Acidification of Forests and forest Soils: Current status

Abstract

Soil acidification is defined as a decrease of the acid neutralization capacity of the soil solids. By this definition, forest soils are generally acidifying under humidic climatic conditions. Some questions are yet to be answered: What is the rate of acidification? What is the cause and to what extend does acidification influence intensity parameters, like soil solution characteristics? The latter is of greater importance when evaluating the effects of forest soil acidification on trees, deeper soil layers and groundwaters. Evaluating the causes of soil acidification by establishing proton budgets reveals that atmospheric deposition of acidifying compounds, like sulfuric and nitric acids and ammonia, is the dominant cause for the acidification of forest soils in Central Europe. The relative importance of other processes (e.g. tree uptake of excess cations) depends mainly on the rate of deposition and the management regime. In northern Scandinavia and parts of North America, tree uptake may be the dominant cause of forest soil acidification. Effects of forest soil acidification on the exchangeable cation pools as well as on the composition of the soil solution may be differentiated. While processes like tree uptake and acidic deposition can reduce the base saturation of the cation exchange capacity of the soil in a similar way, the input of acidity by atmospheric deposition in connection with the input of mobile anions has specific effects on the chemical composition of the soil solution of acid forest soils, esp. on the content of inorganic Al. The release of cation acids like Al ions leads to the acidification of deeper soil layers and groundwaters and poses a significant stress on tree root functioning in terms of nutrient uptake and root growth. Strong evidence exist that base cation depletion, high Al-levels and unfavourable Mg/Al and Ca/Al ratios of the soil solutions are involved in the forest decline phenomena in Central Europe. Clearly the emission of acidifying compounds in Europe needs further significant reduction. However, direct dose/response relations are not to be expected, since the desorption of previously stored sulfate in the soil may prevent a quick recovery of soil chemical status following reduction of the deposition load. In addition to emission control, liming and fertilization of forest soils are appropriate and necessary tools for the stabilization of forest ecosystems.