Forests act as absorbers and reservoirs for atmospheric carbon dioxide (CO₂). As CO₂ concentrations in the atmosphere rise in the future, will terrestrial forest ecosystems produce additional carbon sequestration? Can humans continuously increase carbon sequestration through afforestation? The conventional answer has been “YES.” However, based on six years of field monitoring, scientists have refuted this mainstream view with robust data analysis.
On the strength of the first comprehensive forest ecosystem phosphorus budget, the study reveals the key mechanism by which phosphorus cycling in the ecosystem hinders the forest’s response to increasing atmospheric CO₂ concentrations. It points out that in response to future climate change, plants need more active phosphorus acquisition strategies to enhance the availability of phosphorus in soil.
The findings were published in Nature on June 5 in an article titled “Microbial competition for phosphorus limits CO₂ response of a mature forest”. The study was jointly conducted by scientists from China, Australia, Switzerland, Norway, Spain, the Netherlands, the United States, the United Kingdom, and Germany. The lead author of the paper is Dr. JIANG Mingkai, a terrestrial ecosystem ecologist from the College of Life Sciences at Zhejiang University.
Where is phosphorus and where does it go?
Green plants convert CO₂ and water into organic matter through photosynthesis, releasing oxygen in the process. Forest carbon sequestration plays a vital role in mitigating the greenhouse effect by absorbing CO₂ from the forest and fixing it in vegetation or soil.
With global climate change, carbon sequestration plays an increasingly crucial role, but its function is limited by the availability of soil nutrients. “The productivity of tropical and subtropical forests is generally constrained by the availability of soil phosphorus. However, there is still a knowledge gap on how ecosystem phosphorus cycling limits forest productivity, thereby restricting its carbon sequestration potential in the context of elevated atmospheric CO₂ concentrations. This uncertainty is one of the critical bottlenecks in predicting the feedback of the land-atmosphere carbon cycle in Earth system models,” said Dr. JIANG Mingkai.
To gain a clearer picture of forests’ carbon sequestration and the role of phosphorus, the international team of scientists conducted a six-year study. Using EucFACE—a world-class experiment that simulates the impacts of future carbon-dioxide-rich climates on Australia’s native forests, they experimented on a mature forest of eucalyptus trees over 100 years old in a remnant native Cumberland Plain woodland on an ancient alluvial floodplain in western Sydney to investigate how ecosystem phosphorus cycling responds to elevated atmospheric CO₂ concentrations.
The ecosystem P budget
The scientists undertook a meticulous “budgeting” process. They comprehensively gauged the sizes of all major phosphorus pools in the mature forest ecosystem, including canopy vegetation, understory vegetation, litter, soil, and soil microbes, tracking phosphorus movement rates among these pools to build a detailed ecosystem phosphorus budget. “Just like tracking the flow of cash in a bank account, we tracked the circulation of phosphorus in the forest and assessed how phosphorus availability limits plant growth responses,” Dr. JIANG Mingkai explained.
A failed investment
In the vast terrestrial forest ecosystem, plants, microbes, and soil form a complex cycle. Plants absorb CO₂ through photosynthesis, converting it into their nutrients while also absorbing phosphorus from soil to sustain their growth. Microbes can decompose organic matter in soil into inorganic matter, releasing phosphorus for plant absorption. However, when soil phosphorus is insufficient, microbes compete with plants for it.
The phosphorus budget shows that a significant portion of soil phosphorus is occupied by microbes, which compete fiercely for phosphorus. Under increased CO₂ concentrations, plants release more carbon into soil through their roots, but microbes do not release more phosphorus to support plant growth. The anticipated “carbon-for-phosphorus” investment by plants failed, as they did not gain additional growth.
The scientists analyzed plants’ absorption, distribution, and utilization efficiency of phosphorus, comparing variations between CO₂-elevated groups and control groups. The results indicate that soil microbes are a key factor limiting phosphorus cycling and plant phosphorus uptake in mature forest ecosystems. Canopy trees in the forest have evolved to have a high phosphorus use efficiency over a long period of ecological succession. When CO₂ concentrations rise, plants’ phosphorus use efficiency also increases, but soil phosphorus availability does not change significantly.
“Microbial mineralization and retention of soil phosphorus limit the phosphorus uptake rate of canopy trees under elevated atmospheric CO₂ concentrations, thereby restricting the additional carbon sequestration capacity of forests. Plants need more active phosphorus acquisition strategies, such as root exudates, to enhance soil phosphorus availability and achieve better ‘carbon sequestration’ goals,” said Dr. JIANG Mingkai.
This study provides theoretical support for improving the prediction mechanism of carbon-phosphorus interactions in terrestrial system models and offers valuable data for climate change mitigation policies. “We will conduct follow-up research on the community and function of soil microbes to explore how soil microbes limit forest carbon-phosphorus interactions and their carbon sequestration potential under elevated atmospheric CO₂ concentrations,” said Dr. JIANG Mingkai.
（From ZJU NEWSROOM）