Quantification of soil aggregate microstructure on abandoned cropland during vegetative succession using synchrotron radiation-based micro-computed tomography

Information for the microstructure of soil aggregates is crucial for understanding the mechanisms of various soil processes. The quantification of complex aggregate microstructure and its relationship to vegetative restoration, however, remains elusive. The objective of this study was to evaluate the impact of natural revegetation on aggregate microstructure using synchrotron-based X-ray micro-computed tomography and image analysis. Soil samples were collected from an active cropland and from four former croplands that have been abandoned for 6, 12, 23, and 32 years on the Loess Plateau, China. Soil aggregates (3-5 mm) were scanned at a voxel resolution of 3.25 μm, and the aggregate pore structure was visualized and quantified with ImageJ software. The stability of wet aggregates and other soil properties were also evaluated. The amount of soil organic carbon increased significantly and soil bulk density decreased significantly with abandonment age. Aggregate water stability was higher after revegetation but did not differ significantly among the abandoned sites. Total porosity, percentage of pores >75 μm, and the fraction of elongated pores increased, but the number of pores, percentage of pores <75 μm, and fractions of regular and irregular pores decreased after the croplands were abandoned. The fractal dimension, degree of anisotropy and the Euler number all indicated that aggregate microstructures were more connected and developed at all abandoned sites than in the active cropland, but the fractal dimension was more sensitive for monitoring the quality of the soil structure. The results from this study can help to improve our understanding of the soil processes during natural vegetative succession and of the importance of pore morphology in monitoring the quality of soil structure.