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As atmospheric carbon dioxide levels rise, forests need soil phosphorus and potassium to sustain growth

Plants use carbon dioxide (CO2) for growth, along with other nutrients. Low soil nitrogen is widely thought to limit tree growth as CO2 rises. However, a new study suggests that low phosphorous and potassium are actually the limiting nutrients. The results are from mixed pine-broadleaved forests at the Duke Free Air Carbon Enrichment (FACE) site in North Carolina.

Two researchers measure a segment of pine on a table set up in the forest.
Forest Service scientist measures crown characteristics in loblolly pine trees. USDA Forest Service photo by Chris Maier.

Several forest-scale FACE experiments show that increased uptake of soil nitrogen is the primary, short-term mechanism supporting high rates of tree growth under elevated CO2. Once soil nitrogen is depleted, more must be added if high rates of growth and carbon sequestration are to be sustained.

At the Duke FACE site, tree grew faster when exposed to elevated CO2, as expected. However, there was variation in the CO2 response, and the variation correlated with soil nitrogen availability.

To test if nitrogen was limiting growth, the researchers applied nitrogen fertilizer annually during the last five years of a 15-year study. They found that there was no additional growth with added nitrogen, although because of the higher levels of CO2, the trees continued to grow faster.

When the researched analysed pine leaves at the molecular level, they found that ratios of phosphorus and potassium to nitrogen were below adequate levels for optimal pine growth. In other words, the leaves of the pine trees were low in essential nutrients, which indicates that low availability of these nutrients in the soil was limiting growth. Each plant nutrient has its own biogeochemical cycle, but these cycles can also affect each other. In this case, nitrogen fertilization made phosphorus and potassium limitations worse. The limitations were either created by elevated CO2 or already present in the forest.

The results show that trees can only respond to increased CO2 when they have a full complement of nutrients. Understanding the spatial and temporal variation across the suite of essential nutrients - not just nitrogen - is essential when modeling long-term forest response to elevated CO2.

Principal Investigators
Chris A. Maier, Team Leader, Research Biological Scientist
Kurt Johnsen, Plant Physiologist
Peter Anderson, Biological Scientist
RWU
4160 - Forest Genetics and Ecosystems Biology
Publication
The response of coarse root biomass to long‐term CO2 enrichment and nitrogen application in a maturing Pinus taeda stand with a large broadleaved component
External Partners
Aubrey Knier, Duke University
Ram Oren, Duke University
Sari Palmroth, Duke University
Dohyoung Kim, University of Notre Dame
Heather McCarthy, University of Oklahoma