Structure development in aggregates of poorly developed soils through the analysis of the pore system

Abstract

Aggregate porosity is the result of structural development, in turn controlled by pedogenic processes, which allow a soil to acquire specific characteristics and to lose those more related to the parent material. Porosity in sediments can be evaluated through simple packing models that provide an ideal porosity (Φ ideal ), which is based on soil particle size distribution only. As such they are not suitable for complex systems such as soil and aggregates. We evaluated therefore if the disagreement between ideal porosity and experimental data (Φ HgT ) is related to chemical properties and to the presence of specific diagnostic horizons, hence to soil development. The porosity underestimate, evaluated as Φ HgT /Φ ideal ratio, was the highest (up to 5-times) in Inceptisols and in their cambic horizons or in mollic epipedons. Thus, where pedogenesis is expressed enough to influence soil properties to such an extend to define diagnostic horizons, the ideal porosity model failed the most in describing the complexity of the void arrangement in the aggregates. Organic matter (OM) is a key factor in determining structural complexity, not only through its accumulation in surface horizons, as evidenced by the correlation between Φ HgT /Φ ideal and organic carbon (r = 0.58), but also in deeper horizons. At the soil surface, independently from soil orders, OM contributed to structure development also through the poorly decomposed fractions. However, as also suggested by the weak linear dependence of the organic carbon-to-total nitrogen ratio on the porosity underestimate (r 2 = 0.51), it was evident that neither the quantitative, nor the qualitative data could fully explain the effects OM had on structure. Some specific spatial arrangement of particles, such as the typical hierarchical aggregation of mollic epipedons, must be taken into account to better understand the OM effects. In the subsoils, where the weathering process has led to a more complex structure, the effect of OM is enhanced by the concomitant presence of charged mineral surfaces. In Inceptisols, calcium is a co-dominant factor in structural development (r = 0.84), as expected when the formation of clay polyvalent cation-organic matter complexes occurs. In the least developed soils, pedogenic processes had a minimal effect on soil properties, and the aggregate porous system still retains some of the characteristics of the parent material, leading to a less pronounced underestimate by the ideal porosity model. ► Packing model provides ideal porosity (Φ ideal ) lower than measured porosity (Φ HgT ). ► The highest disagreement (Φ HgT /Φ ideal ) was found where pedogenesis was more intense. ► Φ HgT /Φ ideal was related to the accumulation of soil organic matter. ► However, OM effect is maximised when other pedogenic processes have proceeded enough.