Microbial contribution to sulphate mobilization after liming an acid forest soil

SUMMARY Sulphate release from unsterilized and γ‐irradiation sterilized soil samples was studied in leaching funnels over a 22‐week incubation period. In unsterilized soil samples, lime addition to the acid forest topsoil caused an immediate increase in extractable sulphate, but enhanced sulphate release rates were detectable only during the first 4 weeks of incubation. Soil samples from an adjacent five‐year‐old limed plot displayed significantly higher sulphate release rates than the control throughout the experiment. The cumulative amount of sulphate extracted from the unsterilized treatments was modelled with parabolic functions 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. Mineral‐N release was also greatest in the samples from the old limed plot while fresh lime additions reduced net N‐mineralization due to microbial immobilization. In the sterilized samples, γ‐irradiation initially caused an increase in extractable sulphate and ammonium which is mainly attributed to the destruction of microbial biomass. After the second week, no further differences in extractable sulphate were detectable between the treatments. After 22 weeks, between 7 and 9 mg SO 4 2− ‐S kg −1 soil was released from the sterilized samples which was 25–50% of the respective amounts in the unsterilized samples. A similar relationship was found for N. The data indicate that even 5 years after lime addition the observed increased sulphate concentrations in the soil solution can be attributed to enhanced microbial activity.