The Root-Soil Water Relationship Is Spatially Anisotropic in Shrub-Encroached Grassland in North China: Evidence from GPR Investigation

To analyze the root-soil water relationship at stand level, we integrated ground-penetrating radar (GPR), which characterized distribution of lateral coarse roots (>2 mm in diameter) shrubs (Caragana microphylla Lam.), with soil core sampling, mapped content (SWC) distribution. GPR surveys and sampling were carried out two plots (Plot 1 2017 Plot 2 2018) same size (30 × 30 m2) sandy semi-arid shrubland northern China. First, survey area was divided into five depth intervals, i.e., 0–20, 20–40, 40–60, 60–80, 80–100 cm. Each interval then three zones horizontal direction, including root-rich canopy-covered area, canopy-free root-poor to indicate different surface distances canopy. The generalized additive models (GAMs) used correlation between root density SWC after spatial autocorrelation each variable eliminated. Results showed that varies vertical directions. Vertically, more are distributed high fewer low SWC. Namely, is positively correlated direction. Horizontally, is, however, complex. In 1, significant (p < 0.05) negatively middle intervals (20–40 cm 40–60 cm). 2, also 0.01) but non-monotonic correlated, increasing, mean decreased first increased. Moreover, discussed possible mechanisms, e.g., uptake, 3D distribution, preferential flow along roots, growing stages, might lead spatially anisotropic level. This study demonstrates advantages ecohydrology studies field scale challenging for traditional methods. reported here complement existing knowledge about environments shed new insights on modeling complex ecohydrological processes zone.