According to a new study, researchers have discovered a hitherto unknown mechanism that causes increased use of forest waters, improves understanding of soil biogeochemical control of water cycles, and highlights threats to plants from water stress.
In a study published in the magazine Scientific progress, researchers report that the mechanism works this way: sulfuric acid and nitric acid fall to the ground when fossil fuels are burned, causing soil to acidify. When this happens, a significant amount of calcium in the soil is washed away from the soil and then the plants suffer from calcium deficiency. Calcium deficiency causes plants to increase water use.
Lixin Wang, Assistant Professor at the School of Sciences at IUPUI, is the senior author of this research and his Ph.D. student Matthew Lanning is the first author.
This research is funded by the National Science Foundation's hydrological science program. Other authors of the research team include Todd Scanlon and Howard Epstein of the University of Virginia, Matthew Vadebonche of the University of New Hampshire, Mary Beth Adams of the United States Forest Service and Daniel Drukenbrod of the University of Ryder.
Calcium plays a unique role in plant cells by regulating minute pores, called the stomach, in the leaves or stems of plants, Wang said. If the plants do not have enough calcium, they cannot close these pores and their use of water increases. In addition, when plants suffer from calcium deficiency, they will pump more water through transpiration, the process of moving water through the plant and evaporating it from the leaves to meet their demand for calcium, he said.
"We hypothesized that leaching of the soil calcium from acid deposition would increase widespread use of plant water," said Lanning. "We are presenting evidence from a long-term acidification experiment throughout the catchment area, showing that changing the supply of calcium to the soil by acid deposition can significantly increase water use."
Researchers have found a wealth of evidence showing that acid-induced calcium recovery not only increases the use of plant water, but significantly reduces the soil water pool of the treated watershed.
"When plants always use a lot of water, it means that there will be less water left for humans," Wang said. "It also means that these plants are very susceptible to drought. If drought occurs and they cannot close their stomachs, they are subjected to high mortality rates due to water stress."
Traditionally, the use of forest water is considered to be a function of meteorological factors, species composition and soil water availability. The impact of soil biogeochemistry on large-scale use of forest waters has not been studied.
Nitrate and sulphate deposition is the main driver of soil acidification in the northeastern United States and eastern Europe, where atmospheric substances exceed the acidity generated by soil. In the United States and much of Europe, nitrate and sulphate emissions are limited by law, but the impact of acid deposition is still of global importance, especially in areas below large cities or high-income agricultural areas.
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