An elemental balancing act

Elements such as carbon, nitrogen, and phosphorus move through the Earth’s atmosphere, water, living creatures, and soil in natural cycles. All three elements are vital to life, and serve as fertilizer to help plants grow. But what happens as the balance of these key nutrients changes?

“The general rat ios of carbon, phosphorus, and nitrogen have been in balance for millions of years, more or less,” says Michael Obersteiner, who leads IIASA’s Ecosystems Services and Management Program.

But today, humans are pumping carbon dioxide into the atmosphere, dumping increasing amount s of nitrogen and phosphorus into the soil, and at the same time rapidly exhausting the world’s limited phosphorus reserves. Carbon and nitrogen are abundantly available, and becoming more so. But the relative amount of phosphorus is declining. That means that in many places, the relative amounts of these three nutrients may change drastically. “The Earth’s plant life evolved with these stoichiometric ratios. If we change them, especially in natural ecosystems, we might get some unexpected results,” says Obersteiner.

What those results might be is the subject of a new international research project sponsored by the European Research Council (ERC), to explore the impacts of this growing imbalance on the climate, the environment, and food security.

Nutrient balance & the climate

While human beings released over nine billion metric tons of carbon dioxide into the air last year, only about half of that remained in the atmosphere. The rest was pulled into natural carbon sinks including oceans and forests. Earth’s forests are one of the largest carbon sinks, but it is still unclear how much carbon they absorb and under what conditions. A growing body of research suggests that the availability of nitrogen and phosphorus in the soil is one condition that makes a huge difference in that balance.

In a new paper published in the journal Nature Climate Change, Obersteiner and the ERC research team show that the nutrient availability in forest soils is a key factor regulating how much carbon forests can take up from the atmosphere.

“When plants are in nutrient poor conditions, they send out more roots and produce chemicals that can help dissolve nutrients from the soil,” says Obersteiner, “This takes energy, though, and so the plants produce less biomass.” The study, which relied on experimental data from 92 forests around the world, showed that in nutrient‑rich conditions, forests accumulated four times as much carbon as in nutrient‑poor conditions.

In another recent study, 2013 IIASA Post‑Doc Christina Kaiser, IIASA Researcher Oskar Franklin, and colleagues took the question of soil nutrient balance to an even more detailed level, examining how nutrient availability affects the function of microbes that break down forest litter—a process that releases approximately six times more greenhouse gases than humans do.

While trees and plants take up carbon dioxide through photosynthesis, the tiny creatures that live in forest soils break down old leaves, branches, and dead animals, releasing greenhouse gases such as carbon dioxide and methane back into the atmosphere. Previous research had suggested that nutrient imbalances could lead to even greater emissions, as the microbes worked less efficiently. But the new study shows that as nutrient conditions change, the microbes adjust to the new conditions and continue to operate smoothly and emit about the same amount of carbon dioxide. Franklin plans to continue the work as part of the ERC grant.

Improving climate models

To project future climate change, Earth system models have to project how much carbon dioxide will stay in the atmosphere. To do that, they have generally looked at forests as one unified photosynthetic organism—essentially a big leaf—without considering factors such as nutrients or soil quality. However, as Obersteiner says, “We now have evidence that this is not transformed into biomass also crucially depends on soil nutrients.” With a better understanding of forest carbon processes, Obersteiner hopes that the models can also be improved, providing more clarity for policymakers.

“There are so many uncertainties about climate change. Improving our models of the carbon cycle is vital for understanding what might happen in the future,” says Obersteiner. “It’s a big problem, and we know very little about it.”

^{Text by Katherine Leitzell}

Last edited: 29 July 2014