Influence of shrub roots on soil macropores using X-ray computed tomography in a shrub-encroached grassland in Northern China

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.