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

Periodic Reporting for period 3 - QUINCY (Quantifying the effects of interacting nutrient cycles on terrestrial biosphere dynamics and their climate feedbacks QUINCY)

Reporting period: 2018-09-01 to 2020-02-29

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. It has been hypothesised that plants will increase belowground carbon allocation (e.g. by root exudation) under elevated carbon dioxide to stimulate the decomposition of soil organic matter, and thereby the release of nutrients in the rooting zone. The net effect of these rhizosphere processes on ecosystem carbon storage, and thus atmospheric carbon dioxide levels is yet unknown.

The first generation of nutrient-enabled global carbon cycle models shows strongly diverging estimates of the nutrient effect on ecosystem carbon storage, primarily resulting from lacking integration of ecosystem observations and fundamental uncertainties in the representation of governing processes. 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.

To address the latter, QUINCY performs a mesocosm experiment on small-scale beech tree - soil systems to study the effect of elevated carbon dioxide on the resource allocation in these beech trees, its effect on soil organic matter decomposition and formation, and beech nitrogen uptake using an innovative dual stable-isotope labelling approach (see Section on work performed for more details). Based on this and other experimental evidence, QUINCY will further create a novel, predictive framework for ecosystem biogeochemical processes founded on the principle of resource optimisation, shifting the paradigm of biogeochemical modelling towards an active and adaptive biological control of matter flows (see Section on work performed for more details). The aspiration of this approach is that assuming such an active role will help to reconcile model predictions and observations from ecosystem manipulation experiments, which provide a fundamental test to the reliability of terrestrial ecosystem models. QUINCY will then link this understanding to the land surface scheme of the MPI Earth System model to apply the lessons learnt from the project to Earth system modelling used for climate change projections.

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. This is particularly relevant for the development of more accurate land representations in comprehensive Earth system models. 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 first 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 modelling work in QUINCY, which crosses all three work packages sets out to develop the next-generation of nutrient enabled terrestrial biosphere models. 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, whereas the second line investigated the effect of different nutrient acquisition strategies on the carbon economy of plant growth. This is different to the common approach employed in dynamic global vegetation models that typically rely on heuristic formulation of plant growth processes. Within the first reporting period, the research in QUINCY has led to a prototype model, which considers a hierarchy of resource allocation decisions at the level of an individual leaf, the total canopy and the whole plant. This model 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. The research has also led to a prototype predictive model of biological nitrogen fixation as part of the plant nutrient acquisition process, which responds amongst others to ecosystem nutrient status, and succession. Both models can be coupled together and thus provide the fundament for the new global ecosystem model that QUINCY aims to develop. The model framework has been tested against a range of data sources including eddy-covariance data, inventory-based data, satellite data, and isotopic records for a range of global biomes.

As part of WP2, which focuses on soil-vegetation interactions, QUINCY has initiated collaboration with other soil scientists from the host institution (MPI Biogeochemistry) to develop the conceptual groundwork for 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. QUINCY contributed to a case study assessing alternative model structures in determining the response of soil organic matter dynamics to changes in litter fall in order to develop an understanding of the relevance of different soil model structures to describe the so-called priming effect. In collaboration with an associated DFG-funded project, we have begun to develop a next-generation soil biogeochemical model based on extending existing model concepts by the nitrogen and phosphorus cycles.

As part of WP 3, which aims at integrating the research of WP 1 and WP2 with the global modelling framework of the MPI land surface model, primarily technical work has been performed. This consisted of supporting the scientific code developments in WP1 and WP2 and ensures the resultant codes can be readily incorporated into a large-scale biosphere model. The second task of WP 3 was to provide a suitable host model into which the codes generated by WP1 and WP2 could be ingested. The choice was to use the MPI-ESM2 framework with its land surface model JSBACH4. 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. However, the effective coupling of these two codes still requires work.

The experimental work of QUINCY consisted of setting up a mesocosm experiment to better understand the effect of elevated atmospheric carbon dioxide of the growth and nutrient uptake of young beech trees. The experimental set-up involves 16 mesocosms containing 64 beech trees in a natural forest soil, with separated chambers for the above-ground and below-ground compartments to facilitate the continuous measurements of growth and organic matter decay in soil and plant. Preparatory work included a re-engineering of the greenhouse chambers in which the mesocosms are located to facilitate energy-efficient lighting of the trees, continuous water supply and the IT- and measurement infrastructure for continuous meteorological and gas measurements. During summer 2016, beech trees were exposed to ambient and elevated (+200 ppm) carbon dioxide levels. Continuous labelling of the atmospheric carbon dioxide with stable isotope enables the tracking of newly acquired carbon in both plants and soil compartments. The experiment run very stable for 4.5 months and has provided sufficient gas exchange measurements and soil and vegetation samples to address the objectives of the experiment. The gas exchange measurements demonstrate the successful implementation of the experiment. The furthermore demonstrate that elevated CO2 has increased gross photosynthesis in accordance with expectations. Interestingly, net biomass growth was not as strongly enhanced, but belowground allocation and microbial activity was enhanced and soil respiration did increase. Elevated CO2 did increase both, plant N uptake and plant N deficit. Further measurements will determine whether total soil carbon storage has responded positively or negatively to the enhanced plant carbon and nitrogen fluxes. The experiment was repeated in 2017 for the entire growing season, with minor modifications to the set-up to and notably a poorer soil. While the general patterns of the response were confirmed by the experiment, detailed intepretation is contingent to lab samples that still need to be processed. To address the outstanding question of the role of root exudation and mycorrhizal exudation, we added a new experiment in the growing season 2018 to test the change in productivity and root exudation for trees with different mycorrhizal association.
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 will give 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.