Changes in leaf nitrogen and phosphorus content from observations and a land surface model: evidence for increasing nutrient imbalance in Europe

2021 ◽  
Author(s):  
Silvia Caldararu ◽  
Katrin Fleischer ◽  
Lin Yu ◽  
Sönke Zaehle
Keyword(s):  
Nutrient Limitation ◽  
Nutrient Availability ◽  
Land Surface ◽  
Land Surface Model ◽  
Nutrient Content ◽  
N Deposition ◽  
Surface Model ◽  
Leaf Habit ◽  
Anthropogenic Nitrogen ◽  
Leaf N

<p>Increasing atmospheric CO<sub>2</sub> concentrations can be a driver for higher ecosystem productivity across the globe but nutrient availability may limit subsequent biomass growth. Concurrently, increased anthropogenic nitrogen (N) deposition introduces a relatively large amount of N into the system, thus potentially alleviating N limitation. However, this new N input could push ecosystems into being limited by other resources, most importantly phosphorus (P) in mid- and high-latitude systems, leading to what has been termed an NP imbalance. While the ecological theory behind the processes described above has been discussed on many occasions, it is yet unclear what the actual spatial and temporal patterns of such an imbalance are, as well as the ecpological processes and drivers behind such observed patterns.</p><p>Here, we use leaf N and P data from a large European monitoring network, ICP forests, in conjunction with a land surface model, QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system), to explore the patterns and drivers behind nutrient limitation at European forest sites. The overall trend in observed leaf N and P content as well as N:P ratio show an increasing nutrient limitation from 1990 to 2015, as well as a shift towards P limitation. However, the observed spatial patterns of change in leaf nutrient content vary strongly with soil nutrient availability, N deposition and leaf habit. The effect of leaf habit suggests that leaf growth strategies  play an important role in dealing with nutrient availability and controlling observed ecosystem responses. </p><p>We use the QUINCY model to explore the drivers behind the observed leaf nutrient trends. We perform simulations with fixed levels of atmospheric CO<sub>2</sub> as well as in the absence of anthropogenic nitrogen deposition. We show that the decrease in leaf N and P content is attributable to increased atmospheric CO<sub>2,</sub> while the changes in N:P stoichiometry are reproducible with increased N deposition. Additionally, the model can only predict observed trends when representing physiologically-realistic responses of leaf stoichiometry to nutrient availability. The use of a process-based model allows us to attribute drivers to the observed changes in leaf nutrient content. This research helps the development of data-constrained, process-based models which can potentially be used to predict changes in ecosystem nutrient limitation, and implicitly growth and carbon storage, under future scenarios</p>

scholarly journals Long-term ecosystem nitrogen limitation from foliar δ15N data and a land surface model

2021 ◽  
Author(s):  
Silvia Caldararu ◽  
Tea Thum ◽  
Lin Yu ◽  
Melanie Kern ◽  
Richard Nair ◽  
...  
Keyword(s):  
Land Surface ◽  
Nitrogen Limitation ◽  
Land Surface Model ◽  
N Deposition ◽  
Spatial And Temporal Variation ◽  
Surface Model ◽  
Terrestrial Carbon ◽  
N Content ◽  
N Limitation ◽  
Leaf N

The effect of nutrient availability on plant growth and the terrestrial carbon sink under climate change and elevated CO2 remains one of the main uncertainties of the terrestrial carbon cycle. This is partially due to the difficulty of assessing nutrient limitation at large scales over long periods of time. Consistent declines in leaf nitrogen (N) content and leaf δ15N have been used to suggest that nitrogen limitation has increased in recent decades, most likely due to the concurrent increase in atmospheric CO2. However, such datasets are often not straightforward to interpret due to the complex factors that contribute to the spatial and temporal variation in leaf N and isotope concentration. We use the land surface model QUINCY, which has the unique capacity to represent N isotopic processes, in conjunction with two large datasets of foliar N and N isotope content. We run the model with different scenarios to test whether foliar δ15N isotopic data can be used to infer large scale nitrogen limitation and if the observed trends are caused by increasing atmospheric CO2, changes in climate or changes in sources of anthropogenic N deposition. We show that while the model can capture the observed change in leaf N content and predicts widespread increases in N limitation, it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ15N observed in the data across the globe. The addition of an observed temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ15N. Our results show that leaf δ15N observations should not, on their own, be used to assess global scale N limitation and that using such a dataset in conjunction with a land surface model can reveal the drivers behind the observed patterns.

