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Scientists Detect Acid Deposition Causing Increased Forest Water Use

A previously unknown mechanism causes an increase in forest water use, and as scientists explain in a recent study, it is caused by acid deposition. The new study offers insights into soil biogeochemical control, and how acid deposition causes water stress which can have dire consequences for the forest systems.

A group of researchers discovered a mechanism which affects water acidification in the forests and causes increased forest water use. Their findings were published in the journal Science Advances, and describe what happens when sulfuric and nitric acid reach the soil when fossil fuels are burned. When that happens, the soil absorbs the acid and becomes more acidic in composition. This process causes the amount of soil calcium to wash off, which causes plants to suffer from calcium deficiency. Because of this, trees use more water.

The study was led by Lixin Wang, an associate professor from the School of Science at IUPUI and Ph.D. student Matthew Lanning as the first author.

Trees use calcium which uniquely regulates the minute pores, known as stomata, located in the leaves or stems of plants. According to Wang, if plants don’t have enough calcium from the soil, these pores can’t close which causes them to use an increased amount of forest water. The fossil fuels and acids that are absorbed by the soil eliminate the presence of calcium, causing the deficiency in the trees. This causes a special mechanism to activate, wherein plants use more water through transpiration – a process in which water moves through a plant and evaporates from leaves – to try to pump up more calcium.

“We hypothesized that the leaching of the soil calcium supply, induced by acid deposition, would increase large-scale vegetation water use,” Lanning said in a statement. “We present evidence from a long-term whole watershed acidification experiment demonstrating that the alteration of the soil calcium supply by acid deposition can significantly intensify water use.”

Scientists found significant evidence which shows that the calcium deficiency is a result of acid deposition, causing the increased forest water use. This mechanism also causes a decreased soil water pool, which leaves humans with less corresponding drinking water.

“When plants are always using a lot of water, it means there will be less water left for people,” Wang said. “It also means that these plants are very sensitive to drought. If a drought comes, and they can’t close their stomata, they are subject to high levels of mortality due to water stress.”

Scientists have for a long time considered forest water use a part of meteorological factors, as well as species composition and the availability of fresh soil water. Scientists haven’t yet analyzed the impacts of soil biogeochemistry globally on forest water, which means acidification may have taken a greater toll on fresh water than we know.

However, what scientists know is that nitrate and sulfate deposition as a result of burning fossil fuels are the main culprit behind soil acidification in the northeastern United States and eastern Europe. While nitrate and sulfate emissions have been widely restricted as a result of various regulations, the impacts of acid deposition are still a concern, especially in areas with rich agriculture.