TEMPORAL CHANGES IN SOIL MICROBIAL BIOMASS CARBON IN AN ARABLE SOIL - CONSEQUENCES FOR SOIL SAMPLING

Sugar beet, winter wheat and winter barley were planted within a crop rotation on an arable soil with conventional soil management. Soil samples were taken monthly from different depths of the whole plough layer (0-10, 10-20 and 20-30 cm) during a 56 month period. The samples were analysed for microbial biomass carbon using the substrate-induced respiration technique. Temporal changes in the amount of microbial biomass carbon were observed. Within a year, microbial biomass-C varied from low values (-15% of total mean) in winter to high values (+15% of total mean) in summer. Relative deviations from the annual means were calculated for each month in the year to demonstrate these fluctuations. Temporal changes in microbial biomass-C depended on the sources of sample variation (5 years, 3 crops, 3 sampling depths). The highest relative deviation from the annual mean microbial biomass-C was attributable to the factor ''year''. Less variations were caused by ''crops'' and ''sampling depth''. Soil microbial biomass-C remained constant during frost periods. From the observed temporal changes, recommendations for a suitable date for soil sampling are given, which allows a representative estimation of the mean annual microbial biomass-C content in arable soils.