosystem science

“…1), they are purely observational measurements and lack the specific advantages of an EME. A small number of EC sites do use a treatment, albeit on an unreplicated treatment scale (El-Madany et al, 2021;Zhao et al, 2022). EC experiments are laborious to construct and expensive to operate in tall stature ecosystems, and even in shorter ecosystems the footprint of an EC tower is a challenge for application of many global change relevant treatments.…”

Monitoring nature's calendar from space: Emerging topics in land surface phenology and associated opportunities for science applications

“…This leads to the so‐called N‐P imbalance and stoichiometry that are expected to have large impacts on ecosystem properties and dynamics of carbon and plant growth (Janssens et al, 2010 ; Nair et al, 2019 ). One landscape‐scale nutrient manipulation study in the Mediterranean tree‐grass ecosystem that involves the use of eddy‐covariance flux towers, phenocams, and satellite observations illustrated that nitrogen‐added treatment would accelerate the senescence rate and advance phenocam/satellite detected EOS compared with N:P balanced treatments (El‐Madany et al, 2021 ; Luo et al, 2020 ). This was attributed to the fact that soil water depleted more rapidly in the nitrogen‐added treatment during the dry‐down period due to enhanced leaf biomass production (Luo et al, 2020 ).…”