1. Impacts of wheel traffic on the physical properties of a Luvisol under reduced and conventional tillage Saur Marie-Laure 1, Destain Marie-France 1, Roisin Christian 2, Mercatoris Benoit 1 1 Department of Biosystem Engineering, Gembloux Agro-Bio Tech, University of Liège, 2 Passage des Déportés, 5030 Gembloux (Belgium) 2 Department of Agriculture and Natural Environment, Walloon Agricultural Research Centre, 9 Rue de Liroux, 5030 Gembloux (Belgium) Introduction Compaction is one of the major degradation of agricultural soils [1]. Due to the traffic of heavy machines (Fig. 1), this phenomenon basically leads to a decrease of soil porosity and an increase of soil strength.  Agricultural consequences : Crop yield drops since compaction is harmful for root growth, germination, mesofauna and bacterial life.  Environmental consequences : Hydraulic conductivity drops which increases risks of runoff (Fig. 2), contamination of surface water, erosion and emission of greenhouse gases. In a context of sustainable and precision agriculture, the objectives of this research project [2] are (i) to study the influence of different agricultural techniques on the soil compaction susceptibility and (ii) to characterise the impact of wheel traffic on the physical properties of soils. Materials et methods 1.Experimental approach :  Soil samples were taken in a Luvisol on plots under conventional (CT) and long-term reduced tillage (RT).  Automatic cone index [3] measurements were performed to map the soil resistance and identify the different depths (Fig. 3).  Bulk density, precompression stress (Fig. 4) and pore size distribution (Fig. 5) were combined. These measurements were performed before andafter machine passage (beet harvester, tractor). 2. Numerical modelling :  A finite element method coupled to a modified Cam-Clay model was used to predict the elastoplactic soil behaviour.  The model parameters were calibrated from the experimental measurements. Efforts to reduce the impact of wheel traffic on agricultural soil :  The model highlights the need to decrease tyre pressure and machine weight.  Reduced tillage benefits should be relativised in the presence of heavy machines in wet soil conditions as soil will not be loosened by tillage anymore resulting in strongly compacted soil years after years. In order to extend the validation of the model in different situations, measurements will be performed at different water contents and for different types of agricultural machinery. [1] D’Or, D., Destain, M.-F., 2014. Toward a tool aimed to quantify soil compaction risks at a regional scale: application to Wallonia (Belgium). Soil Tillage Res. 144, 53-71. [2] Destain, M.-F., Roisin, C., Dalcq, A.-S., Mercatoris, B.C.N. 2016. Effects of wheel traffic on the physical properties of a Luvisol. Geoderma 262, 276-284. [3] Roisin, C., 2007. A multifractal approach for assessing the structural state of tilled soils. Soil Sci. Soc. Am. J. 71, 15-25. Figure 6 : Porosity distribution in CT (conventional tillage) and RT (reduced tillage) Figure 7 : Before/after passage of a tractor in CT (conventional tillage) and RT (reduced tillage) Figure 1 : Beet harvester Figure 2 : Runoff due to soil compaction Conclusions Results 1.Experimental approach :  Plough pan at 30-cm depth was identified under (CT) and still present after long term RT (Fig. 6).  Subsoil under RT showed a natural regeneration process of the microporosity (Fig. 6 and 7).  After agricultural machine passages (Fig. 7) : - In CT, measurements revealed large modifications of soil structure in the topsoil and slight changes through depth. - In RT, the modifications were more limited through depth. 2. Numerical modelling :  The study of variations of machine weight and tyre pressure (Fig. 8) showed that the machine weight has an influence in the topsoil and the subsoil, whereas the tyre pressure affects only the topsoil. Figure 4 : Oedometer test Figure 3 : Mapping of an automatic cone index in CT (conventional tillage) and RT (reduced tillage) Figure 5 : Mercury intrusion porosimetry RT CT Figure 8 : Modelling of the variation of machine weight and tyre pressure through soil depth