PhD project "Can terrestrial nutrient limitation resolve the "weak land carbon" conundrum-"

Position PhD project "Can terrestrial nutrient limitation resolve the 'weak land carbon' conundrum?" Employer Max Planck Institute for Biogeochemistry (MPI‑BGC) Location Jena, Germany Job 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) 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. The project will start by analysing 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 parameters (e.g. regarding the response of carbon allocation to nutrient stress), utilising 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