Plant nutrient map sheds light on carbon sinks

Natural areas that can absorb huge quantities of carbon dioxide could play a crucial role in combatting climate change. However, our ability to identify and employ these carbon sinks has been hobbled by a lack of information about where plants grow best. Now, research from Stanford University reveals a global map of areas where insufficient nutrients in the soil could limit plant growth.

The study,  published  in Nature Geoscience, provides a new data framework for understanding plant growth and, by extension, predicting what areas of the world will be able to soak up carbon emissions in the future.

"Farmers and foresters spend considerable money and time analyzing the nutrient needs of crops and trees," said study senior author  Rob Jackson , a professor of Earth system science in Stanford’s  School of Earth, Energy & Environmental Sciences  (Stanford Earth). "This study is a step toward doing that for natural forests and grasslands."

The researchers focused on nitrogen and phosphorus - nutrients that plants need in large daily doses. To estimate where those nutrients are in limited supply, the researchers combined global measurements of how much plants grow with experiments looking at how plant growth is related to the nutrient supplies. From this, they determined where around the world plants are growing more slowly than expected due to limited nutrients.

Map shows natural areas that are highly dependent on nitrogen for growth (red) and highly dependent on phosphorous for growth (blue).

Their results suggest that plants growing on approximately 40 percent of Earth’s natural land surface are limited by inadequate phosphorus, and plants on about 20 percent of land are limited by inadequate nitrogen. The researchers couldn’t diagnose which nutrients might be limited on the remaining 40 percent of land area, which could mean plants in those ecosystems are equally dependent on nitrogen and phosphorous or only weakly dependent on either nutrient alone.

Overall, the findings could have significant implications for conservation and planning aimed at easing climate change. Tropical and subtropical areas suffer relatively strong phosphorus limitations that may impede their ability to soak up extra carbon dioxide from fossil fuel emissions. Boreal forests and tundra may benefit from climate warming-induced nitrogen mineralization if not affected by other stresses. Many temperate regions with relatively weak nutrient limitation are candidates to become valuable carbon sinks, especially where damaged ecosystems are restored.

"There is an exciting potential to extend our approach to human-dominated or managed ecosystems, such as commercial plantations and urban forests," said study lead author Enzai Du, an associate professor at Beijing Normal University who was a visiting scholar at Stanford while researching the study. "This could lead to better nutrient management, stronger plant growth and bigger carbon sinks."

Jackson is the Michelle and Kevin Douglas Provostial Professor and a senior fellow at the Precourt Institute for Energy. Stanford co-authors include César Terrer; Adam Pellegrini; Anders Ahlström. Other co-authors are from Utrecht University, the Chinese Academy of Sciences, and Beijing Normal University.

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