Application of biochemical degradation indices to the microbial decomposition of maize leaves and wheat straw in soils under different tillage systems

In a long-term tillage experiment comparing mouldboard plough and rotary harrow since 1967, litterbags with green maize residues and wheat straw were buried, recovered from soil and analysed for biochemical degradation indices. Our objective was to investigate whether lignin units and neutral and amino sugars give information on microbial degradation after burial periods of 6 and 12 months at two different depths (0–5 and 15–20 cm). Tillage and burial depth had no effects on the content and composition of lignin. In contrast, the extended burial period resulted in higher acid/aldehyde ratios of vanillyl units, due to increasing microbial oxidation, and in higher ratios of cinnamyl/vanillyl units, due to the higher resistance of vanillyl units against microbial degradation. The contents of mannose, bacterial muramic acid, and fungal glucosamine were significantly higher in the plough than in the harrow treatment, due to a higher microbial colonisation. For the same reason, the extended burial period led to significant increases in the contents of mannose, glucosamine, muramic acid, and galactosamine as well as in the GM/AX ((galactose + mannose)/(arabinose + xylose)) ratio. The decline in the fungal C/bacterial C ratio indicated that bacterial colonisation of litter followed fungal colonisation with delay. A greater burial depth led to a lower microbial colonisation and consequently had contrasting effects to those of a longer burial period. Treatment effects on maize residues and wheat straw were generally similar, despite the strong differences in composition. The combination of litterbags and biochemical degradation indices gave further evidence that lower tillage intensity reduces microbial turnover and decomposition activity.