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How does phosphorus cycling affect carbon uptake in the Amazon?

June 25, 2016

The Science

Phosphorus (P) has been generally considered to be the most limiting nutrient in lowland tropical forests. Several recent field studies in Amazonia have highlighted the importance of P in tropical forest productivity and function. Despite the importance of P in tropical carbon cycling, most Earth system models do not currently include P cycling and P limitation. In this study, we investigated how P cycling dynamics might affect tropical ecosystem responses to changes in atmospheric CO2 and climate using a P-enabled land surface model.

The Impact

This study demonstrated that the coupling of the P cycle in a land surface model produces a more realistic spatial pattern of simulated ecosystem productivity in the Amazon region. Through exploratory simulations this study points to the need for more tropical field measurements under different temperature and humidity conditions with different soil P availability.

Summary

Carbon–nutrient interactions are increasingly recognized to play important roles in regulating terrestrial carbon cycle responses to increasing CO2 in the atmosphere and climate change. Nitrogen-enabled models in CMIP5 showed that accounting for nitrogen greatly reduces the negative feedback between land ecosystems and atmospheric CO2. None of the CMIP5 models has considered P as a limiting nutrient, although P is typically considered the most limiting nutrient in lowland tropical forests. In this study, scientists from Oak Ridge National Labobratory investigated the effects of P availability on carbon cycling in the Amazon region using a P-enabled land surface model. Model simulations demonstrated that the CO2 fertilization effect in the Amazon region may be greatly overestimated if P cycling is not considered. Exploratory simulations highlighted the importance of considering the interactions between carbon, water, and nutrient cycling (both nitrogen and phosphorus) for the prediction of future carbon uptake in tropical ecosystems.

Objective

Explore how phosphorus (P) cycling interacts with changes in atmospheric CO2 and climate to affect historical and future carbon uptake in the Amazon region.

Approach

Exploratory land model simulations for the Amazon region were performed with and without P dynamics coupling under increasing atmospheric CO2 conditions.

These graphs show the trajectories of vegetation, soil, and total (vegetation plus soil) carbon stocks for the period 1900–2009 associated with historical changes in atmospheric CO<sub>2</sub>. CNP simulated carbon accumulation is about 26% lower than that simulated by the CN model, which is due mainly to low soil P availability in most of the Amazon region, which causes a weaker response of plant growth to increasing atmospheric CO<sub>2</sub>.

These graphs show the trajectories of vegetation, soil, and total (vegetation plus soil) carbon stocks for the period 1900–2009 associated with historical changes in atmospheric CO2. CNP simulated carbon accumulation is about 26% lower than that simulated by the CN model, which is due mainly to low soil P availability in most of the Amazon region, which causes a weaker response of plant growth to increasing atmospheric CO2.

Results/Impacts

  • Model simulations showed that coupling of P dynamics reduced the simulated historical terrestrial carbon sink by about 26%.
  • Results highlight the need for new observations on the effects of elevated CO2 on P dynamics, and the need for more tropical leaf-scale measurements under different temperature and humidity conditions and different levels of soil P availability.

Yang, Xiaojuan, Peter E. Thornton, Daniel M. Ricciuto, and Forrest M. Hoffman. July 16, 2016. “Phosphorus Feedbacks May Constrain Tropical Ecosystem Responses to Changes in Atmospheric CO2 and Climate.” Geophys. Res. Lett., 43(13):7205–7214. doi:10.1002/2016GL069241.