The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990-1992) microbial biomass-C (Cmic), CO₂ evolution, the Cmic:Corg ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(Cmic) was noted, Cmic increasing with increasing soil pH in Oh or Ah horizons. More microbial biomass-C per unit Corg (Cmic:Corg ratio) was detected in limed plots with elevated pH of Oh or Ah horizons as compared to unlimed plots with almost 3 times more Cmic per unit Corg in the limed Oh horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the Cmic:Corg ratio was more pronounced in the upper horizon (Oh)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot. Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the Cmic:Corg ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 μg CO₂-C mg⁻¹ Cmic h⁻¹ (Oh) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased Cmic:Corg ratio.