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 (C-mic), CO2 evolution, the C-mic:C-org ratio and the metabolic quotient for CO2 (qCO(2)) were determined in a Norway spruce stand (Hogrwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C-mic) was noted, C-mic increasing with increasing soil pH in O-h or A(h) horizons. More microbial biomass-C per unit C-org (C-mic:C-org ratio) was detected in limed plots with elevated pH of O-h or A(h) horizons as compared to unlimed plots with almost 3 times more C-mic per unit C-org in the limed O-h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C-mic:C-org ratio was more pronounced in the upper horizon (O-h)). 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, qCO(2), 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 C-mic:C-org ratio or qCO(2). An elevated qCO(2) could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 mu g CO2-C mg(-1) C-mic h(-1) (O-h) the highest qCO(2) value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the Hf ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C-mic:C-org ratio.