New research from the University of New Hampshire shows the effects of climate change on forests in the northeast may be more complicated than previously thought.
Forests store a lot of carbon, keeping it from moving into the atmosphere and warming the planet. Trees play a role, but soil stores about two thirds of the carbon in land ecosystems – more than the carbon in the atmosphere and in vegetation combined.
Microbes in soils decompose that carbon and release it back into the atmosphere. As the planet warms, scientists are trying to understand how the balance of carbon stored and carbon released is changing.
Most experiments studying large-scale environmental change are focused on the effects of one particular change, said Serita Frey, an ecosystem ecologist at the University of New Hampshire.
But this study looked at the interaction between two changes: the warming of soil and the increase of nitrogen. That increase is another effect of burning fossil fuels, with nitrogen put into the atmosphere by fuel combustion and car exhaust and then deposited on land.
“In the real world, these things are happening simultaneously, and there may be interactions that we can't account for by simply studying the single factors in isolation,” she said.
Past studies have shown that warming up soil and adding nitrogen have opposite effects.
Warming up soil stimulates the microbes that live there, allowing them to decompose organic material faster than they usually would. That causes soil to lose carbon over time, putting more into the atmosphere.
More nitrogen being added to the soil builds up organic matter in soil and increases the amount of carbon in soil.
Frey and her team of researchers used data from a study at the Harvard Forest Long-Term Ecological Research Site in Massachusetts that has been running since 2006. By burying heating cables under the soil and spraying fertilizer using backpacks, researchers studied the effects of both phenomena together over the course of 16 years.
The study found that when soils warm up and are fed more nitrogen at the same time, the amount of carbon dioxide that comes out of the soil is much higher. But the amount of carbon inside of the soil is higher, too. Together, the effects balance each other out.
“I like to think about this as a bathtub,” Frey said. “The water level in the bathtub is the amount of organic matter in the soil.”
The faucet, she said, represents the amount of carbon coming into the system. The drain is the loss of carbon through decomposition. When soil is warmer and has more nitrogen, the faucet is running faster and the drain is bigger. But the amount of water in the bathtub stays pretty much the same.
The study provided insight on how carbon might move around in New England soils as climate change continues. But a bigger takeaway, Frey says, is the importance of studying multiple changes at once – and how those changes interact.
“We for the past 20 years have looked at just soil warming. We made some predictions about how much carbon might be lost from the soil in response to climate warming. And I think what we're saying is that we need to look at this in a more holistic way,” Frey said.
Having long-term data from this study, Frey said, could help models that predict future climate scenarios paint a clearer picture.
