Degradation and humification of maize straw in soil microcosms inoculated with simple and complex microbial communities.

The microbial communities in agricultural soils are responsible for nutrient cycling and thus for maintaining soil fertility. However, there is still a considerable lack of knowledge on anthropogenic impacts on soils, their microflora, and the associated nutrient cycles. In this microcosm study, microorganisms involved in the conversion of crop residues were investigated by means of classical microbiological and molecular methods such as denaturing gradient gel electrophoresis (DGGE) of PCR (polymerase chain reaction) amplified 16S rRNA genes. 14C-labelled maize straw was humified by the naturally occurring microflora in native and in ashed soils, from which organic carbon was removed by heating at 600°C. The humic acids synthesized in the microcosms served as indicators of the humification process and were analysed by 13C-NMR spectroscopy. Ashed, autoclaved and native soil exhibited similar microbial and physicochemical dynamics after inoculation with a soil suspension. Bacterial counts and DGGE analyses showed that in the first few weeks a small number of rapidly growing r-strategists were principally responsible for the conversion of maize straw. As the incubation continued, the bacterial diversity increased as well as the fungal biomass. 13C-NMR spectroscopy of 26-week old soil extracts revealed that structures typical of humic substances also evolved from the plant material