Long-term effects of a one-time application of flue gas desulfurization gypsum on the soil pore structure in sodic paddy fields

Saline–sodic soil suffer from a high content of exchangeable sodium ions in the soil colloid and the poor air and water permeability of the soil pore structure. Flue gas desulfurization (FGD) gypsum improves sodic saline–alkali soil by replacing sodium ions with calcium ions. However, research on the original soil pore structure after the modification of sodic saline–alkali soil with FGD gypsum is limited, especially the subsequent long-term effects on soil pores. In this study, we utilized X-ray computed tomography (CT) scans to analyse undisturbed soil samples from depths of 0–20 cm in fields that were treated once with FGD gypsum and planted with rice for various periods (1 year (T1), 3 years (T3), and 10 years (T10)). Samples from untreated soil (T0) were examined. The volume, distribution, and morphology of the soil pores were compared. The results revealed that FGD gypsum markedly increased the soil total porosity (≥70.65 μm) in the 0–10 cm layer, increasing it by 2.92, 2.76, and 2.37 times 1-, 3-, and 10-years posttreatment, respectively, compared with that in untreated soil. With the extended improvement time, the soil total porosity of the sodic soil treated with FGD gypsum decreased but remained significantly higher than that of the soil without FGD gypsum application. In contrast, the total porosity decreased in the 10–20 cm layer. The number of pores smaller than 1 mm increased in the 0–10 cm layer after the FGD gypsum application. The FGD gypsum treatment predominantly influenced the volume of larger pores (diameter > 2 mm), with a minimal effect on smaller pores. The increases in the fractal dimension and tortuosity after the FGD gypsum treatment indicated the formation of a more complex soil pore structure. FGD gypsum improved the soil pore skeleton network by increasing the thickness of the skeleton network and the number of coordination sites and by reducing the vertical pore angle, thus improving soil aeration and water permeability. These findings confirm that the pore structure of sodic alkaline soils can be significantly improved with a one-time application of FGD gypsum, verifying its long-term advantages.