Factors influencing variability of proteolytic genes and activities in arable soils

Microorganisms, capable of proteolysis, are widely distributed in soil but almost nothing is known about the abundance of genes related to protein degradation and the regulation of their activity in terrestrial ecosystems. Therefore, the aim of this study was: (1) to quantify two bacterial genes involved in protein degradation, (2) to investigate factors affecting the abundance of these genes, and (3) to relate this data to potential proteolytic activities. For this purpose, an arable field in southern Germany under integrated management was studied. The uniformly managed field showed pronounced soil heterogeneity with four different soil types. In April, July and October 2003, soil samples were taken from the four soil types at three different depths. We applied a real-time PCR assay for quantification of subtilisin (sub) and neutral metalloprotease (npr) genes, both encoding for extracellular proteases, as well as the 16S rRNA gene representing a rough estimate of the size of the bacterial populations. Potential proteolytic activity was measured using casein as a substrate. Both soil type and time of sampling influenced the size and activity of the bacterial protease genes under investigation. Total nitrogen and carbon availability was, beside soil texture, the main factor responsible for the observed changes in the abundance of proteolytic genes and potential proteolytic activity. Whereas a positive relationship was found between sub and npr gene copy numbers and the number of 16S rRNA gene copies in all cases, a positive relationship between sub and npr coding genes and potential proteolytic activity was only found for sandy soils. This indicates that sandy soils cannot stabilize proteolytic enzymes and the activity of npr and sub genes is strictly dependent on the presence of the corresponding genes. In contrast, in clay soils proteolytic activity was not correlated with the abundance of the genes analyzed, probably due to the stabilization of the proteolytic enzymes.