Including the Phosphorus cycle into the LPJ-GUESS Dynamic Global Vegetation Model (v4.1, r10994) – Global patterns and temporal trends of N and P primary production limitation

Phosphorus (P) is a critical macronutrient for plant growth, often limiting plant production in areas where plant demand is higher than soil supply. In contrast to nitrogen (N), P cannot be sourced from the atmosphere, therefore where it is rare, it becomes a strong constraint of primary production. Due to this, most DGVMs are incorporating a prognostic P cycle in addition to N, improving their ability to correctly predict stocks and fluxes of carbon, and how climate change may impact N and/or P limitations to soil processes and plant productivity. We included the P-cycle into an individual-based DGVM, LPJ-GUESS (v4.1, r10994), in order to improve the model performance with regard to observations of vegetation and soil N and P stocks and fluxes in comparison to the N-only (LPJ-GUESS-CN) model version. The new model version (LPJ-GUESS-CNP) includes soil organic P dynamics, P limitation of organic matter decomposition, P deposition, temperature and humidity-dependent P weathering, plant P demand and uptake, and P limitations to photosynthesis. Using the CNP version of LPJ-GUESS we also estimated global spatial patterns of nutrient limitation to plant growth, as well the temporal change in plant N and P limitation during the 20th and early 21st centuries, evaluating the causes for these temporal shifts. We show that including the P-cycle significantly reduces simulated global vegetation and soil C and N stocks and fluxes, in particular in tropical regions. The CNP model simulation improved the fit to global biomass observations in relation to the CN simulation. The CNP model predicted predominant P limitation of plant growth in the tropics, and N limitation in the temperate, boreal, and high altitude tropical regions. The CNP model also correctly predicted the global magnitude (~ 50 PgP) and the spatial pattern of total organic P stocks. P limited regions cover less land surface area (46 %) than N limited, but are responsible for 57 % of global GPP and 68 % of vegetation biomass, while N limited regions store a larger portion of total carbons stocks (55.9 %). Finally, the model showed that globally primary production limitation to N availability decreased and limitation to P increased from 1901 to 2018, with N being more responsive to temperature, and P to CO2 changes. We conclude that including the P-cycle in models like LPJ-GUESS is crucial for understanding global-scale spatial and temporal patterns in nutrient limitation and improving the simulated carbon stocks and fluxes.