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 sam ples were taken monthly from different depths of the whole plough laye r (0-10, 10-20 and 20-30 cm) during a 56 month period. The samples wer e analysed for microbial biomass carbon using the substrate-induced re spiration technique. Temporal changes in the amount of microbial bioma ss carbon were observed. Within a year, microbial biomass-C varied fro m low values (-15% of total mean) in winter to high values (+15% of to tal mean) in summer. Relative deviations from the annual means were ca lculated for each month in the year to demonstrate these fluctuations. Temporal changes in microbial biomass-C depended on the sources of sa mple variation (5 years, 3 crops, 3 sampling depths). The highest rela tive deviation from the annual mean microbial biomass-C was attributab le to the factor ''year''. Less variations were caused by ''crops'' an d ''sampling depth''. Soil microbial biomass-C remained constant durin g frost periods. From the observed temporal changes, recommendations f or a suitable date for soil sampling are given, which allows a represe ntative estimation of the mean annual microbial biomass-C content in a rable soils.