Influence of exclosure on CT-measured soil macropores and root architecture in a shrub-encroached grassland in northern China

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.