Assessing the usage potential of SoilGen2 to predict clay translocation under forest and agricultural land uses

Long-term impacts of climate evolution and agricultural management on soil properties and thus soil functioning must be assessed by modelling studies. We focus on modelling of the clay depth distribution as a result of natural soil formation and agricultural management by using the soil genesis model SoilGen2 with loess-derived Luvisols in northern France. An essential first step is the calibration of the processes concerning clay leaching (lessivage) in this model, for which we propose a reproducible method consisting of a screening for a realistic parameter range, a sensitivity analysis and finally a model calibration. To be able to separate the effects of natural soil evolution and more recent agricultural activities, we calibrated the model firstly on natural soils still under forest. Four parameters were the most sensitive and in need of calibration, representing the volume fraction of the soil in contact with macropores, a filter coefficient and physical weathering producing clay-sized particles. These parameters were successfully calibrated, but independent validations at Belgian and Norwegian sites with Albeluvisols and Stagnosols developed in loess and marine clay did not always show an improvement on earlier calibrations. This can be related to several factors, most importantly the dominance of processes other than lessivage in Albeluvisols and uncertain reconstructions of the climate. After calibration, a functional sensitivity analysis was carried out to assess the effect of agricultural management on lessivage. Clear effects on clay loss in the topsoil were found for the length of the crop-cover period and the ploughing depth and to a lesser extent for the intensity of soil mixing and the interception evaporation of the crops. As it is not feasible to reconstruct these factors for the entire period of agriculture in this region (several millennia), further analysis should take the form of scenario studies, but this should be preceded by use of field leaching experiments to calibrate SoilGen2 over short time-scales.