Combined effects of long-term tillage and fertilisation regimes on soil organic carbon, microbial biomass, and abundance of the total microbial communities and N-functional guilds

Reduced tillage intensity is known to increase soil organic carbon (SOC) in the topsoil, but can also lead to increased microbially derived nitrous oxide (N2O) emissions. Although the trade-offs between C sequestration and N2O emissions under different agricultural practices have been extensively investigated, the reported results are conflicting and, thus remain unclear. In this study, the effects of different types of tillage [no-till (NT) and conventional mouldboard tillage (CT)] in combination with four fertilisation regimes [unfertilised control (CON), mineral NPK (MIN), organic (compost) (ORG), and mixed (NPK + compost) (MIX)] on SOC and microbial biomass (Cmic) were examined two decades after the experiment was initiated. The abundance of the microbial community and N-functional guilds within the soil profile (up to a depth of 60 cm) was also determined. SOC content was significantly higher in NT than in CT at 0-10 cm depth, with an average difference of 0.6-1.3 % SOC, depending on fertilisation. Organic fertilisation increased the SOC content in both tillage systems up to a depth of 20 cm. Microbial biomass, abundance of total bacterial and archaeal 16S rRNA, and fungal ITS genes decreased with depth, corresponding to the decrease in SOC, and, consequently, were higher in NT than in CT, in the top 20 cm soil for microbial biomass and bacterial abundance and top 10 cm for fungi and archaea. Denitrifiers and ammonium oxidising archaea (AOA) were affected by soil depth and tillage, whereas the distribution of ammonium oxidising bacteria (AOB) was affected by fertilisation and depth. The ratios between (i) the two nitrite-reducing communities (nirS/nirK), (ii) the two N2O-reducing communities (nosZI/nosZII), and (iii) the nitrite-and N2O-reducing communities [(nirK + nirS)/(nosZ + nosZII)] increased significantly with soil depth, indicating niche differentiation caused by differences in nutrients contents and environmental conditions. Overall, stratification of SOC and nutrients in the soil by tillage and fertilisation was the main driver of the differences in the total microbial and N cycling communities. The findings of our study provide novel insights that can aid in development of effective strategies for steering soil microbiome responsible for N2O emissions.