MICROBIAL CONTRIBUTION TO SULFATE MOBILIZATION AFTER LIMING AN ACID FOREST SOIL

Sulphate release from unsterilized and gamma-irradiation sterilized so il samples was studied in leaching funnels over a 22-week incubation p eriod. In unsterilized soil samples, lime addition to the acid forest topsoil caused an immediate increase in extractable sulphate, but enha nced sulphate release rates were detectable only during the first 4 we eks of incubation. Soil samples from an adjacent five-year-old limed p lot displayed significantly higher sulphate release rates than the con trol throughout the experiment. The cumulative amount of sulphate extr acted from the unsterilized treatments was modelled with parabolic fun ctions and reached 4.6%, 6.8% and 8.2% of the initial total S-contents of the control, the lime treatment and limed plot respectively. Miner al-N release was also greatest in the samples from the old limed plot while fresh lime additions reduced net N-mineralization due to microbi al immobilization. In the sterilized samples, gamma-irradiation initia lly caused an increase in extractable sulphate and ammonium which is m ainly attributed to the destruction of microbial biomass. After the se cond week, no further differences in extractable sulphate were detecta ble between the treatments. After 22 weeks, between 7 and 9 mg SO42--S kg-1 soil was released from the sterilized samples which was 25-50% o f the respective amounts in the unsterilized samples. A similar relati onship was found for N. The data indicate that even 5 years after lime addition the observed increased sulphate concentrations in the soil s olution can be attributed to enhanced microbial activity.