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Quantifying the effects of interacting nutrient cycles on terrestrial biosphere dynamics and their climate feedbacks QUINCY

Periodic Reporting for period 4 - QUINCY (Quantifying the effects of interacting nutrient cycles on terrestrial biosphere dynamics and their climate feedbacks QUINCY)

Reporting period: 2020-03-01 to 2021-08-31

Nutrient availability plays a pivotal role in the response of terrestrial ecosystems to increasing atmospheric carbon dioxide and climate change. The so-called carbon dioxide fertilisation effect suggests increasing leaf-level photosynthesis with rising atmospheric carbon dioxide. Whether this enhanced photosynthesis leads to increased plant growth, and thereby potentially also ecosystem carbon storage is of pivotal importance to understand the consequence of human fossil fuel emissions on atmospheric levels of carbon dioxide, and climate change.

The carbon dioxide fertilization effect and the associated potential increase in ecosystem carbon storage is - in many terrestrial ecosystems - limited by the availability of the most important nutrients, nitrogen and phosphorus, which are required for plant and microbial growth. Ecosystem carbon storage further depends on the microbial decomposition of soil organic matter, which leads to carbon dioxide losses to the atmosphere, but at the same time increases the availability of nitrogen and phosphorus for plants and soil microbes. The net effect of these rhizosphere processes on ecosystem carbon storage, and thus atmospheric carbon dioxide levels is yet unknown.

The objective of QUINCY is to clarify the role of the interacting terrestrial nitrogen and phosphorus cycles and their effects on terrestrial carbon allocation and residence times as well as terrestrial water fluxes.

QUINCY will address two prime areas of uncertainty in the current generation of global biosphere models, namely
- the effects of nutrient availability on plant photosynthesis and respiration, and
- the effects of vegetation-soil interactions, namely rhizosphere processes, on plant nutrient availability and soil carbon turnover.

A robust understanding of the effects of nutrients on terrestrial vegetation and its interaction with soil biota is relevant to society as it contributes to better understanding the interplay between the terrestrial biosphere and climate. Notably, QUINCY will contribute to a better understanding of the likely pathways of future terrestrial carbon uptake (and that of other greenhouse gases), and thereby help to guide future anthropogenic emissions targets aiming at limiting climate change.
The work in the last reporting period developed in three distinct work packages, focussing on the simulation of plant growth under nutrient constraints (WP1), modelling and observations on soil-vegetation interactions (WP2), and integration of these aspects into global scale modelling (WP3).

The research in WP1, focussing on the simulation of plant growth under nutrient constraints developed in two distinct lines: The first line looked at modelling plant growth decisions from the point of view of resource economics (Caldararu et al. 2020, New Phytologist), whereas the second line investigated the effect of different nutrient acquisition strategies on the carbon economy of plant growth (Kern, PhD-thesis, TU Munich). This is different to the common approach employed in dynamic global vegetation models that typically rely on heuristic formulation of plant growth processes. The QUINCY prototype mode allows for the dynamic simulation of the seasonality and long-term growth patterns in deciduous or evergreen forests, as well as grasslands in response to different nutrient availability along a climate gradient from boreal to tropical ecosystems, which have been evaluated against a range of data sources including eddy-covariance data, inventory-based data, satellite data, and isotopic records for a range of global biomes (Thum et al. 2019, Biogeosciences). Using this model, we have investigated regional trends in increased nutrient stress by vegetation recorded in 15N observations (Caldararu et al. 2021, Global Change Biology).

As part of WP2, which focuses on soil-vegetation interactions, QUINCY has implemented and tested a next-generation soil biogeochemical model, which accounts for the role of plant-soil microbiota interaction in the decay and formation of plant litter and soil organic matter and integrates nitrogen and phosphorus dynamics (Yu et al, 2019, GMD, Yu et al. 2020, Frontiers). QUINCY also performs an experiment to study the effect of elevated carbon dioxide on plant allocation, soil organic matter decomposition, and nitrogen uptake using an innovative dual stable-isotope labelling approach (Eder, PhD-thesis, ongoing). The experimental set-up involves 16 mesocosms containing 64 beech trees in a natural forest soil. Trees were exposed to ambient and elevated (+200 ppm) carbon dioxide levels with different levels of soil fertility years to probe the effect of nutrient availability on the CO2 response. The experiment demonstrates that elevated CO2 has increased gross photosynthesis in accordance with expectations, but net biomass growth was not as strongly enhanced. Belowground allocation and microbial activity were enhanced and soil respiration did increase, associated with an observable decline in soil carbon.

WP 3 aims to integrate the research of WP 1 and WP2 with the global modelling framework of the ICON land surface model. This model has been successfully ported to run on the host institutions HPC cluster, and the QUINCY code has been designed to smoothly interface with this code. A prototype global scale application of the model exists and will be available for global analysis in few months.
The QUINCY experiment uses a novel technique to trace the fate of newly assimilated carbon in plants and soils, as well as tracing nutrient uptake from fine roots and mycorrhiza using stable-isotope labelling, as well as continuous gas exchange measurements of the above- and below ground compartment. This research gives new insights into the relative importance of changes in total below ground carbon allocation for the nutrient acquisition of plants. The results can also contribute to better understanding the role of mycorrhiza in plant nutrition, as well as to better understanding the effect of changing below ground carbon allocation of plants on the formation, turnover and stabilisation of soil organic material. This knowledge will help to inform the next generation of soil biogeochemical modelling needed for an integrated perspective of the effect of nutrient dynamics on terrestrial biosphere processes.

The new framework for biosphere modelling in QUINCY will put plant and microbial resource use strategies at the centre, and thereby progress over the current generation of biosphere models, which takes a primary physico-geochemical approach to modelling matter turnover in the biosphere. This novel approach will very likely increase the realism with which the tight coupling of the terrestrial carbon, nitrogen, and phosphorus cycling can be represented at the scale of Earth system models. As a result, one may expect a more realistic representation of perturbation responses in the QUINCY model (compared to the current generation of biosphere models), but also a better representation of the spatial distribution of current terrestrial biogeochemical pools and fluxes. Both factors are an important facet of a realistic simulation of the interaction of climate with the terrestrial biosphere, and therefore, one may expect an ecophysiologically more sound representation of biogeophysical, physiological and biogeochemical feedback mechanisms in the Earth system.
QUINCY mesocosm experiment with 16 tree chambers (back) and 4 soil control chambers (front)
Conceptual diagram of the QUINCY project including its three work packages