Congress will soon give careful consideration to renewal of the Clean Air Act and its amendments. In anticipation of this event, a group of scientific experts met last year to discuss their research of acid deposition, often called acid rain. The results of this meeting have been summarized and are now available in a new report from the Ecological Society of America entitled, "Acid Deposition: The Ecological Response."
The group of experts was composed of 40 of the nation's top ecological researchers. They met in March 1999 with three specific goals in mind: to evaluate the long term trends emerging from research data on ecosystem response to acid deposition; to examine whether changes have occurred in the ecological damage caused by acid deposition, and to compare those changes to observations and projections of the 1990 National Acid Precipitation Assessment Program (NAPAP) Report. During this same year, the Clean Air Act Amendments (CAAA) were passed to reduce the adverse effects of acid deposition through phased reductions of annual emissions of its precursors, sulfur dioxide (SO2) and nitrogen oxides (NOx). The scientists also wanted to ascertain whether ecosystems were experiencing any kind of recovery from previous acid deposition.
The most important conclusions drawn from the workshop regard regional differences in both deposition levels and recovery rates. In the Midwest and Northeast, the scientists agreed, emission reductions of sulfur dioxide correspond to decreases in the fluxes of sulfate in both precipitation and surface waters. But in some regions and ecosystems, these reductions may not have resulted in ecosystem recovery.
Several factors may be limiting this recovery. First, acid deposition can deplete base cations from soils, such as calcium and magnesium, that are essential for plant growth. Besides being essential for plants these elements also neutralize further acid inputs.