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Grassland exclosures with fencing are commonly used to prevent shrub encroachment. However, few studies have been conducted on the effect of exclosures on soil macropores and root architecture during shrub encroachment. This study aimed to quantify the effects of exclosures on soil macropore and root characteristics beneath the encroached shrub C. microphylla L. in the Inner Mongolia grassland. Two exclosure treatments, 9EX (shrubland enclosed for 9 years) and 2EX (shrubland enclosed for 2 years), were designed, with free grazing plots serving as controls. A total of twelve soil cores (0–50 cm deep) were excavated, which included three replicates of each treatment and control. Soil and root architecture of the soil cores were explored using X-ray computed tomography method. Results indicated that macroporosity was 1.03–1.66 times greater in the soil under the enclosed C. microphylla L. than in the soil under the grazed C. microphylla L., and the soil macroporosity increased with exclosure age. Macropores were abundant over the 0–400 mm soil layers under the enclosed shrubland but were mainly present at 0–250 mm soil depth under freely grazed shrubland. Exclosure led to significant proliferation of shrub roots compared with grazing. Root density was 1.04–1.86 times greater in the soil under the enclosed C. microphylla L. than in the soil under the grazed C. microphylla L. Roots were concentrated in the 0–250 mm soil layer in the grazed shrubland, whereas roots were abundant over 0–400 mm in the enclosed shrubland. The high number of macropores under the enclosed shrubland was closely associated with a high root density. These results suggest that exclosure enhances shrub root proliferation and therefore increases soil macroporosity and soil pore connectivity, which would be favourable for shrub encroachment.

The relationship between soil macropore and plant roots warrants special attention because it influences the behavior of water in soil. However, the influence of shrub roots on soil macropores in shrub-encroached grasslands is poorly understood. The objectives of this study were to quantify soil macropores and root architecture in a shrub-encroached grassland in northern China, and to reveal the relationship between shrub roots and soil macropore. In this study, treatments were performed that corresponded to three successional states of the shrub C. microphylla L. with three different shrub densities. At each site, three undisturbed soil cores were excavated under the shrub canopies, and a Philips medical scanner was used to simultaneously visualize and quantify the soil and root architectures. Strong positive correlations between root volume, length, and surface area and the solid surface/solid volume ratio were found, and greater root growth was noted in more porous soil. The results highlighted that the soil macropore characteristics corresponded well with the root characteristics of the soils for the three treatments. Soil macroporosity and macropore volume increased with increasing shrub root network density. In addition, the influence of plant roots on soil macropores increased with increasing shrub encroachment. The study confirmed that the large number of macropores found in the soils under shrubs was attributed to the great degree of root development there. The greater degree of macroporosity under shrubs was attributed to the larger root network density, which might cause greater amounts of water to be concentrated in the deep soil layer by macropore flow under shrub patches. The influence of plant roots on soil macropores increased with increasing shrub encroachment.

Grassland exclosure is a widely-used option to prevent from grazing in degraded grasslands for restoration. However, the influence of exclosure on soil macropore of grassland remain scarce. The objective of this study was to quantify the pore architecture of grassland soils under exclosure. Two treatments, 9E (grassland enclosed for 9 years) and 5E (grassland enclosed for 5 years), were designed, with grazing as a control in the experiment. Nine soil columns (0–50 cm deep) were taken at the three sites with three replicates. At each site, three soil columns were from the grassland, and cores were scanned with a Philips Brilliance ICT Medical Scanner. Numbers of macropores, macroporosity, network density, length density, and node density within the 50-cm soil profile were interpreted from X-ray computed tomography to analyze soil pore architecture. The results indicated that exclosure significantly influenced CT-measured soil macroporosity in the Inner Mongolia grassland of northern China. Soils under enclosed grassland had greater macroporosity, length density, total volume, and node density than that of under freely grazed grassland. Macroporosity increased as the enclosure age increased. For soils under enclosed grassland, macropores were concentrated at 0–300-mm soil layers, and macropores were mainly present at 0–100-mm soil depth under freely grazed grassland. The large number of macropores found in soil under enclosed grassland can be attributed to greater root development. Exclosure increases soil macroporosity and improve soil structure.