Disentangling the long-term foliar 15N signal using a land surface model

2020 ◽  
Author(s):  
Silvia Caldararu ◽  
Tea Thum ◽  
Richard Nair ◽  
Sönke Zaehle
Keyword(s):  
Nutrient Limitation ◽  
Land Surface ◽  
Nitrogen Availability ◽  
Land Surface Model ◽  
Ecosystem Dynamics ◽  
Surface Model ◽  
Terrestrial Vegetation ◽  
Climate System Model ◽  
Foliar N

<p>Terrestrial vegetation growth is hypothesised to increase under elevated atmospheric CO<sub>2</sub>, a process known as the CO<sub>2</sub> fertilisation effect. However, the magnitude of this effect and its long-term sustainability remains uncertain. One of the main limitations to the CO2  fertilisation effect is nutrient limitation to plant growth, in particular nitrogen (N) in temperate and boreal ecosystems. Recent studies have suggested that decreases in observed foliar N content (N%) and δ<sup>15</sup>N indicate widespread nitrogen limitation with increasing CO<sub>2</sub>  concentrations. However, the factors driving these two variables, and especially the foliar δ<sup>15</sup>N values, are complex and can be caused by a number of processes. On one hand, if the observed trends reflect nutrient limitation, this limitation can be caused by either CO<sub>2</sub> or warming driven growth. On the other hand, it is possible that nutrient limitation does not occur to its full extent due to plant plastic responses to alleviate nutrient limitation, causing a decrease in N%, but changes in the anthropogenic N deposition 15N signal cause the observed δ<sup>15</sup>N trend. In reality, it is likely that all these factors contribute to the observed trends. To understand ecosystem dynamics it is important to disentangle the processes behind these signals which is very difficult based on observational datasets only.</p><p>We use a novel land surface model to explore the causes behind the observed trends in foliar N% and δ<sup>15</sup>N. The QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system) model  has the unique capacity to track ecologically relevant isotopic composition, including <sup>15</sup>N in plant and soil pools. The model also includes a realistic representation of plant plastic acclimation processes, specifically a representation of nitrogen allocation to and inside the canopy in response to nitrogen availability, so implicitly to changes in CO<sub>2 </sub> concentrations. We test the different hypotheses above behind the observed changes in N% and δ<sup>15</sup>N separately and quantify the contribution of each of the factors towards the observed trend. We then test the different hypotheses against existing observations of N% and δ<sup>15</sup>N from the ICP Forests database and other published datasets such as the global dataset of Craine et al. 2018.</p><p>Our study showcases the use of an isotope-enabled land surface model in conjunction with long-term observations to strengthen our understanding of the ecosystem processes behind the observed trends.</p>

L'apport de la télédétection spatiale à la modélisation des surfaces continentales

2020 ◽  
pp. 052
Author(s):  
Jean-Christophe Calvet ◽  
Jean-Louis Champeaux
Keyword(s):  
Satellite Data ◽  
Land Surface ◽  
Land Surface Model ◽  
Static Model ◽  
Geographic Information ◽  
Surface Model ◽  
Model Parameters

Cet article présente les différentes étapes des développements réalisés au CNRM des années 1990 à nos jours pour spatialiser à diverses échelles les simulations du modèle Isba des surfaces terrestres. Une attention particulière est portée sur l'intégration, dans le modèle, de données satellitaires permettant de caractériser la végétation. Deux façons complémentaires d'introduire de l'information géographique dans Isba sont présentées : cartographie de paramètres statiques et intégration au fil de l'eau dans le modèle de variables observables depuis l'espace. This paper presents successive steps in developments made at CNRM from the 1990s to the present-day in order to spatialize the simulations of the Isba land surface model at various scales. The focus is on the integration in the model of satellite data informative about vegetation. Two complementary ways to integrate geographic information in Isba are presented: mapping of static model parameters and sequential assimilation of variables observable from space.

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