Evaluation of temporal changes in hydrostructural properties of regenerating permanent grassland soils based on shrinkage properties and μCT analysis

In this study, structural recovery of degraded cropland, which was converted to permanent grassland and managed under no-till and no-renovation condition, was evaluated. We combined hydrostructural parameters with shrinkage characteristics and X-ray computed microtomography (μCT) analysis at three depths, to determine the mechanism for soil structural recovery over a 19-year period. The results showed that the total porosity and hydraulic properties were improved, with the age of the regenerating grassland, due to a better rearrangement of soil particles which in combination with the shrinkage induced realignment of pores in the soil matrix, resulted in better pore continuity. Changes in hydraulic stresses were most pronounced in the intensely rooted soil layers with further enhancement of the soil structure during restoration. This was similarly observed in the reconstructed images, which further highlight the strong contribution from the root development and organic matter conversion to the improved pore networks. Our result also showed that ignoring the depth and time dependent shrinkage behaviour of within the soil matrix resulted in overestimation of water retention potential in the first 8 years since the commencement of the restoration programme, and in the underestimation of water retention in the structurally stable soil beyond this period The results from these combined analyses; allowed a more methodical, detailed, quantitative and qualitative assessment of temporal soil structural recovery in naturally recovering grassland soil.