Tropical forests less able to absorb nitrogen

The results have implications for nature’s response to climate change, Sigman said. He explained that nitrogen and phosphorous are usually the nutrients that limit plant growth. Because ecosystems’ ability to absorb man-made carbon dioxide is limited by the availability of other nutrients, such as nitrogen and phosphorus, ecosystems’ response to climate change will depend on the levels of these nutrients.

Because rocks release phosphorus when they weather, scientists had previously expected the forests in the mountainous regions that the researchers studied to be relatively high in phosphorus and deficient in nitrogen. However, Sigman and Hedin found otherwise.

“The expectation that’s been in the field for decades is that a system like this would be nitrogen-poor,” Sigman said. “But these systems weren’t nitrogen-poor. Their streams have very high nitrate concentration.”

The researchers first investigated whether the extra nitrogen was a result of human activity. The scientists examined over a decade’s worth of water samples from Costa Rican forest streams and measured the levels of different forms of nitrogen in the streams. They found that most of the nitrogen appeared in the form of nitrate, an ion containing nitrogen and oxygen.

Sigman explained that there was no change in the nitrate concentrations in the stream samples over time. He added that this suggested levels of nitrogen in the forest were similar from year to year.

The scientists then investigated the levels of specific isotopes of nitrogen and oxygen in the streams. While the ratios of nitrogen isotopes in the samples confirmed that they were stable over time, the ratios of oxygen isotopes in the samples allowed the scientists to conclude that the nitrate was not produced by human activity.

“The oxygen isotopic composition of nitrate ... is strongly affected by how much of that nitrate derives from the atmosphere ... versus nitrogen that’s been turned over in the soil,” Sigman said. “All the nitrate had been turned over in the soils.”

Sigman said that because the forests already draw more nitrogen than they need from the atmosphere, they are unlikely to absorb extra nitrogen from pollution as well. If more nitrogen were added to the ecosystem, “all the systems downstream might be altered,” Sigman said.

Hedin also noted in a press release that their results call into question the future of tropical regions where nitrogen pollution is increasing.

Noting that the “dead zone” in the Gulf of Mexico near the mouth of the Mississippi River — where biodiversity is extremely low — is caused by excess nutrients from fertilizers, Sigman said the group’s research indicated that the risk of these areas in tropical regions may be greater than previously thought.

“On the most dramatic scale, this [dead zone] is the type of stuff you worry about,” Sigman said. “Across tropical regions, we may need to expect them to be less absorbent for pollution nitrogen than we thought of them as before.”

The paper was published in the February issue of Nature Geosciences.