Pore structure of natural and regenerated soil aggregates: An x-ray computed tomography analysis

Quantitative characterization of aggregate pore structure can reveal the evolution of aggregates under different land use and management practices and their effects on soil processes and functions. Advances in X-ray computed tomography (CT) provide powerful means to conduct such characterization. This study examined aggregate pore structure of three differently managed same textured Danish soils (mixed forage cropping, MFC; mixed cash cropping, MCC; cereal cash cropping, CCC) for (i) natural aggregates, and (ii) aggregates regenerated after 20 mo of incubation. In total, 27 aggregates (8-16 mm) were sampled from nine different treatments; three natural soils and three repacked lysimeters without and three with organic matter (OM; ground rape) amendment. Three dimensional X-ray CT images, tensile strength, and organic carbon (OC) were obtained for each aggregate. Aggregate-associated OC differed significantly between the three soils as 2.1, 1.4, and 1.0% for MFC, MCC, and CCC, respectively. Aggregate porosity and pore connectivity were significantly higher for CCC aggregates than for MFC and MCC aggregates. The CCC aggregates had an average pore diameter of 300 mm, whereas MFC and MCC had an average pore diameter of 200 and 170 mm, respectively. Pore shape analysis indicated that CCC and MFC aggregates had an abundance of rounded and elongated pores, respectively, and those of MCC were in-between CCC and MFC. Aggregate pore structure development in the lysimeters was nearly similar irrespective of the soil type and organic matter amendment, and was vastly different from the state of natural aggregates. Aggregate porosity (>30 mm) was observed to be a good predictor for the mechanical properties of aggregates. In general natural aggregates were stronger than lysimeter aggregates.