BIOLOGICAL PROCESSES IN 2 BEECH FOREST SOILS TREATED WITH SIMULATED ACID-RAIN - A LABORATORY EXPERIMENT WITH ISOTOMA-TIGRINA (INSECTA, COLLEMBOLA)

The effect of simulated acid rain on soil biotic processes was studied in a beech forest on moder soil using experimental chambers containing intact soil cores sampled in the Solling research area (Germany). Substrate from a limed area was included in order to analyse the influence of a modified state of base saturation on the reaction of soil organisms to acid rain. Water adjusted to pH 4.15 with H2SO4 was applied to the control treatments to simulate the proton deposition by precipitation, and water adjusted to pH 2.95 was applied to the acid rain treatments to simulate the proton input by stemflow water. In the acid treatment of the natural soil, CO2-C production, C mineralization of the litter colonizing microflora, microbial biomass in the F layer and leaching of nitrate-N were reduced. In contrast, the metabolic quotient of the F microflora as well as leaching of mineral-N (ammonium-N + nitrate-N) were increased by acid rain. According to its effect of CO2-C production in the L layer as well as on the mineralization of C-14-labelled beech litter, the negative effect of acid rain on the early stage of litter decomposition was especially strong. After termination of the acid rain regime, the litter colonizing microflora responded rapidly and their CO-14-2 production approached that of the controls within 3 weeks. The collembolan species Isotoma tigrina reduced C mineralization of the litter colonizing microflora, diminished the biomass and the metabolic quotient of the microflora in the F layer and accelerated the leaching of mineral-N in the pH 4.15 treatment of the natural soil. For many properties measured these effects were significantly different in the lime and in the pH 2.95 treatments. In the acid rain treatment of the natural soil, I tigrina diminished the acid-induced leaching of mineral-N and accelerated the recovery of the litter colonizing microflora after acid rain had been terminated. The mesofauna may thus form an important part of the biological buffering system in the organic layer of acid soils. In the limed soil, the effect of acid rain on many of the propertied measured was less pronounced than in the natural soil. It was thus concluded that liming of acid forest soils increases the buffering capacity of biotic processes against acid stress. However, the buffering effect on the early stage of litter decomposition was diminished by the fact that in the limed soil, in contrast to the natural soil, the litter-colonizing microflora was not able to respond rapidly after termination of the acid rain regime. In contrast to its effect in the natural soil, I tigrina did not accelerate recovery.