Soil management modifies micro-scale abundance and function of soil microorganisms in a Mediterranean ecosystem

The present study tests whether soil management (tillage and fertilizer) modified the small-scale abundance and function of soil microorganisms in response to changes in organic matter quantity and quality. The experimental field, located in the coastal hills of Marche (central Italy), was planted in rotation with Triticum durum in winter and Zea mais in summer. Soil samples were collected in the maize-field soil, in conventional and no-tillage (NT) systems, and in fertilized and unfertilized soil. We analysed total organic C (TOC), total nitrogen (TN) microbial biomass C (MBC), enzymes involved in C- (β-glucosidase, α-glucosidase, β-cellobiohydrolase, β-xylosidase), N- (leucine-aminopeptidase and N-acetyl-β-glucosaminidase), P- (acid phosphatase) and S-cycling (arylsulphatase), as well as functional diversity in the bulk soil, coarse sand, fine sand, silt and clay fractions. Micro-scale investigations revealed great microbial abundance in smaller fractions because of protection offered by microaggregates, whereas the distribution of enzymes reflected the availability of their corresponding substrates. No-tillage treatment significantly increased organic input, mainly in the coarser fractions, enhancing enzyme activities and the functional diversity of the microbial community. This effect was even larger in the absence of fertilizer. At the particle-size level of resolution, adding fertilizer stimulated nutrient cycling. Our results confirmed the hypothesis that no-tillage enlarges the content of particulate organic matter in the coarse sand fraction and stimulates microbial decomposition. In the smaller fractions the enlarged microbial pool and increased soil organic matter with small C/N ratio under NT confirm that this management practice is effective in increasing soil C sequestration capacity.