Long-term influence of different tillage intensities on soil microbial biomass, residues and community structure at different depths

The site-specific contribution of bacterial and especially fungal residues to a tillage-induced C sequestration is largely unknown, although microbial residues contribute a significant percentage to the soil organic C (SOC) pool. In the current study, the co-accumulation of microbial residues and organic matter was investigated in a mature, 15-year-old on-farm tillage experiment (mouldboard plough (MBT), grubber (GRT), no tillage (NT)) on four different arable silt-loam sites in central and southern Germany at 0 to 40 cm soil depth. The GRT and NT treatments increased the stocks of SOC (+7 %) and microbial biomass C (+20 %) in comparison with the MBT treatment. The differences between the GRT and NT were small, but there were more positive effects for the GRT treatment in most cases. Our results indicate significant tillage effects in loess-derived silt loams suitable for rain-fed sugar beet production, although strong site-specific differences occurred for most of the soil chemical and microbiological properties analysed. In the GRT and NT treatments, the increased stocks of SOC were not caused by the preferential accumulation of fungal residues at 0–5 cm depth, whereas ergosterol-free biotrophic arbuscular mycorrhizal fungi (AMF) was promoted at the expense of saprotrophic fungi at 30–40 cm depth. Our results suggest that the relationship between saprotrophic fungi and AMF is an important factor for tillage-induced changes in microbial turnover of SOC.