Every year, trees and plants across the world absorb a vast amount of carbon dioxide from the atmosphere.

But a new study suggests this massive carbon sink could instead become a source of carbon dioxide by the end of the century.

This means we might not be able to rely on plants soaking up our emissions for much longer, the lead author tells Carbon Brief.

Extra carbon dioxide

Through photosynthesis, plants convert carbon dioxide, water and sunlight into the fuel they need to grow, locking up carbon in their branches, stems and leaves in the process.

Research suggests that as human-caused carbon dioxide emissions accumulate in the atmosphere, plants will grow more quickly because the rate of photosynthesis speeds up. This is called ‘carbon dioxide fertilisation’.

This argument is sometimes used in parts of the media to suggest that additional carbon dioxide is beneficial for the Earth as extra food for plants.

But research published this week in Nature Geoscience suggests that plants won’t have enough nutrients to make full use of the extra carbon dioxide in the atmosphere. So any benefits will be limited, say the authors.

Nutrient needs

Plants need the right mix of nutrients to grow. Two of the most important nutrients are nitrogen and phosphorus. But there isn’t an endless supply in soils for plants to use, lead author Dr Will Wieder, from the National Centre for Atmospheric Research in Colorado, tells Carbon Brief:

“Many ecosystems appear to be co-limited, meaning that both nitrogen and phosphorus are important for plant growth. There are places where one element or the other may be slightly more limiting, but at the end of the day plants need both to build roots, leaves and wood. This is why many fertilizers used in gardens and farms come with both nitrogen and phosphorus.”

While nitrogen is abundant in the air we breathe, most plants can only take it up from the soil. Nitrogen gets into the soil by being ‘fixed’ from the air by microbes and certain plants, such as soy, Wieder says. Phosphorus primarily originates from rocks, and reaches the soil when they are worn down by the weather.

Nutrients can come from a little further afield as well, Weider adds:

“Both nitrogen and phosphorus can be moved around and transported through the atmosphere as dust or air pollution. The subsequent deposition of nitrogen and phosphorus also can contribute new nutrients to an ecosystem.”

Limits to growth

Most climate models used for the latest Intergovernmental Panel on Climate Change (IPCC) report assume that enough additional nitrogen and phosphorus would be available for extra plant growth. But this might not actually be the case, Weider says:

“This ‘new’ nitrogen and phosphorus would have to come from somewhere, and we found it is unlikely to be supplied from outside the ecosystem, meaning that the increases in plant growth would have to be met through accelerated recycling of nutrients within ecosystems.”

When Wieder and his colleagues included realistic amounts of nitrogen and phosphorus in their models, they found it limited the boost to plant growth from the extra carbon dioxide.

In the graph below, the black line shows the increase in plant growth the IPCC models project under a high-emissions scenario. Without limiting nitrogen and phosphorus, plant growth increases by 63% by 2100.

When the team included future limits to nitrogen (red line), they found extra plant growth dropped to 29%. With limits to both nutrients (blue line), the boost to growth dropped even further to 20%.

Carbon storage

Limits to the amount of carbon plants can take up is only part of the story. Warmer temperatures mean carbon held in soils and dead plants will decompose more quickly, releasing more carbon back into the atmosphere, the study says.

This could mean that, on balance, more carbon dioxide is released from the soil than is absorbed by the plants, Weider says:

“We’re going to go from terrestrial ecosystems sponging up carbon dioxide out of the atmosphere to actually having them contribute to the problem.”

You can see this in the graph below. Without realistic nutrient limits, models project an increase in the amount of carbon stored in land (black line). But with limited nitrogen (red) and nitrogen and phosphorus (blue), the land releases more carbon than it absorbs, turning it from a carbon sink to a carbon source.

By 2100, the amount of carbon released by the land could be equivalent to 14 years of current human-caused emissions, the study suggests. These emissions would increase atmospheric carbon dioxide by 66 parts per million (ppm).

You can see the projected change in land carbon storage across the world in the maps below. The reds and yellows in the upper map show areas that historically have held the most land carbon. The dark browns in the lower map show areas with the biggest drop in the amount of carbon the land can store.

Regions of the world where large increases in plant growth are expected are likely to be most limited by availability of nutrients, Wieder says. These include tropical forests, savannahs, boreal forests and tundra ecosystems.

Bigger problem

The findings suggest we can’t count on plants to offset the impact of our emissions as much as previously thought, Wieder concludes:

“So far, we have greatly benefited from plants removing carbon dioxide from the atmosphere. But if a lack of nutrients limits their ability to keep soaking up carbon dioxide, then climate change becomes an even bigger problem than we thought.”

The carbon cycle is a critical part of regulating Earth’s climate. With the land’s ability to take up carbon diminishing this century, the study suggests society will need to compensate by finding other ways to cut back on emissions if we’re to keep temperatures in check.

Main image: Sunrise over a meadow.

Wieder, W.R. et al. (2015) Future productivity and carbon storage limited by terrestrial nutrient availability, Nature Geoscience, 10.1038/ngeo2413