Influence of storage of soil samples on microbial biomass and its activity

Sampling of soil causes changes in its natural environment: among others, s oil structure and aeration status are disrupted and soil temperature and mo isture are usually affected. This results in changes of soil microbial biom ass and disruption of important biological processes. Although in general n ot recommended (Anderson, 1987), soil samples are often stored after sampli ng and before analyses, as for various reasons they cannot be processed imm ediately. The objective of this study was to assess the impact of soil stor age on microbial biomass and its activities. Soils were sampled in several sites in the Czech Republic, mostly from plots of long-term field experimen ts (soils Jaro, Lipa, Luka, Iva) or from ordinary fields (soils from Netreb ice and Chelcice). Microbial biomass, bazal and glucose-induced respiration s and nitrifying activity were determined using standard methods; the resul ts are expressed per gram of dry soil. As shown in Fig. 1, storage of most soils at 4 +/- 2 degrees C influenced microbial biomass. After one week sto rage, both increase and decrease of microbial biomass were recorded. During further storage (after 18 as well as 60 weeks) decrease of microbial bioma ss was found. However, only small fluctuations of microbial biomass but lar ge increases of bazal respiration were found in soil stored at constant moi sture at laboratory conditions (18 to 25 degrees C) for 56 weeks (Tab. I). Microbial biomass in air-dried soil first increased, but then decreased, wh ile bazal respiration was slightly increased (Tab. II). Glucose-induced res piration fluctuated at the beginning of soil storage at 4 +/- 2 degrees C a nd decreased later on (Fig. 2). Nitrifying enzyme activity was determined i n refrigerated (4 +/- 2 degrees C) as well, as in frozen (-20 degrees C) so ils, the latter defrosted quickly (2 hours) and/or slowly (2 days) before t he measurements. As shown in Fig. 3, it was enhanced in soils after 2 and 1 1 weeks of storage at 4 +/- 2 degrees C. When the frozen soils were defrost ed quickly, the activity was significantly lower, while in soils defrosted slowly it was significantly higher in comparison with soils stored at 4 +/- 2 degrees C. The results showed that microbial biomass and its activities can be substantially changed during the storage of soils after sampling. It was also found that various soils (that is soils from different sites or d ifferently fertilized soils) can respond differently. It is likely that aft er sampling there is a few days' fluctuation period of control and regulato ry factors, which is then reflected in fluctuation of microbial biomass and activities. Although Stenberg et al. (1998) recommend freezing when soils are to be stored, our results show that microbial activities in defrosted s oils can be substantially changed. When processing freezed soils, the criti cal step is defreezing. We suggest that refrigeration of soils at about 4 d egrees C is the most careful mode of storage but not even in this case unco ntrolled changes in soils can be excluded.