Carbon Absorption by Plants Limited by the Abundance of Soil Nutrients


Climate models suggest oceans absorbed 30% of human-emitted CO2, with plants taking another 30%, but scientists argue models are too optimistic.

As carbon dioxide levels continue to climb in the Earth’s atmosphere, most current climate models project that the world’s oceans and trees will keep soaking up more than half of the extra CO2. However, researchers have spotted a problem with the capacity of plants to absorb more CO2. It’s now expected that this capacity will be much lower than previously thought, due to the limitations in soil nutrients.

The scientists published their findings in the journal Nature Climate Change. Plants use up CO2 during photosynthesis and it’s been assumed that they will be able to offset increases in atmospheric CO2 caused by the burning of fossil fuels. Previously, climate models estimated that the world’s oceans absorbed about 30% of the CO2 generated by humans in the last 150 years and that plants have taken another 30%.

However, the new study by ecologists Peter Reich and Sarah Hobbie at the University of Minnesota in St Paul, suggests that these estimates are far too optimistic. Plants need soil nutrients, like nitrogen and phosphorous in order to grow. Few studies have tested whether soils contain enough of these nutrients to fuel plant growth in proportion to the rising levels of CO2.


In a 13-year field experiment on 296 open-air plots, the researchers grew perennial grassland species under ambient and elevated concentrations atmospheric CO2 and soil nitrogen. The scientists tried to create conditions that they believed would be similar to the atmosphere of 2070 above their plots. From 2001 to 2010, the grasses growing under heightened CO2 levels grew only half as much in untreated as in enriched nitrogen soils.

Since researchers do not have a firm grasp on the complexities of nitrogen and carbon cycle interactions, the vast majority of models do not adequately reflect nutrient limitation. The 13-year record of a single ecosystem in this study will provide valuable insight.

Another study, which included nutrient cycling modeling across the globe, predicted that plants would absorb 23% less carbon than projected by other models. The nutrient dynamics are complex, especially in forest ecosystems, which are expected to be an important part of carbon uptake.

Reference: “Decade-long soil nitrogen constraint on the CO2 fertilization of plant biomass” by Peter B. Reich and Sarah E. Hobbie, 30 September 2012, Nature Climate Change.
DOI: 10.1038/nclimate1694

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