Limited influence of tillage management on organic matter fractions on silt soils under cereal/root crop based rotations

Reduced tillage (RT) management may increases the top soil organic carbon (SOC) mainly by increasing labile organic matter (OM) in particular due to a reduction of the mixing of freshly added OM in the soil and due to enhanced physical protection of particulate organic matter (POM) inside the micro-aggregates. However, most previous studies through-out the literature have focused on true or nearly complete no-tillage farming systems with cereal based monocultures and experimental data is absent for cereal-root crop rotations. We here investigated the effect of reduced tillage (RT) on the storage and distribution of SOC and soil organic nitrogen (SON) over different soil fractions in loess arable silt soils under temperate climate. We collected top soil samples (0-10 cm) from three pairs of adjacent reduced and conventional tilled (CT) fields located at Heestert, Court-St-Etienne and Villers-le-Bouillet (Belgium). These soil samples were physically fractionated according to two different methodologies, i.e. a microaggregate isolation method and a ultrasonication-sedimentation method in order to separate SOM into fractions. RT management was found to be effective in increasing the OC and ON content of the top soil (0-10cm) at these three sites. Physical fractionation showed that while the free particulate OC and ON can be increased by up to 70% by adopting RT management, findings from these experiments suggest a modest increase in the relative contribution of free POM under the typical arable crop rotations in Western Europe. Furthermore the intra-microaggregate OM accounted up to 50% of the difference in one out of the three experiments investigated here. However, in the other two field experiments we measured no accumulation of iPOM and this may be attributed to the repeated disturbance of soil by harvest of the root crop (sugar beat or potato). It is concluded that under Western European arable cereal-root crop rotations RT systems do not result in the storage of more SOC and SON in the entire soil profile within a 5–20 year period, but rather, redistribution towards more labile OM in the top layer (0-10 cm) takes place