Consequences on macroporosity and bacterial diversity of adopting a no-tillage farming system in a clayish soil of Central Italy

Conservation agricultural practices, such as no-tillage, crops rotation and balanced fertilization are increasingly adopted for maintaining soil fertility, improving crops health and reducing soil erosion. The aim of this study was to evaluate the effects of the long-term adoption of contrasting tillage (no-tillage or conventional tillage) and N-fertilization (0 or 90 kg/ha N) practices on soil porosity and active bacterial communities in cropping system plots (sunflower–wheat or maize–wheat rotation) established on a clayish soil and under Mediterranean climate. Soil porosity was evaluated by micromorphological observations of soil thin sections. The composition and structure of the active bacterial communities were estimated by a culture-independent approach (reverse transcription – polymerase chain reaction – denaturing gradient gel electrophoresis, RT-PCR-DGGE) exploiting the 16S rRNA of bacteria and nirK, nirS and nosZ transcripts of denitrifiers. Finally, canonical correspondence analysis (CCA) was used to correlate microbial data with soil physical and chemical characteristics. When repeated for a long period, no-tillage has significantly increased soil compaction compared to the conventional tilled soil. Soil compaction was likely responsible for creating a selective environment for active bacterial species. On the other hand, tillage favoured the richness and diversity of active soil bacteria by increasing the rate of diffusion of O2 and the energy sources availability. A wide variability of active nirK denitrifiers was found in each soil management, while nirS denitrifiers were more closely related to lower porosity conditions. N fertilizer management seemed to affect mainly the active nosZ denitrifiers. Our results suggested that conservation tillage practices on heavy clayish soils are not free of relevant side effects on soil porosity and bacterial soil communities