Image analysis and soil micromorphology applied to study physical mechanisms of soil pore development: An experiment using iron oxides and calcium carbonate

The importance of soil structure for terrestrial ecosystems is well known. A large bulk of scientific literature addresses factors influencing soil structure and soil pore development, but little is still known concerning the underlying physical mechanisms of their action. In this work we attempted to investigate physical mechanisms of soil pore development as consequence of the addition of calcium carbonate (CaCO3) and iron (Fe) oxides on two soils with different shrinkage-swelling capacity subjected to several wetting and drying cycles. Analysis was conducted using soil 2D image analysis and soil micromorphology. An aggregate stability test was also performed. Our results showed changes in the pore size distribution, in some cases very large, and allowed the identification of specific mechanisms of pore modification induced by Fe and micrite pedofeatures produced by the mobilization in suspension of Fe oxides and CaCO3. These physical mechanisms were triggered by (i) Fe concretions which, in the soil samples with high shrinkage-swelling capacity, induced the formation of new pores, (ii) Fe coatings which filled and reduced the pore space, (iii) CaCO3 segregations which induced a pore size redistribution fragmenting the pore space, and (iv) CaCO3 coatings which seemed to induce a cumulative effect on porosity cementing the walls of newly-formed pores in the soil samples with high shrinkage-swelling capacity. Our results, even if obtained on experimental samples, give a contribution in the understanding of the physical role of Fe oxides and CaCO3 pedofeatures in pore formation in soils in field and show the need to reassess physical simulation tests in order to quantitatively investigate combined effects of factors influencing soil structure formation.