Melanie A. Thurner (Kern)

The terrestrial biosphere sequesters currently about 25% of anthropogenic CO₂ emissions, which reduces human-induced climate change. Models and observations suggest that this is largely caused by a CO₂ fertilization effect on plants, which may be reduced in the future due to progressively increasing nitrogen (N) limitation. However, the extent to such so-called progressive N limitation (PNL) occurs is highly uncertain, and causes major disagreement among model projections of future land carbon (C) uptake. Recent studies have shown that most terrestrial biosphere models (TBMs) have the tendency to overestimate PNL on plant growth under elevated CO₂ (eCO₂) by lacking a representation of the potential of plants to acclimate to environmental changes, particularly with regard to plant N nutrition and plant N acquisition.
In this context, I focus on two important plant N acquisition strategies, which rely on symbioses, i.e. symbiotic N fixation and mycorrhizal fungi, that are only insufficiently represented in TBMs yet to answer the questions if and to what extent N controls plant growth under current conditions, as well as under rising atmospheric CO₂, to improve predictions of future land C uptake.

During my PhD I developed a plant N acquisition model within the TBM QUINCY that allows plants to actively invest C into (i) root uptake of mineral N, (ii) mycorrhizal fungi that export N to their host plants, and (iii) symbiotic N fixers. Following the basic assumption that plants target maximum growth, the model calculates (potential) C costs for each strategy, and allocates available C accordingly to gain most N by investing less C.
As part of the QUINCY project I now investigate the effects of the newly implemented model structure on plant N acquisition and future land C uptake.