Pore morphology changes under tillage and no-tillage practices
Résumé
The relative influence of biological and mechanical processes on the structure of cultivated soils was estimated by investigating the macroporosity of the surface layers of a silty soil during a maize growing season. The soil was subjected to different cultivation techniques (conventional tillage, moderate tillage, and no tillage) and fertilization modes (mineral or organic) for five years. A typological model was developed (i) to identify the macropores by 2D image analysis in undisturbed soil samples, and (ii) to characterize their morphology with two shape parameters (elongation and regularity indexes). Based on the three usual pore categories (tubular, planar and packing pores), five groups were defined with an additional distinction within the packing pore category (discrete, continuous and planar packing). This typology, as based on a set of images that reached the representative elementary area (REA), proved appropriate to describe the structural modifications linked to the crop management practices. It was shown that mechanical soil working produces ‘continuous packing pores' whereas the absence of work induces ‘discrete packing pores'. The study of the intra-annual structural dynamics enabled the identification of the processes responsible for those modifications. With conventional tillage, the progressive substitution of continuous packing pores with discrete packing pores can be interpreted as the resumption of biological fragmentation after mechanical fragmentation (harrowing). Without management, the soil is only subjected to biological fragmentation, so neither quantitative nor morphological evolution of the macroporosity was observed during the growing season, with the soil structure depending more on spatial heterogeneity than on seasonal changes.
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