Microbial biomass in arable soils of Germany during the growth period of annual crops

Results from several field studies involving numerous measurements were used to describe the change of soil microbial biomass C (Cmic) and N (Nmic) during the growth period of annual crops (years 1988-1992, 1994, 1995) under the temperate climatic conditions of central Europe. The data were taken from our own investigations as well as from the literature. Only studies with at least eight measurements on one plot during the growth period were used. The total number of farms (cash crop-production farms) was 7, that of experimental plots was 15. The evaluation of these results through regression analysis demonstrated that Cmic and Nmic from the beginning of a year increased only slightly until summer and subsequently decreased until autumn to their initial levels. This increase on an average corresponded to a C assimilation of approx. 100 kg ha⁻¹ and an N immobilization of approx. 20 kg ha⁻¹ (30 cm)⁻¹. The increase in Nmic alone could not explain N immobilization rates frequently observed in different studies using ¹⁵N-labeled fertilizers. Most of the labeled N that was immobilized (>50 kg N ha⁻¹) might have accumulated in the matrix of soil organic matter (SOM). Therefore, the changes in microbial biomass may be of less importance for changes in soil N storage as frequently assumed.