Effects of atmospheric deposition on forest nutrient cycles

Atmospheric depositions of nitrogen, sulfur, and hydrogen ion have probably increased over the last few decades in many parts of the world. Because most forests are deficient in nitrogen, such increases can be beneficial. Little is known about the changes (if any) in atmospheric deposition of phosphorus and base cations, but there are indications that base cation inputs have declined recently. This, coupled with atmospheric H/sup +/ inputs (acid deposition) and internal H/sup +/ generation within forest ecosystems, results in a net loss of cations from most forest ecosystems. The extent to which an individual cation is leached depends upon the relative abundance of that cation on soil exchange sites and its selectivity for adsorption. Thus, scarce or tightly bound cations may be conserved despite very intense leaching rates, while more abundant or loosely bound cations are leached. In extremely acid soils, all base cations may be conserved, while hydrogen and aluminum ions are leached. 2 tables.