Representation of P cycle in Joint UK Land Environment Simulator

Most Land Surface Models (LSMs), the land components of Earth system models (ESMs), include representation of N limitation on ecosystem productivity. However only few of these models have incorporated phosphorus (P) cycling. In tropical ecosystems, this is likely to be particularly important as N tends to be abundant but the availability of rock-derived elements, such as P, can be very low. Thus, without a representation of P cycling, the responses of tropical forest in areas such as Amazonia to rising atmospheric CO₂ conditions remains highly uncertain. In this study, we introduced P dynamics and its interactions with the N and carbon (C) cycles into the Joint UK Land Environment Simulator (JULES). The new model (JULES-CNP) includes the representation of P stocks in vegetation and soil pools, as well as key processes controlling fluxes between these pools. We evaluate JULES-CNP at the Amazon nutrient fertilization experiment (AFEX), a low fertility site, representative of about 60% of Amazon soils. We apply the model under ambient CO₂ and elevated CO₂. The model is able to reproduce the observed plant and soil P pools and fluxes under ambient CO₂. We estimate P to limit net primary productivity (NPP) by 24% under current CO₂ and by 46% under elevated CO₂. Under elevated CO₂, biomass in simulations accounting for CNP is 5% lower compared CN and C-only simulations. Our results highlight the potential for high P limitation and therefore lower CO₂ fertilization capacity in the Amazon forest under low fertility soils.