PhD project offered by the IMPRS-gBGC in July 2026

Can terrestrial nutrient limitation resolve the "weak land carbon" conundrum?

Sönke Zaehle , Kai U. Totsche

Project description

Land-atmosphere carbon exchange is one of the most uncertain components of the Earth system. A robust understanding is essential for reliable climate forecasts and for designing effective mitigation policies (Canadell et al., 2021). A recent study by Randerson et al. (2025) showed that the newest generation of Terrestrial Biosphere Models (TBMs)-the tools that feed the Global Carbon Project (Tian et al., 2025; Friedlingstein et al., 2026) and climate-change projections-systematically over-estimate the net carbon uptake of land. Paradoxically, these models do capture the observed rise in terrestrial productivity, suggesting that something else is missing from the simulations.

This PhD project will test the hypothesis that nutrient limitation (especially nitrogen and phosphorus) and the resulting shifts in plant carbon allocation can explain why TBMs are too "optimistic" about land carbon uptake. If vegetation is constrained by nutrients, it cannot convert all the extra photosynthesis into long-term carbon storage, and soils may release more CO₂ than the models currently predict.

This PhD project will start by analyzing simulations from the recent Global Carbon Project ensembles, including the QUINCY model developed by the TBM group in the BSI department (Thum. et al. 2019). The project will then design model experiments with the QUINCY model to address the "weak land carbon" conundrum. Depending on the profile of the successful applicant, further steps of the thesis can include further developing new model parameterisations (e.g. regarding the response of carbon allocation to nutrient stress), utilizing observational constraints, including long-term forest monitoring data or isotope data to constrain modelled regional carbon uptake, or employing ensemble-methods and global observational constraints on the global carbon and nitrogen cycle. The outcome will be a better-constrained terrestrial nutrient cycle and improved confidence in Earth system model projections under future climate scenarios.

This project offers hands-on experience in applying, analysing and developing state-of-the-art biosphere models, data-science skills through the handling of model ensembles, statistical model-data comparison, and uncertainty quantification, as well as interdisciplinary expertise in plant physiology, soil biogeochemistry, and climate modelling.

Working group & collaborations

The candidate will be part of the Biogeochemical Signals department and will have the opportunity to collaborate with researchers from multiple institutions in Germany and beyond.

Requirements for the PhD project are

Applications to the IMPRS-gBGC are open to well-motivated and highly-qualified students from all countries. Prerequisites for this PhD project are:

Master's degree in bio(geo)chemistry, environmental science, geosciences, physics, atmospheric science, engineering, remote sensing or other disciplines related to environmental sciences.
Background in terrestrial carbon cycle, and experience in ecosystem or atmosphere modelling
Computational skills

Experience with programming languages like R or python
Knowledge of higher programming languages such as C or FORTRAN are an asset
Experience with gridded datasets like ERA5 or similar

Excellent oral and written communication skills in English

The Max Planck Society (MPS) strives for gender equality and diversity. The MPS seeks to increase the number of women in those areas where they are underrepresented and therefore explicitly encourages women to apply. The MPS is committed to increasing the number of individuals with disabilities in its workforce and therefore encourages applications from such qualified individuals.

References

QUINCY: https://www.bgc-jena.mpg.de/en/bsi/bsi-data/software/quincy-software

Canadell et al. (2021): Global Carbon and other Biogeochemical Cycles and Feedbacks. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., et al. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 673–816, doi: 10.1017/9781009157896.007.

Tian, H., et a. (2024). Global nitrous oxide budget (1980–2020). Earth System Science Data, 16(6), 2543-2604. https://doi.org/10.5194/essd-16-2543-2024.

Friedlingstein, P., et al (2026): Global Carbon Budget 2025, Earth Syst. Sci. Data, 18, 3211–3288, https://doi.org/10.5194/essd-18-3211-2026.

Thum, T., et al. (2019) A new model of the coupled carbon, nitrogen, and phosphorus cycles in the terrestrial biosphere (QUINCY v1.0, revision 1996), Geosci. Model Dev., 12, 4781–4802, https://doi.org/10.5194/gmd-12-4781-2019.