Microbial biomass and activities in soil as affected by frozen and cold storage

When monitoring microbial biomass and activities in soil, the storage conditions of the soil samples prior to analysis may be decisive for the results. Studies made on freshly collected soils are generally preferred but these are not always possible for practical reasons, since sampling is often restricted to short periods of the year, e.g. due to climatic conditions. The most commonly used methods to store soils for microbiological analyses are refrigeration or freezing of field moist soil. There are, however, studies that warn against any kind of storage, although other studies do not indicate any drawbacks to it. We have compared the microbial biomass and activities in 12 different, annually frozen, agricultural mineral soils when fresh and when stored at +2+/-2 degrees C and at -20+/-2 degrees C for 1 d and for 1, 3, 6 and 13 months. The results showed that the effects of freezing generally were smaller than those of refrigeration. The biomass estimated by chloroform fumigation-extraction and biomass index estimated by substrate induced respiration differed in that chloroform fumigation extracted carbon had decreased with 27% after 3 months at +2 degrees C, while substrate induced respiration showed only small deviations from the results from fresh soils. Basal respiration rate and potential denitrification activity showed a similar pattern, with a pronounced decrease in values for refrigerated soils. The nitrogen mineralisation capacity was the only measure that was greatly influenced by freezing. After 6 months N mineralisation in the frozen soils was 25% higher than that of the fresh soils. Potential ammonium oxidation and the degradation rate of the herbicide linuron were affected only a little or not at all by storage for 13 months. We concluded that storage at -20 degrees C for 13 months does not affect the microflora in annually frozen soils in any decisive way. We have also discussed the possible reasons for the contradictory results between different studies made on storage effects. (C) 1998 Elsevier Science Ltd. All rights reserved.