Acidification of forest soils: Model development and application for analyzing impacts of acidic deposition in Europe

Acidification is considered to be an unfavourable process in forest soil. Timber logging, natural accumulation of biomass in the ecosystem, and acidic deposition are known sources of acidification. Acidification causes a risk of damage to plant roots and subsequent risk of a decline in ecosystem productivity.

A dynamic model is introduced for describing the acidification of forest soils. In 1-year time steps the model calculates the soil pH as a function of the acid stress and the buffer mechanisms of the soil. Acid stress is defined as the hydrogen ion input into the top soil. The buffer mechanisms counteract acidification by providing a sink for hydrogen ions. The concepts buffer rate and buffer capacity are used to quantify the buffer mechanisms. The model compares (a) the rate of acid stress (annual amount) with the buffer rate, and (b) the accumulated acid stress (over several years) with the buffer capacity. These two comparisons give an estimate of the soil acidity.

The model was incorporated into the Regional Acidification Information and Simulation (RAINS) model system of the International Institute for Applied Systems Analysis for analyzing the acidic deposition problem in Europe. This system links information on energy production, pollutant emission, pollutant transport, and pollutant deposition. The data on acid stress entering the soils was obtained from other submodels. Data on buffer rate and buffer capacity were collected from soil maps and geological maps.

The model system as a whole is now available for analyzing the impact of different emission scenarios. The soil acidification model assumes sulfur deposition estimates from the other submodels as input, and as output it produces estimates of the acidity of European forest soils in a map format. Additionally it computes the total area of forests in Europe with the estimated soil pH lower than any selected threshold value. Sources of uncertainty in the soil acidification model are listed and briefly evaluated.