Than Previously Thought
RIVERSIDE, Calif. - University of California Riverside Earth Scientist Martin Kennedy and colleagues report in the Proceedings of the National Academy of Sciences that acid rain, by leaching essential metal nutrients (such as potassium, calcium, and magnesium) from topsoil, may pose a far graver threat to forests than has been previously estimated. This result would especially interest ecologists, biologists, geologists, and policy makers.
"Our work shows that in unperturbed natural ecosystems a very small pool of these nutrients is available and this comes from the atmosphere, mostly as dilute amounts dissolved in rain that then get deposited in topsoil," said Kennedy. "The tight budget of these nutrients is a concern because if the budget is perturbed, the forests are at risk."
If deprived of a certain critical nutrient, such as calcium, a tree faces the risk of dying. In parts of Germany, for example, trees are already dying not from the direct effects of the acid, but from magnesium deficiency, this magnesium loss from the soil stemming from leaching by acid rain. Such leaching results in the loss of topsoil nutrients to groundwater and eventually to rivers.
Kennedy noted that plant roots cannot access all nutrient elements in the soil; some elements are bound in minerals and rocks. "In our study, we were attempting to determine what fraction of the total elements available in the soil the plants could access. We found it was a very small proportion."
It has long been thought that trees obtain their essential metal nutrients from weathered rock particles deep in the soil. But by demonstrating that the trees obtain these nutrients almost exclusively from atmospheric sources, Kennedy and colleagues suggest that the trees cycle a small pool of nutrients that are continually replaced by dilute atmospheric sources.
The scientists experimented on trees in the unpolluted forests of southern Chile (see Fig. 1). "We went to the cleanest atmosphere on earth and looked for a stable ecosystem so that we could find the closest thing to a long-term sustainable forest," Kennedy said (see Fig. 2). "There, we sampled soils, stream water, rain and plants, and analyzed the strontium isotope composition."
Strontium isotopes indicate very accurately which fraction came from the rain and which fraction came from the rock. "We found that in the dominant tree species - the southern Beech - approximately 90% of the strontium, and thus other similar nutrient elements, were brought in by the rain and did not come from soils or rocks, as just about everyone had assumed," said Kennedy.
The researchers also applied a distinctive artificial chemical tracer to the soils in a small portion of the Chilean forest. The tracer mimics the natural nutrients in the soils and trees with the advantage that it can be measured and observed as it moves throughout the soil plant system. By sampling the trees and soil over time and by analyzing the samples for the tracer, the scientists found that within three years most of the tracer was quickly leached from the topsoil. The loss of this element within such a short amount of time surprised the researchers because it implies that a far smaller pool of nutrients is available to the trees from the upper soil than they had imagined.
"The small size of this upper soil nutrient pool has important implications for industrially influenced forests in the northeastern United States and in Europe," said Kennedy. "These forests may be more vulnerable to the effects of acid rain than we had previously thought."
Hydrogen ions from the acid in acid rain replace the nutrient elements in the soil. For every unit of acid added to the soil, an equivalent amount of nutrient elements is removed. As a result, more nutrients get leached from the soil than arrive from weathering of rocks or precipitation (see Fig. 3).
Kennedy explained that the Chilean site was invaluable for the research because the cleanest forests indicate how the system should work if left alone. In forests in the northeast United States on the other hand, the system is already disturbed.
"Our study not only challenges the dominant paradigm that rocks and soil mineral weathering provide a majority of some important plant nutrients like calcium and potassium," said Kennedy, "but it also proposes that our 'stable' old growth forests are the most at risk from acid rain, and that it is a bigger problem, potentially, than we ever imagined."