Characteristics of Soil and Acid Deposition at Natural Forest in Mountainous Region

Abstract

To gain a better insight into acid deposition-soil interaction, which is thought to be one of the major causes of the forest decline, natural forest of fir in the mountainous region (HOUMAN-SANGUN in Fukuoka) has been taken as a model field. Fir trees in this region have recently been damaged to some extent probably due to acid deposition. Samples, which include deposition and surface waters flowing down through this mountaunous region, were collected and analyzed every 2 weeks over 6 months period between 1990 and 1991. Surface soils in the forest were also examined.
The pH-value and concentrations of SO₄^2- and NH₄⁺ in the forest rain at 920 m in altitude were, on average, 4.4, 3.7μg/ml, and 0.5μg/ml, respectively. Concentrations of each ionic component were also about same as those observed in the plains in this area. The effect of deposition on the forest environment, however, should not be underestimated, because there is rainfall in the forest region of 1.44 times as much as that in the plains.
The EC-values and ionic species concentrations in the surface waters were almost constant during the monitoring period, the average pH-value was 6.3. Concentrations of Ca²+, NH₄⁺, and NO3-in the surface waters were found to be 4.8, 0.04, and 0.5 times, respectively, of those in the forest rain.
The forest surface soil, the pH-value of which was 4.1, was found to be more acidic than the forest rain and coming close to the pH region, where the buffer action of Al is caused. Cation exchange capacity of the soil was 32 meq./100g, while the total exchangeable capacity of Ca and Mg, which represents a buffer capacity of soil, was 1.23 meq./100g. In the forest soil in this region the exchangeable H was highly correlated to the exchangeable Al. These observations indicate that this forest soil is acidified, and is very sensitive to acid deposition.
The mass-balance of ionic species in this forest region during the monitoring period was taken into account, and the inputs of H and NH₄⁺ from the atmosphere as rainfalls were considerably excess over the outputs of these species to the surface water, while for Ca²⁺ and Na⁺, their outputs were over their inputs. It is noteworthy that the total equivalent of the excess outputs of Ca2+ and Na+ are almost equal to that of the excess inputs of H⁺ and NH⁴⁺. The NH₄⁺ species of atmospheric origin undergoes oxidation-denitrization in soil, and serves as a Hdonor. This means that the excess inputs of H+ are balanced by the excess outputs of Ca²⁺ and Na⁺.