Throughfall and plant nutrient concentration response to simulated acid rain treatment

Over a 30-mo period forest microcosms were used to evaluate the impact of simulated acidic precipitation pH treatments of 5.7, 4.5, 4.0, and 3.5 annual average on the nutrient content of three tree species growing in the microcosm as well as the throughfall passing through the forest canopy. Throughfall pH responded significantly to the pH treatment and it was observed that while there was some neutralizing capacity in the canopy, it was not sufficient to completely ameliorate the treatments applied. Concentrations of Ca, Cl, and PO₄ increased in throughfall; however, these increases were generally associated with the lowest pH treatments. The principal anions, NO₃ and SO₄, introduced into the system by the precipitation treatments, exhibited different response patterns with throughfall NO₃ concentration declining with time and SO₄ increasing. While throughfall analysis suggested an increase in foliar leaching of certain elements, foliage, stem, and root analysis failed to suggest any consistently significant response for any element or tree species evaluated. Leaching of dry deposited materials and compensatory nutrient cycling are suggested as possible explanations for the observed increases in throughfall concentration in the absence of reductions in plant tissue concentrations.