The influence of phosphorus cycling and temperature acclimation of photosynthesis on the land carbon cycle

Terrestrial carbon (C) cycle models applied for climate projections simulate a strong increase in net primary productivity (NPP) due to elevated atmospheric CO2 concentration during the 21st century. These models usually neglect the limited availability of nitrogen (N) and phosphorus (P), nutrients that commonly limit plant growth and soil carbon turnover. In addition, these models treat the temperature dependence of photosynthesis rigidly, although warming experiments illustrate a high variability in the temperature response of photosynthesis due to acclimation (adjustments by an indiviudal plant in response to an environmental stimulus). Both aspects, nutrient limitation and temperature acclimation, are discussed as major uncertainties in projecting future land C uptake. To investigate how the projected C sequestration and global productivity is altered when nutrient limitation or temperature acclimation of photosynthesis are considered, I incorporated representations for P cycling and for temperature acclimation into the Max Planck Earth System model (MPI-ESM). In direct comparison with simulations which do not consider nutrient cycles, the accumulated land C uptake between 1860 and 2100 (SRES A1B scenario) is 13 % and 16 % lower in simulations with N and P cycling, respectively. The combined effect of both nutrients reduces land C uptake by 25 % compared to simulations without N or P cycling. Nutrient limitation in general may be biased by the model simplicity, but the ranking of limitations is robust against the parameterization and the inflexibility of stoichiometry. After 2100, increased temperature and high CO2 concentration cause a shift from N to P limitation at high latitudes, while nutrient limitation in the tropics declines. The increase in P limitation at high-latitudes is induced by a strong increase in NPP and the low P sorption capacity of soils, while a decline in tropical NPP due to high autotrophic respiration rates alleviates N and P limitations. The quantification of P limitation remains challenging. The poorly constrained processes of soil P sorption and biochemical mineralization are identified as the main uncertainties in the strength of P limitation, while changes in P release rate by chemical weathering may become of importance on multi-centennial time scale. In contrast, acclimation of photosynthesis has only a minor effect on global productivity. Temperature acclimation slightly increases extra-tropical productivity under present climatic conditions by 5% and under future climatic conditions by 7%. The effect of temperature acclimation on tropical ecosystems is elusive. On the one hand, it is under debate if tropical trees do acclimate to temperature at all, as temperatures at low latitudes do not vary much during the growing season. On the other hand, if they acclimate, the sign of their response to warming depends on how the temperature response of the ratio between rubisco activity limited and electron transport limited photosynthesis is treated in the model. More experimental research is needed to evaluate whether this ratio is influenced by temperature or not. My findings indicate that global land C uptake in the 21st century is likely overestimated in models that neglect P and N limitations. In the long-term, insufficient P availability might become an important constraint on C cycling at high latitudes. Temperature acclimation is of minor importance for assessing the response of the global C cycle to warming, unlike previously suggested. Accordingly, I argue that the P cycle must be included in global models used for C cycle projections, but I recommend to wait with the implementation of temperature acclimation until a better process understanding has been achieved.