Intensifying phosphorus limitation accelerates the global decline of the CO₂ fertilization effect

For decades, the Earth's land has acted as a massive shield against climate change, absorbing roughly one-third of the carbon dioxide humans release into the air. This ability relies on a phenomenon scientists call the carbon dioxide fertilization effect. Think of it as a natural boost: as the atmosphere fills with more carbon dioxide, plants generally grow faster and larger because they have more of the raw material they need to build leaves and wood. This extra growth has helped slow the pace of global warming. However, recent observations suggest this helpful boost is fading. The question that has kept researchers up at night is why. For years, the leading theory was that the plants were running out of water, or that the air was becoming too dry for them to thrive. But a new study suggests the story is more complicated, pointing to a different, often overlooked shortage that is quietly undermining the planet's ability to soak up carbon.

A team of researchers, led by Jing Peng at Sun Yat-sen University, set out to investigate this fading boost. They combined long-term satellite images of plant growth with complex computer simulations that track how carbon, nitrogen, and phosphorus move through the soil and plants. Their goal was to see if the decline in plant growth was driven by dry conditions or by a lack of nutrients, specifically phosphorus. Phosphorus is a vital mineral that plants need to store energy and build DNA, but unlike nitrogen, which can be pulled from the air, phosphorus must be mined from rocks in the soil. The researchers found that while water stress plays a role, it is not the whole story. In fact, in many parts of the world, the plants are not suffering from a lack of rain. Instead, they are hitting a wall because they cannot get enough phosphorus to keep up with the extra carbon they are trying to use.

The study focused on the years between 1982 and 2015, a period when atmospheric carbon dioxide levels rose steadily. The team measured how much extra plant growth occurred for every increase in carbon dioxide, a rate that has been dropping globally. They discovered that this drop was happening fastest in specific regions where phosphorus limitation was becoming more intense. These areas, which include parts of the tropics and northern high latitudes, saw the carbon dioxide fertilization effect weaken at a rate of about 2.28 percent for every 100 parts per million increase in carbon dioxide. This was slightly faster than the global average and noticeably faster than in other regions. Crucially, the researchers checked the weather data for these specific areas and found that they were not experiencing harsher droughts or hotter temperatures than the rest of the world. In some cases, the climate was actually milder. This mismatch meant that the faster decline in plant growth could not be blamed on the weather.

To understand what was happening inside the plants, the researchers looked at the chemical balance of their leaves. They found that in the regions where the growth boost was fading fastest, the ratio of nitrogen to phosphorus in the leaves was shifting in a way that signaled a severe shortage. As plants tried to grow faster due to the extra carbon dioxide, they demanded more phosphorus, but the soil could not supply it fast enough. This created a bottleneck. The computer simulations showed that this nutrient shortage was becoming just as powerful a constraint on plant growth as the dryness of the air. In these phosphorus-limited zones, the lack of nutrients was effectively canceling out the benefits of the extra carbon dioxide. The plants were trying to grow, but they were running out of the essential ingredients needed to build their bodies.

The impact of this finding varies depending on the type of forest. The study showed that mixed forests, which are common in northern high latitudes, were the most vulnerable. In these areas, the ability of plants to use extra carbon dioxide for growth dropped by nearly 5 percent for every 100 parts per million rise in carbon. In contrast, evergreen broadleaf forests in the tropics, which have adapted over millions of years to phosphorus-poor soils, showed much less decline. These tropical trees have developed clever strategies to recycle phosphorus efficiently, allowing them to hold onto their growth advantage longer. However, the rapid decline in the northern forests is a significant concern because these ecosystems are currently major drivers of the global carbon sink. If their ability to absorb carbon is weakening faster than expected, the planet's natural defense against climate change is less robust than previously thought.

The researchers also tracked the flow of nutrients through the entire ecosystem, from living trees to dead leaves and the soil beneath them. They found that the ratio of carbon to phosphorus was increasing across all these pools, meaning that carbon was accumulating in the system much faster than phosphorus could be replenished. This imbalance suggests a self-reinforcing cycle: as plants grow, they lock up more carbon, but the phosphorus needed to sustain that growth remains scarce. The dead leaves falling to the ground are becoming richer in carbon but poorer in phosphorus, which slows down the decomposition process and further limits the return of nutrients to the soil. This creates a feedback loop where the very act of growing faster eventually leads to a greater shortage of the nutrients required to keep growing.

This study challenges the long-held assumption that water is the primary limit on how much carbon the land can absorb. While dryness certainly matters, the research indicates that in many places, the real bottleneck is the availability of phosphorus. The authors suggest that current computer models used to predict future climate change often overlook this dynamic nutrient cycling. By failing to account for how phosphorus limitation intensifies over time, these models may be overestimating how much carbon the Earth's forests will be able to store in the coming decades. If the land cannot absorb as much carbon as we hope, the concentration of carbon dioxide in the atmosphere will rise faster, leading to more rapid warming. The findings serve as a reminder that the Earth's systems are deeply interconnected, and solving the climate puzzle requires looking not just at the air and water, but at the invisible chemistry of the soil beneath our feet.