Soil-mediated effects of atmospherically deposited sulphur and nitrogen

Abstract

The soil mediates both terrestrial and aquatic effects of sulphur (S) and nitrogen (N) deposition in a number of ways. Atmospherically deposited S in excess of biological requirements often accumulates by SO ^2- ₄ adsorption onto Fe and Al oxides in highly weathered soils, causing the release of OH ^- , which can neutralize incoming H ⁺ , or an increase in cation exchange capacity. Atmospherically deposited N seldom exceeds biological requirements; when it does, NO ^- ₃ is rapidly leached since no adsorption reactions analogous to those for sulphate exist. When the biological and adsorption capacities are exceeded, cation leaching is accelerated by the mobile SO ^2- ₄ or NO ^- ₃ anions. The effects of this increased cation leaching must be assessed within the context of natural processes of cation removal such as leaching by internally produced carbonic, organic or (occasionally) nitric acid; natural atmospheric S inputs; and cation export by vegetation harvest. The proportion of individual cations available for leaching is regulated by soil exchange processes and biological demand, both of which tend to minimize the losses of those particular cations most tightly bound and, or, in shortest supply. The ultimate effects of atmospheric S and N deposition will be highly site-specific in nature, depending on both inherent site characteristics and amounts and duration of atmospheric inputs. Thus, benefits of S or N deposition may outweigh negative effects in some sites, whereas the reverse may be true for other sites, making broad, global generalizations hazardous.