Atmospheric Deposition and Canopy Interactions

Abstract

Element inputs by atmospheric deposition form a major contribution to a number of element fluxes of forest ecosystems. During the last few decades, inputs from the atmosphere have significantly altered the geochemical cycles of forest ecosystems especially in heavily polluted areas of Central Europe where forests have remained major sinks for air pollution. The deposition of acids such as sulphuric and nitric acids was a major environmental concern during recent decades (Galloway 1995). Acid deposition has caused the acidification of soils and freshwaters in large areas of North America and Europe (Johnson et al. 1991). The effects of soil acidification are the leaching of base cations from the soil (Matzner and Murach 1995), the release of aluminium (Al) ions and heavy metals into soil solution (Tyler 1983), reduced decomposition of soil organic matter, and reduced growth of fine roots (Godbold et al. 2003). The release of acid soil solutions to the hydrosphere is detrimental to aquatic ecosystems. During the last two decades, deposition of acids has decreased substantially in Central Europe due to improved emission controls and the closing down of industry in eastern Germany after the reunion (Meesenburg et al. 1995). However, the deposition of nitrogen (N) compounds has decreased only slightly and has become an increasingly important fraction of the total deposition of acids (Wright et al. 1995). Despite reduced deposition of free acidity, the acid load to soils is still high because of high deposition of ammonium (NH 4 + ). The fate of elevated N-input on forest ecosystem remains partly unknown, but besides the impact on the acid/base balance of ecosystems there are some indications of increased tree growth, reduced root/shoot ratio, nutrient imbalances, reduced frost hardiness and elevated foliage consumption by insects (Binkley and Hogberg 1997; Aber et al. 1998; Meiwes et al. 1999). In N-saturated ecosystems, soils have increased susceptibility for losses of nitrate (NO 3 − ) to the hydrosphere and of trace gases to the atmosphere (Aber 2002).