Comparison of four tillage systems in organic farming. Effect of soil structure modification and organic matter repartition on microbial biomass and soil respiration

In agricultural field, the environmental conditions of soil micro-organisms and their ability to degrade soil organic matter and release nitrogen are strongly modified by the type of tillage. We studied the influence of 4 tillage systems in organic farming on the soil microbial biomass and itspotential activities of C and N mineralization. In order to connect soil structure resulting from these tillage systems with the soil microbial biomass and to extrapolate microbial measurements at the field scale, we adopted a particular sampling scheme based on the morphological characterisation of the soil structure by description of soil profiles. We sampled in three layers (0-7, 7-15 and 15-30 cm) clods with distinct physical characteristics: porous clods with visible porosity (tau clods) and compacted clods without visible porosity (delta clods). On each clod we measured distinct physico-chemical (C and N content, C/N, pH and total porosity) and microbiological characteristics (soil microbial biomass SMB, C and Nmin). This sampling method revealed the influence of soil structure and organic matter repartition on SMB and its activity and allowed a more precise assessment of tillage managements on the soil microbial functioning during the transition period from ploughing to no ploughing.