The influences of phosphorus cycling on the tropical ecosystem carbon cycle-A process-based modeling study

Abstract

The biogeochemical processes of phosphorous (P), carbon (C), and nitrogen (N) in the Earth system are fully coupled, which shapes the structure, functioning, and dynamics of terrestrial ecosystems. However, incorporating P-related processes into terrestrial biosphere models (TBMs) is still in an early stage. Tropical forests store more than half of the world's terrestrial carbon (C) pool and account for one-third of global net primary productivity (NPP). With their significant contribution to the global C cycle, tropical forests maintain critical negative feedback to climate warming through absorbing atmospheric CO2. A few TBMs-based estimates indicate increasing productivity in tropical ecosystems throughout the 21st century due to the CO2 fertilization effect. However, phosphorus (P) limitation on vegetation photosynthesis and productivity have not been considered by most current TBMs. In this dissertation, P impacts on C fluxes and the C-N-P interactions were investigated at both site and tropical scales. We examined how P limitation has affected C fluxes of tropical rainforests during 1860-2018. Our model results showed that consideration of the P cycle reduced the CO2 fertilization effect on tropical rainforests gross primary production (GPP) by 25% and 45%, NPP by 25% and 46%, and net ecosystem production (NEP) by 28% and 41% relative to CN-only and C-only models. During the period from the 1860s to the 2010s, the DLEM-CNP estimated that for per unit area, the tropical rainforest GPP increased by 17 %, plant respiration (Ra) increased by 18%, NPP increased by 16%, heterotrophic respiration (Rh) increased by 13%, and NEP increased by 121%, respectively. Additionally, factorial experiments with DLEM-CNP showed that the enhanced GPP and NPP benefiting from the CO2 fertilization effect had been offset by 147% and 135% due to deforestation from the 1860s to the 2010s. Using future environmental factors, we examined pan-tropic GPP, NPP, and carbon use efficiency (CUE) changes during 2020-2100. Results showed that the P limitation on the CO2 fertilization effect would reduce future tropical GPP and NPP. Under the SSP585 scenario, the CO2 fertilization effect would reach plateaus and the tropical ecosystem’s capability to respond to CO2 increase would weaken after 2060. Under future environmental conditions during 2020-2100, DLEM-CNP estimated that under the SSP126 scenario, the tropical GPP, NPP, and CUE would slightly increase, with a substantially interannual variation. Under the SSP585 scenario, the tropical GPP and NPP would increase by 44% and 21% from 2020 to 2100, respectively; the CUE shows a decrease of 15% under the SSP585 scenario. The CO2 fertilization effect is the dominant factor that would likely increase the future GPP and NPP in the tropics. The climate effect is found to be the most significant factor that would decrease the CUE under both SSP126 and SSP585 scenarios. Our study revealed strong interactions among C, N, P processes, indicating that the inclusion of the P cycle in the current TBMs is essential to better understand the impacts of global change on terrestrial ecosystems.