Effect of vegetation type on microstructure of soil aggregates on the Loess Plateau, China

Several types of vegetation restoration have been implemented on the Loess Plateau in China to control soil erosion and improve soil quality. Different revegetation types, however, have varying effects on soil structure; effects on the pore network of aggregates are especially not well understood. We used synchrotron-based high-resolution X-ray micro-computed tomography to quantify the microstructure of soil aggregates under four types of revegetation and an active cropland on the plateau. Five aggregates (3-5 mm) collected from the topsoil at each site were scanned at a voxel resolution of 3.25 μm, and the aggregate pore structure was visualized and quantified with ImageJ. Total porosities, >75 μm porosities, fractions of elongated pores, 3D mass fractal dimensions, and connectivity were higher and the numbers of pores, <75 μm porosities, the mean pore-shape factors, and the fractions of regular and irregular pores were lower in the revegetated sites than the control plot. Total porosities, macro-porosities, micro-porosities, fractions of regular pores, and 3D mass fractal dimensions differed significantly among the revegetated sites. We suggest that the fraction of elongated pores can be used as an important indicator for monitoring the recovery of soil structure. 3D mass fractal dimensions differed more than connectivity in the aggregates in the same samples, and thus could be a more sensitive indicator of changes in the pore network. Age and revegetation type both significantly affected the development of soil structure, but revegetation type was more important for the recovery of soil structure. We used a soil structural index (SSI) obtained by principal component analysis to assess the overall quality of soil structure. SSI values were higher in all revegetated sites than the cropland site and differed among the revegetated sites in the order: shrubland > grassland > woodland > pastureland. We recommend shrub plantation and natural grassland for the revegetation of degraded land on the Loess Plateau.